Proteins III (Practice MC Questions)

Which of the following statements about enzymes is TRUE?

A. Enzymes increase activation energy

B. Enzymes change the equilibrium constant

C. Enzymes make endergonic reactions exergonic

D. Enzymes raise Gibbs free energy

E. Enzymes lower activation energy but do not change ΔG

E

The activation energy (Ea) represents:

A. The equilibrium constant of the reaction

B. The energy released when the reaction occurs

C. The overall Gibbs free energy of the products

D. The difference in reactant and product concentrations

E. The energy barrier that must be overcome for a reaction to occur

E

ΔG of a reaction determines:

A. The rate at which the reaction proceeds

B. The amount of enzyme required

C. Whether the reaction is thermodynamically favorable

D. The activation energy required

E. The enzyme concentration at equilibrium

C

A reaction with ΔG < 0 is:

A. Nonspontaneous

B. Endergonic

C. At equilibrium

D. Energy-neutral

E. Exergonic

E

A catalyst affects which of the following?

A. The ΔG of the reaction

B. The equilibrium constant

C. The concentration of product formed at equilibrium

D. The activation energy and reaction rate

E. The ratio of reactants to products

D

Which of the following represents the correct sequence of steps in an enzyme-catalyzed reaction?

A. ES → E + S → EP

B. E + S → E + P → EP

C. E + P → ES → EP → E + S

D. E + S → ES → EP → E + P

E. ES → EP → E + S

D

Enzymes release reaction products because:

A. Products have higher affinity for the enzyme than substrates

B. Products are more stable within the enzyme pocket

C. Products lower the enzyme's activation energy

D. Products have lower affinity for the enzyme than substrates

E. The enzyme changes its pH after the reaction

D

In enzymology, a catalyst does not affect:

A. The activation energy

B. The Gibbs free energy change (ΔG)

C. The speed of the reaction

D. The transition state

E. The rate constant

B

The equilibrium constant (Keq) of a reaction depends on:

A. The reaction pathway

B. The enzyme concentration

C. The activation energy

D. The relative energies of reactants and products

E. The pH and temperature

D

In enzyme catalysis, Ea is associated with ________, while ΔG is associated with ________.

A. Reactant concentration; product stability

B. Kinetics; thermodynamics

C. Enzyme turnover; binding affinity

D. Gibbs energy; entropy

E. Specificity; inhibition

B

Enzymes primarily act by:

A. Raising the activation energy

B. Lowering the activation energy

C. Changing ΔG of the reaction

D. Increasing Gibbs free energy

E. Decreasing product stability

B

Activation energy (Ea) determines:

A. The final energy of the products

B. The spontaneity of the reaction

C. The rate of the reaction

D. The equilibrium constant

E. The pH of the medium

C

ΔG < 0 implies the reaction is:

A. Endergonic and nonspontaneous

B. Endergonic and spontaneous

C. Exergonic and spontaneous

D. Isoenergetic

E. At equilibrium

C

Enzymes do not affect which of the following?

A. Activation energy

B. Reaction rate

C. Rate constant

D. ΔG (Gibbs free energy)

E. Transition state

D

In enzyme-catalyzed reactions, the equilibrium constant depends on:

A. The catalyst used

B. The pathway of reaction

C. The relative energies of reactants and products

D. The enzyme concentration

E. The pH of the buffer

C

The activation energy corresponds to:

A. ΔG between reactants and products

B. The energy of the transition state

C. The equilibrium constant

D. The heat released

E. The bond strength of substrates

B

Enzymes are classified as:

A. Structural proteins

B. Information molecules

C. Biological catalysts

D. Signaling molecules

E. Genetic regulators

C

In thermodynamic terms, when Keq > 1:

A. Reaction is endergonic

B. Reaction is exergonic

C. ΔG > 0

D. Equilibrium favors reactants

E. Reaction cannot occur

B

The Michaelis-Menten equation describes the relationship between:

A. Reaction rate and enzyme concentration

B. Reaction rate and substrate concentration

C. Product concentration and ΔG

D. Substrate structure and affinity

E. Enzyme inhibition

B

Vmax represents:

