Question

Whenever the cell's ATP supply is depleted, which of the following enzyme’s activity is increased?

• A. Hexokinase
• B. Pyruvate kinase
• C. Glucokinase
• D. Phosphofructokinase-1

Hint:

PFK-1 is a key regulatory enzyme in glycolysis, and its activity is enhanced during low ATP conditions to promote energy production.

Answer 1

The Correct Option is D

Hexokinase is regulated by its own product, glucokinase responds to glucose levels, and pyruvate kinase is mainly controlled by feed-forward activation and hormonal signals -- none of these directly sense a falling ATP level as their main regulatory trigger.

Phosphofructokinase-1 is different: it is allosterically inhibited by high ATP and citrate, and activated by AMP and ADP, so as ATP falls, PFK-1 activity rises to speed up glycolysis and restore the cell's energy supply.

So the enzyme whose activity increases when ATP is depleted is phosphofructokinase-1.

Answer 2

Another way in is to recall which step of glycolysis is the committed, rate-limiting step, since that is usually where energy-charge regulation is concentrated.

1. Hexokinase: Catalyses an early, largely committed step for glucose entering metabolism generally, but its control point is product inhibition, not a direct energy-charge switch.
2. Pyruvate kinase: Sits at the last step of glycolysis and helps control output, but the pathway's main energy-charge-sensing control point lies further upstream.
3. Glucokinase: Functions mainly as a glucose sensor in the liver, not as an energy-charge regulator.
4. Phosphofructokinase-1: Catalyses the committed, rate-limiting step of glycolysis and is the pathway's primary checkpoint for cellular energy charge, directly speeding up when ATP is low and AMP/ADP rise.

The rate-limiting, energy-charge-sensing position in glycolysis belongs to phosphofructokinase-1.

The correct answer is phosphofructokinase-1.