Biomolecules and enzymes: calcium-calmodulin and phosphorylase kinase: practice question

A Biochemistry question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Biochemistry · Biomolecules and enzymes: calcium-calmodulin and phosphorylase kinase

Sample item

During a sprint, glycogen breakdown in skeletal muscle rises within seconds, before adrenaline-driven signalling can act. Which signal directly links contraction to activation of phosphorylase kinase?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

B. Calcium bound to calmodulin, a subunit of phosphorylase kinase

Phosphorylase kinase contains a calmodulin subunit, so the Ca2+ released from the sarcoplasmic reticulum on stimulation partly activates it. This links contraction to glycogenolysis within seconds, and adrenaline-cAMP signalling adds to it later. Muscle has no glucagon receptors.

Option by option

  • A. Fructose 2,6-bisphosphate

    Fructose 2,6-bisphosphate regulates glycolysis and gluconeogenesis, and does not act on phosphorylase kinase.

  • B. Calcium bound to calmodulin, a subunit of phosphorylase kinase (correct)

    Correct. Ca2+ binding to the calmodulin subunit activates phosphorylase kinase during contraction.

  • C. Glucagon acting through cAMP

    Muscle lacks glucagon receptors, so glucagon does not regulate muscle glycogenolysis.

  • D. Insulin-activated protein phosphatase 1

    Insulin favours dephosphorylation and glycogen synthesis, which opposes glycogenolysis.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is B, Calcium bound to calmodulin, a subunit of phosphorylase kinase. Phosphorylase kinase contains a calmodulin subunit, so the Ca2+ released from the sarcoplasmic reticulum on stimulation partly activates it.

Why is "Fructose 2,6-bisphosphate" not the answer?

Fructose 2,6-bisphosphate regulates glycolysis and gluconeogenesis, and does not act on phosphorylase kinase.

Why is "Glucagon acting through cAMP" not the answer?

Muscle lacks glucagon receptors, so glucagon does not regulate muscle glycogenolysis.

Why is "Insulin-activated protein phosphatase 1" not the answer?

Insulin favours dephosphorylation and glycogen synthesis, which opposes glycogenolysis.

What concept does this question test?

It tests Biomolecules and enzymes: calcium-calmodulin and phosphorylase kinase in Biochemistry. This links contraction to glycogenolysis within seconds, and adrenaline-cAMP signalling adds to it later. Muscle has no glucagon receptors.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Biomolecules and enzymes: calcium-calmodulin and phosphorylase kinase?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.