A. The substrate concentration at half-maximal velocity

B. The reaction velocity at infinite substrate concentration

C. The enzyme turnover rate

D. The lowest measurable velocity

E. The rate constant

B

Km measures:

A. The catalytic turnover number

B. The enzyme concentration

C. The enzyme's affinity for the substrate

D. The number of active sites

E. The reaction's Gibbs free energy

C

A low Km indicates:

A. High substrate affinity

B. Low substrate affinity

C. Enzyme inhibition

D. Irreversible binding

E. High enzyme concentration

A

The Lineweaver-Burke plot linearizes:

A. Km vs. enzyme

B. Reaction rate vs. product concentration

C. Michaelis-Menten equation

D. Enzyme inhibition

E. ΔG vs. temperature

C

The slope of a Lineweaver-Burke plot equals:

A. Vmax/Km

B. Km/Vmax

C. 1/Km

D. 1/Vmax

E. Vmax × Km

B

In a Michaelis-Menten curve, as [S] → ∞, velocity approaches:

A. 0

B. Km

C. ΔG

D. Vmax

E. ½ Vmax

D

The enzyme's catalytic efficiency improves when:

A. Km increases and Vmax increases

B. Km decreases and Vmax increases

C. Km increases and Vmax decreases

D. Km = Vmax

E. Vmax decreases

B

The hexokinase in the brain has:

A. A high Km for glucose

B. A low Km for glucose

C. No affinity for glucose

D. Noncompetitive kinetics

E. Inhibition by ATP

B

The liver glucokinase differs from brain hexokinase because it:

A. Has a higher Km

B. Has a lower Km

C. Is inhibited by glucose

D. Works only in anaerobic conditions

E. Cannot form ES complex

A

The enzyme active site usually exists as:

A. A flat surface

B. A random coil

C. A cleft or crevice

D. A hydrophobic channel

E. A crystalline cavity

C

Which residues bind substrates but do NOT catalyze reactions?

A. Catalytic residues

B. Active residues

C. Binding residues

D. Transition residues

E. Functional residues

C

Catalytic residues:

A. Position substrates

B. Stabilize water

C. Participate in bond making and breaking

D. Only stabilize the transition state

E. Do not interact with substrates

C

Non-covalent interactions in enzyme-substrate binding include all EXCEPT:

A. Hydrogen bonds

B. Salt bridges

C. Hydrophobic interactions

D. Disulfide bonds

E. Van der Waals forces

D

Temperature and pH affect enzymes by:

A. Changing the substrate structure only

B. Altering enzyme stability and binding

C. Modifying ΔG

D. Changing the number of enzymes

E. Increasing reaction equilibrium

B

The lock-and-key model suggests:

A. Substrate induces conformational change

B. Enzyme is rigid and complementary to substrate

C. Enzyme and substrate are identical

D. Metal cofactors control the binding

E. Active sites are flexible

B

The induced-fit model proposes that:

A. The enzyme structure is rigid

B. The substrate binds permanently

C. The enzyme changes shape to fit the substrate

D. No interaction occurs until after catalysis

E. The enzyme hydrolyzes itself

C

Metal cofactors in active sites mainly:

A. Denature enzymes

B. Compete with substrates

C. Orient charged groups and stabilize transition states

D. Block substrate binding

E. Form covalent bonds with enzyme backbone

C

Compared to metal catalysts, enzymes are:

A. Less specific

B. More specific and regulated

C. Chemically inert

D. Not affected by inhibitors

E. Stable at any pH

B

Enzyme active sites usually exclude:

A. Water (unless it's a reactant)

B. Ionic residues

C. Binding residues

D. Substrate molecules

E. Cofactors

A

Irreversible inhibitors typically:

A. Reversibly bind to active sites

B. Covalently modify active site residues

C. Alter enzyme conformation temporarily

D. Bind at allosteric sites

E. Change substrate structure

B

Reversible inhibitors:

A. Permanently inactivate enzymes

B. Dissociate rapidly from the enzyme

C. Require covalent modification

D. Cannot bind to enzyme-substrate complex

E. Only occur in metal catalysts

B

Which of the following describes competitive inhibition?

