Biology
72 Solved Past Papers
2014 – 2024 Archives
Enzymes Past Papers
Solved past paper MCQs for Enzymes from official UHS, NUMS, SZABMU, DUHS, and KMU examinations. Includes verified distractor autopsies and step-by-step cognitive explanations.
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The enzymes enable the conversion of substrates into products by: [KMU-2024]
A
Changing equilibrium in the direction of the substrate
B
Increasing the activation energy
C
Increasing the substrate concentration
D
Lowering the activation energy
View Answer & Propolis Autopsy
Correct Key: Option DDiagnostic Explanation
Concept:
A catalyst speeds up a chemical reaction without altering the thermodynamics or being consumed in the process.
Solution:
Chemical reactions require an initial burst of energy to destabilize current bonds so new ones can form (the transition state).
Enzymes function entirely by providing an alternative reaction pathway that requires much less initial energy.
By lowering the activation energy, a significantly larger fraction of molecules have enough energy to react, vastly accelerating the conversion of substrates into products.
Why other options are incorrect:
Enzymes NEVER change the chemical equilibrium; they only reach it faster. Increasing activation energy would slow the reaction down. Enzymes cannot magically conjure or increase substrate concentration.
Activation of all the following enzymes need a regulatory molecule except: [NUMS-2024]
A
Pepsinogen
B
Trypsinogen
C
Erypsin
D
Chymotrypsinogen
View Answer & Propolis Autopsy
Correct Key: Option CDiagnostic Explanation
Concept:
Many powerful digestive proteases are synthesized as inactive precursors (zymogens) to prevent them from destroying the cells that created them. They require a regulatory molecule to cleave and activate them.
Solution:
Pepsinogen (activated by HCl), Trypsinogen (activated by enterokinase), and Chymotrypsinogen (activated by trypsin) are all inactive zymogens (indicated by the '-ogen' suffix).
Erypsin, however, is a collective term for a mixture of specific peptidases found in intestinal juice that are already active and do not require a separate regulatory molecule to unmask their active sites.
Why other options are incorrect:
Pepsinogen, trypsinogen, and chymotrypsinogen are classic examples of enzymes that are absolutely dependent on regulatory cleavage for activation.
Induced fit model of enzyme activity suggests that an enzyme: [UHS-2024]
A
Cannot modify its active sites
B
Can bind to a single substrate
C
Can catalyze related reaction
D
Usually belong to non-regulatory enzyme
View Answer & Propolis Autopsy
Correct Key: Option CDiagnostic Explanation
Concept:
The Induced Fit model, unlike the rigid Lock and Key model, introduces structural flexibility to the active site.
Solution:
Because the active site can physically mold and adapt its shape upon binding, it is not restricted to one single, perfectly identical molecule.
This flexibility allows the enzyme to accommodate and catalyze reactions for a class of structurally related substrates (e.g., hexokinase can phosphorylate different six-carbon sugars).
Why other options are incorrect:
Option A is the exact opposite of the Induced Fit model (which is built on modifying the active site). Option B describes the absolute rigidity of the Lock and Key model. Regulatory enzymes actually heavily rely on induced fit mechanics.
What happens to the enzyme after an enzyme-catalyzed reaction? [KMU-2024]
A
Reduced to inactive form
B
Becomes inert
C
Changes into substrate
D
Used for another reaction
View Answer & Propolis Autopsy
Correct Key: Option DDiagnostic Explanation
Concept:
A fundamental property of any catalyst, biological or inorganic, is that it is not consumed by the reaction it facilitates.
Solution:
During catalysis, the enzyme temporarily binds to the substrate to form an ES complex.
Once the bonds are rearranged, the product is released.
The enzyme itself remains structurally and chemically unchanged, meaning it is instantly ready and used for another reaction. This reusability is why enzymes are only needed in very small concentrations in the body.
Why other options are incorrect:
Enzymes do not become inert or inactive simply by performing their job. They certainly do not magically transform into their own substrates.
An enzyme is capable of acting on a wide range of related substrates. Which of the following is a property of this enzyme? [NUMS-2024]
A
Inflexible active site
B
Regulatory enzyme
C
Non-regulatory enzyme
D
Highly specific enzyme
View Answer & Propolis Autopsy
Correct Key: Option CDiagnostic Explanation
Concept:
Enzyme specificity ranges from absolute specificity (acting on only one substrate) to group/relative specificity (acting on structurally related substrates).
Solution:
Non-regulatory enzymes: Many non-regulatory enzymes (such as general digestive enzymes like chymotrypsin, pepsin, and lipases) possess flexible active sites that accommodate a wide range of structurally related substrates (e.g., cleaving different peptide or ester bonds).
Regulatory (Allosteric) enzymes: These act as critical rate-limiting pacemakers of metabolic pathways. They must be highly specific for their substrate and regulatory effectors to prevent uncontrolled metabolic flux.
An enzyme acting on a broad range of related substrates conforms to the Induced Fit Model (flexible active site) and functions as a Non-regulatory enzyme.
Why other options are incorrect:
A (Inflexible active site): An inflexible active site (Lock and Key model) would confer rigid, absolute specificity to only one exact molecule, not a wide range.
B (Regulatory enzyme): Regulatory enzymes are highly specialized and strictly specific for precise metabolic control.
D (Highly specific enzyme): Directly contradicts the question stem stating the enzyme acts on a "wide range of related substrates".
Which of the following statement is incorrect regarding rate of enzymatic action? [UHS-2024]
A
Increase in enzyme concentration increases the rate
B
Increase in enzyme concentration reduces the rate
C
All enzymes work at their maximum rate at optimum
D
All enzymes work at their maximum rate at optimum pH temperature
View Answer & Propolis Autopsy
Correct Key: Option BDiagnostic Explanation
Concept:
Enzyme kinetics relies on collision theory. More enzymes mean more available active sites for substrates to collide with.
Solution:
If substrate concentration is abundant, adding more enzyme molecules directly increases the number of available active sites.
This leads to a higher frequency of successful collisions, linearly increasing the reaction rate.
Therefore, the statement that an increase in enzyme concentration reduces the rate is fundamentally backwards and mathematically incorrect.
Why other options are incorrect:
Option A is a true statement. Options C and D are true; peak performance (maximum rate) is achieved when environmental factors (pH and temperature) are exactly at their optimal levels.
Catalase is a vital protective enzyme found universally in the peroxisomes of nearly all aerobic living cells to neutralize toxic hydrogen peroxide.
Solution:
Because catalase operates inside the general intracellular environment of the cell (which is kept near physiological neutrality), its structure is optimized for this environment.
Therefore, catalase functions optimally at a neutral pH of approximately 7.0 (specifically around 7.6).
Why other options are incorrect:
A pH of 1 or 3 is extremely acidic (similar to stomach acid) and would instantly denature the intracellular catalase protein. pH 5 is also too acidic for peak catalase efficiency.
