GATE 2020 BT Q.39: solution and answer

Official answer: DSource: Official answer key, GATE 2020 BT (IIT Delhi)

1. Cells control metabolic flux mainly at key enzymes, by changing the activity of existing enzyme molecules or the amount of enzyme. The question asks which of P, Q and R are genuine regulatory strategies.

2. [P] Phosphorylation-dephosphorylation is reversible covalent modification. Kinases add phosphate and phosphatases remove it, switching an enzyme between active and inactive forms, for example glycogen phosphorylase and glycogen synthase. P is used.

3. [Q] Allostery is the binding of an effector at a site other than the active site, which changes the enzyme's conformation and activity, for example $\text{PFK-1}$ is activated by $\text{AMP}$ and inhibited by $\text{ATP}$ and citrate. Q is used.

4. [R] Feedback inhibition is the end product of a pathway inhibiting an early committed enzyme, for example $\text{CTP}$ inhibiting aspartate transcarbamoylase. R is used. It usually works through an allosteric site, but it is still a recognised regulatory strategy in its own right.

5. All three are used, so options A, B and C each leave out a valid strategy and are wrong. The answer is D.