A. Inhibitor binds to allosteric site

B. Inhibitor binds only to ES complex

C. Inhibitor binds at active site, competing with substrate

D. Inhibitor binds irreversibly

E. Inhibitor binds after product release

C

Competitive inhibition causes:

A. No change in Km

B. Increased Vmax

C. Decreased Km

D. Increased Km, unchanged Vmax

E. Decreased Km, decreased Vmax

D

Example of a competitive inhibitor:

A. Cyanide

B. Digitalis

C. DIPF

D. Sulfanilamide

E. Nitric oxide

D

Noncompetitive inhibition occurs when:

A. Inhibitor binds to active site

B. Inhibitor binds only to ES complex

C. Inhibitor binds at a distinct allosteric site

D. Substrate concentration decreases

E. Product concentration increases

C

In noncompetitive inhibition:

A. Km increases

B. Km decreases

C. Vmax decreases, Km unchanged

D. Vmax increases

E. Both Km and Vmax decrease

C

Example of a noncompetitive inhibitor used medically:

A. Sulfanilamide

B. Digitalis

C. Nitric oxide

D. Penicillin

E. Methotrexate

B

Cyanide toxicity occurs because it:

A. Inhibits acetylcholinesterase

B. Blocks Na⁺/K⁺ ATPase

C. Prevents aerobic respiration by noncompetitive inhibition

D. Alters pH in mitochondria

E. Competes with glucose

C

Irreversible inhibitors differ from reversible ones because they:

A. Bind weakly to active sites

B. Are overcome by adding more substrate

C. Form covalent bonds with the enzyme

D. Bind allosterically

E. Only affect Vmax

C

Allosteric regulation involves:

A. Binding of ligands to active site

B. Denaturation of enzyme

C. Binding of ligands to a site distinct from the active site

D. Covalent modification of substrates

E. pH changes only

C

Homotropic allosteric regulation occurs when:

A. The regulator is different from the substrate

B. The regulator is a product or substrate of the reaction

C. The enzyme is covalently modified

D. The enzyme is denatured

E. A protein binds irreversibly

B

Heterotropic allosteric regulation involves:

A. The regulator being identical to the substrate

B. The regulator not participating in the enzymatic reaction

C. Permanent modification of the enzyme

D. Feedback inhibition

E. Metabolic activation

B

Covalent modification is most commonly achieved through:

A. Methylation

B. Glycosylation

C. Phosphorylation

D. Oxidation

E. Denaturation

C

Phosphorylation can:

A. Only deactivate enzymes

B. Only activate enzymes

C. Either activate or deactivate enzymes

D. Always decrease enzyme affinity

E. Destroy the active site

C

An example of enzyme activation by phosphorylation is:

A. Acetylcholinesterase

B. Glycogen phosphorylase

C. Na⁺/K⁺ ATPase

D. Dihydropteroate synthetase

E. Hexokinase

B

Protein kinase A is activated when:

A. cAMP binds and releases catalytic subunits

B. cAMP inhibits catalytic subunits

C. Phosphate groups are removed

D. ATP is hydrolyzed by cAMP

E. Glycogen is degraded

A

The rate-limiting step in a pathway:

A. Occurs at the end of the pathway

B. Is the fastest reaction

C. Determines the overall rate of the pathway

D. Cannot be regulated

E. Is unaffected by inhibitors

C

Feedback inhibition occurs when:

A. The end product inhibits an earlier enzyme

B. The substrate activates a later enzyme

C. An enzyme binds covalently to its substrate

D. The rate-limiting enzyme is degraded

E. ATP concentration drops

A

Feed-forward regulation occurs when:

A. A product inhibits an earlier step

B. An intermediate activates a later enzyme

C. Enzyme expression increases

D. ATP activates the first enzyme

E. pH triggers enzyme folding

B