The digestive system is compartmentalized. The stomach is acidic, but the small intestine must be alkaline to protect the tissue and activate a new set of enzymes.
Solution:
The pancreas secretes lipase into the duodenum (small intestine) to digest fats.
The pancreas also secretes heavy concentrations of bicarbonate to neutralize incoming stomach acid, raising the intestinal pH to an alkaline level.
As an evolutionary result, pancreatic lipase operates best in a basic medium, specifically at a pH of roughly 8.0 (to 9.0).
Why other options are incorrect:
pH 2 is optimal for stomach enzymes (pepsin). pH 4 and 6 are acidic and would result in poor efficiency or denaturation of pancreatic enzymes.
Enzymatic activity can be inhibited by: [KMU-2024]
A
Heavy metal ions
B
Methane
C
Mutase
D
Noble gases
View Answer & Propolis Autopsy
Correct Key: Option ADiagnostic Explanation
Concept:
Certain external chemicals can act as severe, irreversible poisons to biological systems by violently attacking the fragile tertiary structure of proteins.
Solution:
Heavy metal ions (such as Mercury \( \text{Hg}^{2+} \), Lead \( \text{Pb}^{2+} \), and Silver \( \text{Ag}^+ \)) are highly toxic inhibitors.
They forcefully bind to the reactive sulfhydryl (-SH) groups of the amino acid cysteine within the enzyme.
This physically breaks the crucial disulfide bridges that hold the enzyme together, causing catastrophic and irreversible denaturation.
Why other options are incorrect:
Methane and noble gases are largely inert in biological systems and do not break protein bonds. Mutase is a class of isomerase enzymes, not an inhibitor.
Competitive inhibition relies on molecular mimicry. Because the bonds formed are temporary, the system can be manipulated by changing concentrations.
Solution:
Competitive inhibitors mimic the substrate and bind to the active site using weak, non-covalent interactions (like hydrogen bonds).
Because these bonds are weak, the inhibitor frequently attaches and detaches.
If the normal substrate concentration is heavily increased, it will outcompete the inhibitor for the active site, proving that the inhibition is reversible.
Why other options are incorrect:
Inhibitors decelerate, not accelerate, reactions. It competes with the substrate (for access to the enzyme), it does not compete with the enzyme itself. By definition, true competitive inhibitors are reversible (unlike heavy metals).
Which enzyme is secreted in the active form? [KMU-2024]
A
Amylase
B
Trypsin
C
Pepsin
D
Chymotrypsin
View Answer & Propolis Autopsy
Correct Key: Option ADiagnostic Explanation
Concept:
Cells that produce protein-digesting enzymes must protect themselves from being digested by their own secretions. Enzymes that digest safe materials don't need this protection.
Solution:
Proteases (like trypsin, pepsin, and chymotrypsin) are highly dangerous to the cells that produce them because cells are made of proteins. Thus, they are secreted as inactive zymogens (-ogens).
Amylase, however, digests starch (carbohydrates). Since the salivary glands and pancreas are not made of starch, amylase poses no threat to the internal cellular machinery.
Therefore, amylase can be safely secreted completely in its active form.
Why other options are incorrect:
Trypsin, Pepsin, and Chymotrypsin are all proteases secreted as inactive trypsinogen, pepsinogen, and chymotrypsinogen, respectively.
Ions of heavy metal are harmful for living organisms because of their ability to: [NUMS-2024]
A
Cleave nucleic acid
B
Interfere with fat metabolism
C
Break glycosidic bonds
D
Destabilize proteins
View Answer & Propolis Autopsy
Correct Key: Option DDiagnostic Explanation
Concept:
Heavy metals are profound cellular poisons because they target the fundamental architectural pillars of proteins.
Solution:
The 3D folding of many critical enzymes is maintained by strong covalent disulfide bridges between cysteine amino acids.
Heavy metal ions (like \( \text{Hg}^{2+} \), \( \text{Pb}^{2+} \)) have an immense chemical affinity for the sulfur in these sulfhydryl (-SH) groups.
When they bind to the sulfur, they physically rip apart the disulfide bridges, causing the protein to unfold. This ability to violently destabilize proteins leads to rapid cell death.
Why other options are incorrect:
While heavy metals can indirectly affect all metabolism by destroying the enzymes involved, their direct, primary chemical mechanism of toxicity is the irreversible denaturation and destabilization of proteins, not specifically attacking fats, nucleic acids, or carbohydrates.
Enzyme catalase protects the plant cell from: [NUMS-2024]
A
Dehydration
B
Photorespiration
C
Toxic effect of alcohol metabolism
D
Protein loss
View Answer & Propolis Autopsy
Correct Key: Option CDiagnostic Explanation
Concept: Peroxisomal enzyme activity in cellular detoxification.
Solution: Catalase breaks down toxic hydrogen peroxide (\(H_2O_2\)) generated during oxidative reactions (including organic metabolite breakdown) into harmless water and oxygen, protecting the cell from oxidative damage.
In cellular respiration, enzymes require organic helper molecules to transfer electrons safely between metabolic pathways.
Solution:
NAD (Nicotinamide Adenine Dinucleotide) is an organic, non-protein molecule derived from Vitamin B3 (niacin).
It binds loosely and temporarily to redox enzymes (dehydrogenases) to accept and donate electrons (as \( \text{NADH} \)).
Because it is an organic, detachable helper molecule, it is officially classified as a Coenzyme.
Why other options are incorrect:
NAD is not a protein, so it cannot be an enzyme. It is derived from a vitamin, but it is not the vitamin itself. It does not act as a hormone (signaling molecule) in the endocrine system.
Non protein but inorganic detachable co-factor is called: [SZABMU-2023]
A
Prosthetic group
B
Co-enzyme
C
Activator
D
Apoenzyme
View Answer & Propolis Autopsy
Correct Key: Option CDiagnostic Explanation
Concept:
Co-factors are classified by two distinct properties: their chemical composition (organic vs. inorganic) and how tightly they bind to the enzyme (detachable vs. permanent).
Solution:
If a co-factor is inorganic (usually a metal ion like \( \text{Mg}^{2+} \) or \( \text{Zn}^{2+} \)) and detaches easily after the reaction, it is explicitly defined as an Activator.
Why other options are incorrect:
A Co-enzyme is detachable but is strictly organic (like NAD). A Prosthetic group is permanent/covalently bonded (can be organic or inorganic). The apoenzyme is the protein part itself.
All of the following are related to enzymes except: [ETEA-2023]
A
Remain unchanged after reaction
B
Speed up reaction
C
Increase the activation energy
D
Possess the active site
View Answer & Propolis Autopsy
Correct Key: Option CDiagnostic Explanation
Concept:
Enzymes are biological catalysts. Their entire thermodynamic function is to make reactions easier to achieve, not harder.
Solution:
Enzymes speed up chemical reactions.
They possess an active site where the substrate binds.
They remain structurally unchanged at the end of the reaction, ready to be used again.
