Enzymes Regulation Questions and Answers

Answer: D

Correct answer: Heterotropic allosteric enzymes have the substrate and modulator same

That is the wrong way round. In a homotropic enzyme the substrate is itself the modulator - which is what produces cooperative binding and the sigmoid curve. In a heterotropic enzyme the modulator is a different molecule, typically the end product of the pathway.

Answer: C

Correct answer: Sigmoid curve

An allosteric enzyme has several subunits, and binding substrate to one of them changes the shape of the others - cooperativity. The result is that activity rises slowly at low substrate concentration, then steeply, then levels off, giving a sigmoid curve rather than the hyperbola of Michaelis-Menten kinetics. The steep middle section is what makes such enzymes good switches: a small change in substrate produces a large change in rate.

Answer: D

Correct answer: 5

Cells regulate enzymes in five main ways: allosteric control, covalent modification (phosphorylation and the rest), proteolytic activation of zymogens, the use of isoenzymes, and control of the amount of enzyme present by induction and repression. They differ chiefly in speed - allosteric control acts in milliseconds, induction over hours.

Answer: A

Correct answer: First enzyme

Regulation is applied to the first enzyme of a pathway - the committed step. Switching off there stops the whole sequence at once and wastes nothing, whereas inhibiting a later step would leave intermediates accumulating uselessly. It is also the enzyme the end product inhibits in feedback control.

Answer: C

Correct answer: Those involved in regulating enzyme activity

Regulatory enzymes are those whose activity can be turned up or down in response to a signal - by an allosteric modulator, by covalent modification, or by proteolytic activation. They sit at the control points of pathways, and the rest of the enzymes simply follow the flux they set.

Answer: C

Correct answer: Both

Regulation operates at two levels, and both are needed. Activity control - allosteric effects, phosphorylation - adjusts existing enzymes within seconds. Control of synthesis - induction and repression - changes how much enzyme there is at all, over a much longer period. Fast control handles moment-to-moment demand; slow control matches the cell’s enzyme complement to its circumstances.

Answer: D

Correct answer: All of the above

An allosteric enzyme has an active site where catalysis happens, a separate allosteric site where the regulator binds, and the modifiers that bind there. All three are part of the description - what defines the class is that the two sites are distinct and that binding at one changes the other.

Answer: A

Correct answer: 2

Effectors are of two kinds: positive (activators), which increase the enzyme’s affinity for substrate, and negative (inhibitors), which decrease it. Both act by binding at a regulatory site and shifting the enzyme between its more and less active conformations.

Answer: C

Correct answer: Multiple forms of enzyme

Isoenzymes are different molecular forms of an enzyme that catalyse the same reaction but differ in structure and in kinetic properties. They arise from different genes or different subunit combinations, and having several lets the same reaction be tuned differently in different tissues - which is what makes the LDH pattern useful in diagnosis.

Answer: C

Correct answer: Sigmoid shaped

An allosteric enzyme has several subunits, and binding substrate to one of them changes the shape of the others - cooperativity. The result is that activity rises slowly at low substrate concentration, then steeply, then levels off, giving a sigmoid curve rather than the hyperbola of Michaelis-Menten kinetics. The steep middle section is what makes such enzymes good switches: a small change in substrate produces a large change in rate.