However, they function strictly by lowering the activation energy barrier. They NEVER increase the activation energy.
Why other options are incorrect:
Remaining unchanged, speeding up the reaction, and possessing an active site are all fundamental, factual properties of enzymes. Increasing activation energy is the exact opposite of what a catalyst does.
The coenzyme NAD is made up of: [ETEA-2023 Retake]
A
Vitamins
B
Sugar
C
Amino acids
D
Nucleotides
View Answer & Propolis Autopsy
Correct Key: Option DDiagnostic Explanation
Concept:
To understand the function of a coenzyme, we must look at its underlying biochemical structure and its full chemical name.
Solution:
NAD stands for Nicotinamide Adenine Dinucleotide.
As the name implies, it is structurally composed of two distinct nucleotides joined together by their phosphate groups.
One nucleotide contains an adenine base, and the other contains nicotinamide.
Why other options are incorrect:
While NAD is functionally derived from a vitamin (niacin), its physical structural building blocks are nucleotides. It is not made of amino acids (proteins) or simple sugars alone.
What alters the shape of enzyme and makes them functional: [ETEA-2023 Retake]
A
Cofactor
B
Other enzymes
C
Proteins
D
Both A and C
View Answer & Propolis Autopsy
Correct Key: Option ADiagnostic Explanation
Concept:
Many enzymes are synthesized in an incomplete, non-functional state (apoenzyme) and require an external trigger to complete their specific 3D active site.
Solution:
Apoenzymes are inactive on their own.
When a specific cofactor (like a metal ion or coenzyme) binds to the apoenzyme, it physically alters the protein's conformational shape.
This shape change properly aligns the active site, converting the useless apoenzyme into a fully functional, active holoenzyme.
Why other options are incorrect:
While some enzymes (kinases) can activate other enzymes via phosphorylation, the universal structural requirement that completes an enzyme's shape to make it a holoenzyme is a cofactor.
Biological macromolecules are divided into four main classes: carbohydrates, lipids, proteins, and nucleic acids. Catalytic molecules fall into a specific class.
Solution:
Virtually all metabolic and digestive enzymes in living organisms are composed of long chains of amino acids folded into complex 3D structures.
Therefore, by definition, enzymes are universally classified as Proteins.
(Note: While RNA ribozymes exist, historically in basic biology curricula, the foundational rule is 'All enzymes are proteins').
Why other options are incorrect:
Lipids (fats) and carbohydrates (sugars) serve mainly as structural components or energy storage; they lack the complex chemical versatility required to form a catalytic active site.
What is 'flavin adenine dinucleotide'? [ETEA-2023 Retake]
A
Coenzyme
B
Enzyme
C
Catalyst
D
Cofactor
View Answer & Propolis Autopsy
Correct Key: Option ADiagnostic Explanation
Concept:
Flavin Adenine Dinucleotide (FAD) is a crucial electron carrier utilized primarily in the Krebs cycle during cellular respiration.
Solution:
FAD is an organic, non-protein helper molecule.
It works closely with specific oxidoreductase enzymes to accept electrons (becoming \( \text{FADH}_2 \)) and then transport them to the electron transport chain.
Because it is a detachable, organic molecule derived from a vitamin (riboflavin), its exact biochemical classification is a Coenzyme.
Why other options are incorrect:
FAD is not a protein, so it cannot be an enzyme. While it is technically a sub-type of cofactor, 'coenzyme' is the most accurate and specific classification for organic dinucleotides like FAD and NAD.
The active sites of enzymes are composed of? [UHS-2023]
A
Few nucleosides
B
Few nucleotides
C
Few amino acids
D
Few saccharides
View Answer & Propolis Autopsy
Correct Key: Option CDiagnostic Explanation
Concept:
Despite an enzyme being a massive protein structure containing hundreds of monomers, the business end of the molecule is remarkably small.
Solution:
Since enzymes are proteins, their foundational building blocks are amino acids.
The active site is a tiny, highly specific geometric pocket on the enzyme's surface.
It is composed of a strictly arranged configuration of just a few amino acids (typically 3 to 12 residues) whose R-groups perfectly interact with the substrate.
Why other options are incorrect:
Nucleotides/nucleosides form DNA and RNA. Saccharides form carbohydrates. Only amino acids form the active sites of protein enzymes.
Lock and Key Model for enzyme action proposed by Emil Fischer suggests that: [SZABMU-2023]
A
Enzymes can modify their active sites
B
Enzymes are unbiased for the substrate
C
An enzyme can catalyze variety of reactions
D
Enzymes are restricted to one reaction type
View Answer & Propolis Autopsy
Correct Key: Option DDiagnostic Explanation
Concept:
The foundational premise of Emil Fischer's 1894 model is extreme structural rigidity. A physical lock cannot change its shape to accommodate different keys.
Solution:
The Lock and Key model dictates that the active site is a rigid, pre-formed structure that exactly matches only one specific substrate.
Because it can physically only bind to one specific chemical, the enzyme is completely restricted to one reaction type.
Why other options are incorrect:
Modifying active sites is the hallmark of Koshland's Induced Fit model. Catalyzing a variety of reactions or being 'unbiased' directly violates the strict specificity of the Lock and Key concept.
Enzymes work by lowering the ____ of the reactions they catalyze: [SZABMU-2023]
A
Activation energy
B
Potential energy
C
Heat energy
D
Kinetic energy
View Answer & Propolis Autopsy
Correct Key: Option ADiagnostic Explanation
Concept:
For molecules to react, they must collide with enough force to disrupt existing electron clouds and reach an unstable transition state.
Solution:
The minimum amount of energy required to achieve this transition state is the activation energy.
Enzymes act as physical templates, binding substrates in the perfect orientation and putting stress on their bonds.
This incredibly efficient process severely lowers the activation energy required, allowing the reaction to proceed rapidly at physiological temperatures.
Why other options are incorrect:
Enzymes do not reduce the innate kinetic or potential energy of the molecules, nor do they cool the reaction down (heat energy). They exclusively target the energy barrier of the transition state.
Metabolic pathways are tightly controlled by specific rate-limiting enzymes that can be turned on or off via allosteric regulation, preventing the over-accumulation of products.
Solution:
Hexokinase is the crucial first enzyme in the glycolysis pathway.
It phosphorylates glucose to trap it inside the cell.
It is highly regulated via feedback inhibition (inhibited by its own product, glucose-6-phosphate), making it a classic textbook example of a regulatory (allosteric) enzyme.
Why other options are incorrect:
Maltase, sucrase, and urease are simple digestive/hydrolytic enzymes. They generally operate continuously based solely on substrate availability and lack the complex allosteric regulatory sites found in pathway gatekeepers like hexokinase.
Which one of the following factors does not affect the rate of enzyme action? [ETEA-2023]
A
Enzyme concentration
B
Water concentration
C
Substrate concentration
D
Temperature
View Answer & Propolis Autopsy
Correct Key: Option BDiagnostic Explanation
Concept:
Enzyme kinetics are governed by factors that directly alter the frequency of collisions between the active site and the substrate, or the physical integrity of the protein itself.
Solution:
Temperature affects collision kinetic energy and denaturation.
Enzyme and substrate concentrations dictate how many active sites are available and filled.
Water concentration (in standard physiological environments) acts merely as a ubiquitous solvent. While severe dehydration stops life, mild fluctuations in water concentration do not directly or fundamentally alter the kinetic curve of enzyme action like the other targeted factors do.
Why other options are incorrect:
Temperature, enzyme concentration, substrate concentration, and pH are the four universally taught primary factors that directly dictate and alter the rate of enzymatic reactions.
Enzymes present in mammals work best at: [ETEA-2023]
A
\( 20^{\circ}\text{C} \)
B
\( 30^{\circ}\text{C} \)
C
\( 40^{\circ}\text{C} \)
D
\( 50^{\circ}\text{C} \)
View Answer & Propolis Autopsy
Correct Key: Option CDiagnostic Explanation
Concept:
Enzymes evolve to function optimally at the internal body temperature of the organism they belong to.
Solution:
Mammals are endothermic (warm-blooded) and generally maintain a core body temperature around \( 37^{\circ}\text{C} \).
Enzymes in mammalian bodies have an optimum temperature range closely matching this, typically between \( 37^{\circ}\text{C} \) and \( 40^{\circ}\text{C} \).
Since \( 37^{\circ}\text{C} \) is not an option, \( 40^{\circ}\text{C} \) represents the closest upper limit of optimal physiological activity before significant denaturation begins.
Why other options are incorrect:
\( 20^{\circ}\text{C} \) and \( 30^{\circ}\text{C} \) are too cold for peak mammalian enzyme kinetics. \( 50^{\circ}\text{C} \) is too hot and would cause rapid thermal denaturation of mammalian proteins.
Most enzymes work best at the following temperature: [NUMS-2023]
A
\( 30^{\circ}\text{C} \)
B
\( 40^{\circ}\text{C} \)
C
\( 50^{\circ}\text{C} \)
D
\( 20^{\circ}\text{C} \)
View Answer & Propolis Autopsy
Correct Key: Option BDiagnostic Explanation
Concept:
The rate of an enzyme-catalyzed reaction increases with heat due to higher kinetic energy, up to a strict limit where the protein structure breaks down.
Solution:
For most human/mammalian enzymes, reaction rates peak just before the onset of thermal denaturation.
This optimal kinetic peak occurs around \( 37^{\circ}\text{C} \) to \( 40^{\circ}\text{C} \).
At \( 40^{\circ}\text{C} \), molecular collisions are maximized while the 3D protein structure remains largely intact.
Why other options are incorrect:
\( 50^{\circ}\text{C} \) destroys (denatures) most human enzymes. \( 20^{\circ}\text{C} \) and \( 30^{\circ}\text{C} \) do not provide enough kinetic energy for the reaction to reach its maximum possible rate.
In term of enzyme action, 'maximum temperature' refers to a temperature at which: [NUMS-2023]
A
Enzymes start to denature
B
Enzymes start to re-nature
C
Enzymes work best
D
Enzymes are reactivated
View Answer & Propolis Autopsy
Correct Key: Option ADiagnostic Explanation
Concept:
Enzyme activity is governed by three cardinal temperatures: Minimum, Optimum, and Maximum temperatures.
Solution:
Optimum Temperature: The temperature at which an enzyme exhibits its maximum catalytic activity (works best, typically ~37??C in humans).
Maximum Temperature: The upper critical temperature at which enzymes start to denature and lose their functional tertiary/quaternary structure due to heat-induced disruption of hydrogen and ionic bonds.
Minimum Temperature: The lowest temperature at which enzyme activity is detectable.
Therefore, 'maximum temperature' specifically refers to the point where enzymes start to denature.
Why other options are incorrect:
B & D (Re-nature / Reactivated): Heat denaturation is generally irreversible for globular enzymes.
C (Enzymes work best): This defines the Optimum Temperature, not the Maximum Temperature.
Adding substrates will not make a difference to the enzyme catalyzed reaction: [ETEA-2023 Retake]
A
When few enzyme molecules are left unsaturated
B
When the substrate cools down
C
When the substrate heats up
D
When many enzyme molecules remain unsaturated
View Answer & Propolis Autopsy
Correct Key: Option ADiagnostic Explanation
Concept:
Enzyme kinetics follows a saturation curve. The reaction rate is limited by the physical number of active sites available.
Solution:
Initially, adding substrate speeds up the reaction because there are plenty of empty enzymes.
However, once all active sites are occupied (saturated), the enzyme is working at its maximum velocity (\( V_{max} \)).
When very few enzyme molecules are left unsaturated, the system is essentially at full capacity. Adding more substrate at this point will not yield any significant increase in the reaction rate because there are no available active sites to process it.
Why other options are incorrect:
If 'many' enzymes are unsaturated (Option D), adding substrate will make a huge difference. Heating or cooling the substrate changes kinetic energy, but doesn't directly address the specific chemical concept of enzyme saturation limits.
To successfully block an active site, a competitive inhibitor must physically mimic the normal substrate.
Solution:
In biochemical terminology, molecules that share a deeply similar structural framework and functional group arrangement are often termed structural analogs or structural homologues.
Because they are structurally homologous to the true substrate, competitive inhibitors can wedge themselves perfectly into the highly specific geometry of the active site.
Why other options are incorrect:
'Analogous' generally implies a similar function but a different structure (which would fail to fit the active site). Size alone (simply being larger or smaller) is irrelevant; the precise 3D shape must match the substrate.
____ reduce the enzyme productivity by blocking the substrate entering into the active site due to similar shapes. [NUMS-2023]
A
Competitive inhibitors
B
Non-competitive inhibitors
C
Co- enzymes
D
Activators
View Answer & Propolis Autopsy
Correct Key: Option ADiagnostic Explanation
Concept:
Enzyme inhibition can occur via a direct physical blockade of the catalytic site.
Solution:
Competitive inhibitors possess a 3D molecular geometry that is remarkably similar to the natural substrate.
Due to this structural mimicry, they compete for and bind directly to the active site.
This acts as a physical plug, strictly blocking the true substrate from entering and consequently reducing overall enzyme productivity.
Why other options are incorrect:
Non-competitive inhibitors bind elsewhere (allosteric sites) and change the enzyme's shape, rather than directly blocking the entrance. Co-enzymes and activators are essential helpers that increase, not reduce, productivity.
Non- competitive inhibitors react with enzymes at: [NUMS-2023]
A
Allosteric site
B
Active site
C
Passive site
D
Regulatory site
View Answer & Propolis Autopsy
Correct Key: Option ADiagnostic Explanation
Concept:
Inhibitors that do not mimic the substrate must find an alternative location on the enzyme to exert their effect.
Solution:
Non-competitive inhibitors bind to a secondary, distinct pocket on the enzyme's surface.
This secondary site is universally referred to in biochemistry as an Allosteric site (from Greek allo meaning 'other' and steric meaning 'shape/space').
Binding here triggers a global conformational shift that warps the active site, destroying its ability to bind the substrate.
Why other options are incorrect:
Competitive inhibitors bind to the active site. While an allosteric site acts as a 'regulatory site', the term 'Allosteric site' is the specific, universally accepted terminology for the binding location of non-competitive inhibitors in standard curricula.
An example of competitive inhibitor molecule is: [ETEA-2023]
A
Insecticide
B
Sulphonamide
C
Cyanide
D
Metal ions
View Answer & Propolis Autopsy
Correct Key: Option BDiagnostic Explanation
Concept:
Many clinical drugs are designed as competitive inhibitors to specifically target and starve bacterial metabolic pathways.
Solution:
Sulphonamides (sulfa drugs) are a classic textbook example of competitive inhibition.
They are structurally very similar to para-aminobenzoic acid (PABA), a natural substrate bacteria use to synthesize folic acid.
The drug competitively binds to the bacterial enzyme instead of PABA, halting folic acid synthesis and killing the bacteria.
Why other options are incorrect:
Cyanide is an irreversible, non-competitive inhibitor that binds strongly to cytochrome c oxidase. Insecticides (like organophosphates) and heavy metal ions generally form strong covalent bonds, acting as irreversible/non-competitive poisons.
The inhibition, in which the inhibitor does not combine directly with the enzyme but binds to the enzyme substrate complex is called: [ETEA-2023]
A
Reversible inhibition
B
Competitive inhibition
C
Non-competitive inhibition
D
Uncompetitive inhibition
View Answer & Propolis Autopsy
Correct Key: Option DDiagnostic Explanation
Concept:
Inhibitors are classified by exactly when and where they bind to the enzyme during the catalytic cycle.
Solution:
If an inhibitor only recognizes and binds to the enzyme after the substrate has already attached (forming the Enzyme-Substrate or ES complex), it traps the substrate inside.
This highly specific mechanism is strictly defined as Uncompetitive inhibition.
It prevents the ES complex from converting into products and cannot be overcome by adding more substrate.
Why other options are incorrect:
Competitive inhibitors bind the free enzyme. Non-competitive inhibitors can bind both the free enzyme and the ES complex. Binding exclusively to the ES complex is the hallmark of uncompetitive inhibition.
Which of the following is an example of hydrolase? [ETEA-2023 Retake]
A
Transmethylase
B
Oxygenase
C
Phosphatase
D
Peroxidase
View Answer & Propolis Autopsy
Correct Key: Option CDiagnostic Explanation
Concept:
Hydrolases are a major class of enzymes that use water molecules to break chemical bonds (hydrolysis).
Solution:
A Phosphatase is an enzyme that removes a phosphate group from its substrate.
It accomplishes this cleavage by introducing a molecule of water (\( \text{H}_2\text{O} \)) across the bond, effectively hydrolyzing it.
Therefore, it securely belongs to the hydrolase category.
Why other options are incorrect:
Transmethylases transfer methyl groups (transferases). Oxygenases and peroxidases facilitate oxidation-reduction reactions (oxidoreductases). None of these rely on water to cleave bonds like hydrolases do.
Enzymes are biological macromolecules constructed from specific monomeric subunits that allow them to fold into highly complex, active structures.
Solution:
Almost all known biological enzymes are proteinaceous in nature (with the rare exception of ribozymes, which are RNA).
They are composed of long chains of amino acids that fold into a tertiary or quaternary structure, which creates the functional active site.
Why other options are incorrect:
Carbohydrates, lipids, and vitamins do not form the structural backbone of enzymes. (Vitamins often act as precursors to coenzymes, but are not the enzyme itself).
Zinc ion is attached to the active site of the enzyme carboxypeptidase. The zinc ion functions as: [UHS-2022]
A
An inhibitor molecule
B
A coenzyme molecule
C
An activator
D
Controller of allosteric site
View Answer & Propolis Autopsy
Correct Key: Option CDiagnostic Explanation
Concept:
Many enzymes require non-protein helper molecules for their catalytic activity. When this helper is an inorganic metal ion, it has a specific designation.
Solution:
A detachable inorganic co-factor is known as an activator.
Metal ions like \( \text{Zn}^{2+} \), \( \text{Mg}^{2+} \), and \( \text{Fe}^{2+} \) often act as activators by temporarily binding to the enzyme and helping to draw electrons away from the substrate, facilitating the reaction.
Why other options are incorrect:
Coenzymes are organic molecules (like vitamins). Inhibitors decrease enzyme activity, whereas zinc is required for carboxypeptidase to function. It binds to the active site to aid catalysis, not an allosteric regulatory site.
The catalytic activity of an enzyme is restricted to its small portion: [SZABMU-2022]
A
Regulation site
B
Allosteric site
C
Active site
D
Passive site
View Answer & Propolis Autopsy
Correct Key: Option CDiagnostic Explanation
Concept:
Although an enzyme is a massive macromolecule made of hundreds of amino acids, only a tiny fraction of its surface is actually involved in the chemical reaction.
Solution:
This highly restricted, specific region is called the active site.
The active site is a uniquely shaped cleft or crevice composed of a small number of specific amino acids (usually 3 to 12) where the substrate binds and is converted into a product.
Why other options are incorrect:
The allosteric/regulation site is used for turning the enzyme on or off via effector molecules, not for the main catalytic conversion of the substrate. 'Passive site' is not a recognized biological term.
The catalytic site of enzyme is made up of how many amino acids? [BUMHS-2022]
A
50-100
B
3-12
C
100-1000
D
30-50
View Answer & Propolis Autopsy
Correct Key: Option BDiagnostic Explanation
Concept:
The bulk of an enzyme's protein structure exists primarily to maintain the exact 3D framework required for a tiny functional region.
Solution:
The active/catalytic site is a very small region on the surface of the enzyme.
Different amino acids are specifically arranged into this site, but their total number is surprisingly small—typically varying from only 3 to 12 amino acids.
The rest of the hundreds of amino acids merely provide the structural scaffolding to keep those 3-12 amino acids in the exact right spatial orientation.
Why other options are incorrect:
Numbers like 50-100 or 100-1000 refer to the entire length of the polypeptide chain that forms the whole enzyme, not the minute active site.
Enzymes lower the activation energy by stabilizing the transition state of a metabolic reaction due to: [UHS-2022]
A
Distorting the molecules in the allosteric site
B
Changing conditions within the active site
C
Changing conditions within the protein framework
D
Rearranging the fatty acids in active site
View Answer & Propolis Autopsy
Correct Key: Option BDiagnostic Explanation
Concept:
Activation energy is lowered when an enzyme makes it physically and chemically easier for substrates to reach the unstable transition state.
Solution:
When a substrate binds to the active site, the enzyme subtly alters the microenvironment.
By changing conditions within the active site (such as altering the local pH, adjusting charge distribution, or putting mechanical stress on specific bonds), the enzyme stabilizes the transition state, requiring less energy for the reaction to proceed.
Why other options are incorrect:
Changes in the general protein framework or allosteric site are related to regulation, not the direct catalytic mechanism. Active sites are composed of amino acids, not fatty acids.
When substrate combines the active site of an enzyme it induces a conformational changes that enable enzyme to perform: [DUHS-2022]
A
Catalytic function
B
Meiosis
C
Metabolism
D
Analytic function
View Answer & Propolis Autopsy
Correct Key: Option ADiagnostic Explanation
Concept:
The active site of an enzyme is functionally divided into a binding site and a catalytic site. The Induced Fit model explains how these two cooperate.
Solution:
First, the substrate attaches securely to the binding site.
This binding physically induces a conformational (shape) change in the overall active site.
This shift perfectly aligns the specific amino acid residues of the catalytic site against the substrate's chemical bonds, inducing the enzyme to perform its catalytic function (breaking or forming bonds).
Why other options are incorrect:
Meiosis is cellular division. Metabolism is a broad term for all cellular reactions. 'Analytic function' is a made-up term in this context. The direct result of the conformational shift is catalysis.
Considering enzyme action, 'minimum temperature' is the term used when: [NUMS-2022]
A
Enzyme work at their best
B
Enzymes start denaturing
C
Inactive enzyme getting reactivated
D
Enzyme become hyperactive
View Answer & Propolis Autopsy
Correct Key: Option CDiagnostic Explanation
Concept:
At incredibly low temperatures (near freezing), molecular kinetic energy is so low that substrates rarely collide with enzymes. The enzyme is functionally inactive, but importantly, its physical structure is intact (not denatured).
Solution:
When heat is slowly reapplied, kinetic energy returns.
The 'minimum temperature' is defined as the lowest possible amount of thermal energy required to jumpstart the collisions and reactivate a temperature-inactivated enzyme.
As temperature rises past this minimum, activity increases until it reaches the optimum.
Why other options are incorrect:
'Working at best' is the optimum temperature. 'Denaturing' happens at the maximum/extreme high temperature. Low temperatures never make an enzyme hyperactive.
The digestive tract has different pH zones to compartmentalize digestion. The stomach is acidic, while the small intestine is highly basic.
Solution:
Pancreatic lipase is secreted by the pancreas directly into the duodenum of the small intestine.
The small intestine's environment is heavily buffered with bicarbonate to achieve a pH of around 9.0.
Therefore, pancreatic lipase has evolved to function perfectly in this highly alkaline medium.
Why other options are incorrect:
Pepsin works in the stomach's extreme acid (pH 2). Sucrase works in a mildly acidic environment (pH 4.5). Enterokinase is also intestinal but Pancreatic Lipase is the classic textbook example of a high alkaline optimum enzyme (pH 9).
In term of enzyme action maximum temperature refers to a temperature at which: [SZABMU-2022]
A
Enzymes start to denature
B
Enzymes work best
C
Enzymes are reactivated
D
Enzymes start to re-nature
View Answer & Propolis Autopsy
Correct Key: Option ADiagnostic Explanation
Concept: Kinetic thermal limits in enzymatic reactions.
Solution: In enzyme kinetics, the peak of the temperature curve represents the point of maximum molecular collisions where enzymes work best, immediately prior to thermal denaturation.
Optimum pH for digestive enzymes of stomach is: [BUMHS-2022]
A
Slightly acidic
B
Highly alkaline
C
Highly acidic
D
Slightly alkaline
View Answer & Propolis Autopsy
Correct Key: Option CDiagnostic Explanation
Concept:
The stomach is a unique digestive compartment designed to aggressively break down tough food particles and kill incoming pathogens.
Solution:
The parietal cells of the stomach lining secrete concentrated Hydrochloric Acid (\( \text{HCl} \)).
This drops the stomach's pH down to a drastically low level of approximately 2.0.
Therefore, the chief digestive enzyme of the stomach, pepsin, requires a highly acidic optimum pH to function effectively.
Why other options are incorrect:
A slightly acidic pH (like 6) is found in the mouth (saliva). Alkaline conditions exist in the small intestine. Only the stomach maintains a highly acidic environment.
Enzymes are generally inactivated rapidly by exposure to UV light and also \( \alpha \), \( \beta \) rays because it alters the: [DUHS-2022]
A
Quantity of enzyme
B
Shape of enzymes
C
pH of enzyme
D
Nature of enzyme
View Answer & Propolis Autopsy
Correct Key: Option DDiagnostic Explanation
Concept: High-energy radiation disrupts protein folding and tertiary stability.
Solution: Ionizing radiation disrupts internal hydrogen and disulfide bonds maintaining the tertiary structure of enzymes. This irreversibly denatures the protein, fundamentally altering the nature of the enzyme and permanently halting catalysis.
The active site of an enzyme is a highly specific physical space designed for one type of molecule. If two similar molecules are present, a chemical tug-of-war ensues.
Solution:
A competitive inhibitor is structurally analogous (has a very similar shape) to the normal substrate.
Because of this similarity, both molecules attempt to bind to the exact same physical location (the active site).
Therefore, the competitive inhibitor is directly competing with the substrate for access to the enzyme.
Why other options are incorrect:
It does not compete with the enzyme (it binds TO the enzyme). It does not compete with the product or coenzyme, as they occupy entirely different roles or phases of the reaction.
____ reduces the enzyme productivity by blocking the substrate from entering into the active site due to similar shapes. [SZABMU-2022]
A
Non-Competitive inhibitors
B
Activators
C
Competitive inhibitors
D
Co-enzymes
View Answer & Propolis Autopsy
Correct Key: Option CDiagnostic Explanation
Concept:
When two molecules share a very similar geometric structure, they can both fit into the same enzymatic binding pocket.
Solution:
Competitive inhibitors are structural imposters of the normal substrate.
By perfectly mimicking the substrate's shape, they successfully wedge themselves into the active site.
This creates a physical roadblock, preventing the true substrate from entering and bringing enzyme productivity to a halt.
Why other options are incorrect:
Non-competitive inhibitors do not have similar shapes and bind elsewhere. Activators and co-enzymes INCREASE enzyme productivity, they do not reduce it.
Enzymes work by lowering the ____ of the reactions they catalyze. [PMC-2020]
A
Kinetic energy
B
Activation energy
C
Heat energy
D
Potential energy
View Answer & Propolis Autopsy
Correct Key: Option BDiagnostic Explanation
Concept:
For a chemical reaction to occur, the reactants must overcome an initial energy barrier before they can be converted into products.
Solution:
This initial energy hurdle is known as the activation energy.
Enzymes stabilize the transition state of the reaction, providing an alternative pathway that requires significantly less activation energy.
By doing so, a much larger proportion of molecules have enough energy to react at biological temperatures.
Why other options are incorrect:
Enzymes do not alter the overall kinetic, heat, or potential energy of the reactants or products; they specifically lower the energy required to reach the transition state.
Lock and key model for enzyme action proposed by Emil Fischer suggests that: [PMC-2020]
A
Enzyme can catalyze variety of reactions
B
Enzymes can modify their active sites
C
Enzymes are unbiased for substrate
D
Enzymes are restricted to one reaction type
View Answer & Propolis Autopsy
Correct Key: Option DDiagnostic Explanation
Concept:
The Lock and Key model implies absolute rigidity and absolute specificity.
Solution:
Proposed by Emil Fischer in 1894, this model states that an enzyme's active site perfectly matches the shape of only one specific substrate, much like a specific key fits only one lock.
Because of this rigid and highly specific physical match, the enzyme is entirely restricted to catalyzing one single, specific type of reaction.
Why other options are incorrect:
The ability to modify active sites belongs to the Induced Fit model. Catalyzing a variety of reactions contradicts the core premise of absolute specificity in the Lock and Key model.
According to ____ model, the active site of enzyme is modified as the substrate interacts with enzyme. [MDCAT-2019]
A
Lock and key
B
Fluid mosaic
C
Induced fit
D
Emil Fischer
View Answer & Propolis Autopsy
Correct Key: Option CDiagnostic Explanation
Concept:
The interaction between an enzyme and its substrate is dynamic, not rigid. The enzyme's active site undergoes conformational changes upon substrate binding.
Solution:
The Induced Fit model, proposed by Daniel Koshland in 1959, suggests that the active site is flexible.
When a substrate combines with the enzyme, it induces a conformational change in the enzyme's structure.
This change aligns the catalytic groups perfectly to perform the reaction efficiently.
Why other options are incorrect:
The Lock and Key model (proposed by Emil Fischer) assumes a rigidly shaped active site that never changes. The Fluid Mosaic model describes cell membranes, not enzymes.
What is common in both competitive and non-competitive inhibition? [MDCAT-2019]
A
Non-Reversible inhibition
B
Reversible inhibition
C
Feedback inhibition
D
Irreversible inhibition
View Answer & Propolis Autopsy
Correct Key: Option BDiagnostic Explanation
Concept:
Enzyme inhibitors are broadly classified into two main categories based on whether their effects are permanent (covalent) or temporary (weak bonds).
Solution:
Both competitive (blocking the active site) and non-competitive (binding to an allosteric site) inhibitors are primary examples of reversible inhibition.
They attach to the enzyme using weak, non-covalent interactions (like hydrogen or ionic bonds).
Because these bonds are weak, the inhibitor can eventually detach, allowing the enzyme to regain its full catalytic function.
Why other options are incorrect:
Irreversible/non-reversible inhibition involves permanent destruction of the enzyme via strong covalent bonds (like poisons or heavy metals). Feedback inhibition is a specific metabolic pathway mechanism, not a structural classification shared universally by all competitive/non-competitive inhibitors.
A student of chemical engineering mistakenly engulfed the toxic compound 'A' which was a potent inhibitor of certain enzyme. He was immediately brought to hospital where the doctor injected intravenously substrate 'B' to minimize the toxic effect of compound 'A'. His life was saved from serious damages. The treatment method shows that compound 'A' was a ____ inhibitors. [MDCAT-2019]
A
Irreversible
B
Competitive reversible
C
Temperature sensitive
D
Non-competitive reversible
View Answer & Propolis Autopsy
Correct Key: Option BDiagnostic Explanation
Concept:
The defining clinical feature of competitive inhibition is that it can be overcome by flooding the system with excess natural substrate.
Solution:
The toxic compound 'A' was blocking the active sites of a vital enzyme.
By injecting a massive dose of normal substrate 'B', the doctors drastically shifted the concentration ratio.
The abundant substrate 'B' out-competed the toxic inhibitor 'A' for the active sites, allowing the enzyme to resume its life-saving function.
This proves the toxin formed weak bonds and fought for the active site, meaning it was a competitive reversible inhibitor.
Why other options are incorrect:
If it were irreversible, the enzyme would be permanently dead, and adding substrate would do nothing. If it were non-competitive, it would be bound elsewhere changing the active site shape, and flooding substrate into a broken active site would also fail.
Phosphatases belong to which group of the following? [ETEA-2019]
A
Ligase
B
Hydrolases
C
Lyases
D
None of the above
View Answer & Propolis Autopsy
Correct Key: Option BDiagnostic Explanation
Concept:
Enzymes are universally classified into six main classes based on the specific type of chemical reaction they catalyze.
Solution:
Phosphatases are enzymes that catalyze the removal of a phosphate group from a molecule.
They achieve this cleavage by using a molecule of water to break the chemical bond (a process known as hydrolysis).
Therefore, phosphatases are strictly categorized under the broader enzyme class of Hydrolases.
Why other options are incorrect:
Ligases join two molecules together (often using ATP). Lyases break chemical bonds without using water. Since they use water to break bonds, they must be hydrolases.
The temperature that promotes the maximum activity of enzyme is referred as: [MDCAT-2018]
A
Fixed temperature
B
Optimum temperature
C
Active temperature
D
Controlled temperature
View Answer & Propolis Autopsy
Correct Key: Option BDiagnostic Explanation
Concept:
As temperature increases, molecular collisions increase, speeding up the reaction rate—but only up to a specific threshold before the protein denatures.
Solution:
The precise temperature at which an enzyme works at its absolute maximum efficiency is called its optimum temperature.
For most enzymes in the human body, this temperature is around \( 37^{\circ}\text{C} \) (normal body temperature).
Beyond this point, the intense thermal kinetic energy breaks the delicate hydrogen bonds maintaining the enzyme's 3D shape, leading to rapid denaturation.
Why other options are incorrect:
'Fixed', 'Active', and 'Controlled' temperatures are not the standard scientific terms used in biochemistry to describe the peak of an enzyme's activity curve.
If molecule can bind to another site of the enzyme rather than the true active site, it is referred as [MDCAT-2018]
A
Competitive inhibitors
B
Non-allosteric inhibition
C
Irreversible inhibition
D
Non-competitive inhibitors
View Answer & Propolis Autopsy
Correct Key: Option DDiagnostic Explanation
Concept:
Inhibition can occur without directly blocking the active site if the enzyme possesses secondary regulatory binding pockets.
Solution:
When an inhibitor binds to a distinct, alternate location on the enzyme (an allosteric site), it alters the global 3D structure of the protein.
This remote structural distortion ruins the shape of the active site so the substrate can no longer fit.
Because it does not compete for the active site itself, it is accurately termed a non-competitive inhibitor.
Why other options are incorrect:
Competitive inhibitors strictly bind to the true active site. Irreversible inhibitors can bind anywhere but the defining feature here is the location (allosteric), not the permanence of the bond.
Enzymes are highly folded, complex molecules that act as biological catalysts to speed up chemical reactions within the body.
Solution:
Enzymes are composed of hundreds of amino acids joined together by peptide bonds.
They fold and coil upon themselves to form a specific three-dimensional, spherical shape known as a globular structure.
This globular shape is essential because it forms the highly specific active site required for substrate binding.
Why other options are incorrect:
Enzymes are not fibrous proteins (which are structural, like keratin or collagen) or lipoproteins, and they generally have very high molecular weights due to their long polypeptide chains.
The reactants on which enzyme work are: [MDCAT-2017]
A
Metabolites
B
Catabolites
C
Substrates
D
Products
View Answer & Propolis Autopsy
Correct Key: Option CDiagnostic Explanation
Concept:
In enzymatic reactions, the initial molecules that undergo transformation are specifically termed in biochemistry to distinguish them from generic chemical reactants.
Solution:
A molecule that is directly acted upon by an enzyme is defined as a substrate.
When substrates bind to enzymes, they undergo an enzyme-induced chemical change and are converted into the final products.
Why other options are incorrect:
Products are the result of the reaction, not the starting material. Metabolites and catabolites are general terms for molecules involved in or resulting from metabolic processes, but 'substrate' is the precise term for the reactant of an enzyme.
Enzymes function solely to lower activation energy and accelerate the rate at which equilibrium is reached. They do not alter the thermodynamics or the final nature of the reaction products.
Solution:
Enzymes are biocatalysts that speed up biochemical reactions.
Because they only alter the reaction pathway (lowering the activation energy), their presence does not affect the inherent chemical properties or the nature of the end product(s).
Why other options are incorrect:
Enzymes are globular (not fibrous), highly specific to their substrates (not non-specific), and remain entirely unchanged at the end of the reaction (they are not 'used up').
Which of the following comprises of inorganic ions? [MDCAT-2017]
A
Apoenzyme
B
Coenzymes
C
Prosthetic group
D
Activators
View Answer & Propolis Autopsy
Correct Key: Option DDiagnostic Explanation
Concept:
Enzyme co-factors are divided into organic and inorganic categories. Inorganic co-factors are typically metal ions necessary for catalysis.
Solution:
Activators are strictly inorganic ions (such as \( \text{Mg}^{2+} \), \( \text{Fe}^{2+} \), \( \text{Cu}^{2+} \), \( \text{Zn}^{2+} \)) that temporarily bind to the enzyme to enhance its activity.
They act as a 'bridge' between the enzyme and the substrate.
Why other options are incorrect:
Coenzymes are organic molecules. Prosthetic groups are covalently bonded (can be organic or inorganic, but 'activator' specifically defines the detachable inorganic ion class). The apoenzyme is the protein part.
Modified form of Lock and Key model was proposed by: [MDCAT-2017]
A
Fischer
B
Rosalind Franklin
C
Koshland
D
Watson
View Answer & Propolis Autopsy
Correct Key: Option CDiagnostic Explanation
Concept:
The rigidity of the early Lock and Key model failed to explain how some enzymes catalyze reactions for multiple related substrates or exhibit allosteric regulation.
Solution:
To resolve this, Daniel Koshland proposed a modified version known as the Induced Fit model in 1959.
He postulated that the active site continuously molds itself to the substrate as it binds, maximizing catalytic efficiency.
Why other options are incorrect:
Emil Fischer proposed the original, rigid Lock and Key model. Watson and Rosalind Franklin are famous for their work on the structure of DNA, not enzyme kinetics.
Which of the following type of inhibitor can be neutralized by adding more substrate into reaction? [MDCAT-2017]
A
Irreversible non-competitive
B
Reversible inhibitor
C
Irreversible competitive
D
Irreversible inhibitor
View Answer & Propolis Autopsy
Correct Key: Option BDiagnostic Explanation
Concept:
The effect of competitive inhibitors relies purely on a statistical game of chance regarding which molecule collides with the active site first.
Solution:
A competitive reversible inhibitor weakly binds to the active site without permanently damaging it.
Because it is a numbers game, if you drastically increase the concentration of the true substrate, the substrate molecules vastly outnumber the inhibitors.
The sheer probability of a true substrate colliding with the active site becomes so high that the effect of the inhibitor is completely neutralized.
(Note: The provided options historically list "Reversible inhibitor" as the correct choice for this competitive mechanism).
Why other options are incorrect:
Any irreversible inhibitor (competitive or non-competitive) forms permanent covalent bonds that destroy the enzyme; adding more substrate cannot save a destroyed active site.
A co-factor tightly bound to the enzyme on the permanent basis is called: [MDCAT-2014]
A
Activator
B
Prosthetic group
C
Apo-enzyme
D
Coenzyme
View Answer & Propolis Autopsy
Correct Key: Option BDiagnostic Explanation
Concept:
Co-factors are non-protein components required for enzyme activity. They are classified based on their chemical nature and how tightly they bind to the apoenzyme.
Solution:
If the non-protein part is covalently and permanently bonded to the enzyme's protein structure, it is defined as a prosthetic group.
This tight binding ensures the group is always present to assist in the enzyme's specific biochemical reaction.
Why other options are incorrect:
A coenzyme is loosely attached and organic. An activator is loosely attached and inorganic. An apoenzyme is the protein portion of the enzyme itself without its cofactor.
Which one of the following is the optimum pH of pancreatic lipase enzyme? [MDCAT-2014]
A
9.70
B
8.00
C
7.60
D
9.00
View Answer & Propolis Autopsy
Correct Key: Option DDiagnostic Explanation
Concept:
Enzymes have evolved to work optimally at the specific pH of their natural environment in the body.
Solution:
Pancreatic lipase is secreted by the pancreas into the small intestine to digest fats.
The environment of the small intestine is highly basic (alkaline) due to bicarbonate secretions.
Therefore, the optimum pH for pancreatic lipase is perfectly adapted to this alkaline medium, which is exactly 9.00.
Why other options are incorrect:
7.60 is the optimum for catalase, and 9.70 is the optimum for arginase. An optimum of 8.00 is slightly too low for pancreatic lipase's peak efficiency based on standard biological tables.
The competitive inhibitors have structural similarity with: [MDCAT-2014]
A
Binding site
B
Active site
C
Coenzyme
D
Substrate
View Answer & Propolis Autopsy
Correct Key: Option DDiagnostic Explanation
Concept:
Enzyme inhibition occurs when a foreign molecule prevents the normal substrate from binding. The mechanism depends on the inhibitor's physical shape.
Solution:
A competitive inhibitor is a chemical imposter.
It possesses a remarkably close structural similarity to the normal substrate.
Because of this mimicry, it fits perfectly into the active site, physically blocking the true substrate from entering and stopping the reaction.
Why other options are incorrect:
The inhibitor mimics the substrate, not the active site itself (it fits into the active site). It does not resemble a coenzyme or the broader binding site.
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