Carbohydrate metabolism: vitamin B6 in glycogen phosphorylase: 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 · Carbohydrate metabolism: vitamin B6 in glycogen phosphorylase

Sample item

Most of the body's pyridoxal phosphate is found in skeletal muscle. The vitamin is mainly bound to which enzyme?

ACystathionine synthase, in transsulfurationBGlycogen phosphorylase, participating in phosphorolysisCGlutamate decarboxylase, in GABA formationDAminolevulinate synthase, in heme synthesis

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

The answer and the reasoning

B. Glycogen phosphorylase, participating in phosphorolysis

A large share of body pyridoxal phosphate is bound to glycogen phosphorylase in muscle. Here the phosphate group, not the aldehyde, participates in acid-base catalysis of the phosphorolytic cleavage of glycogen. This pool is not readily lost in short-term deficiency.

Option by option

A. Cystathionine synthase, in transsulfuration
Cystathionine synthase needs PLP, but it is a minor liver pool compared with muscle phosphorylase.
B. Glycogen phosphorylase, participating in phosphorolysis (correct)
Correct. Muscle glycogen phosphorylase holds most body PLP, using its phosphate group in catalysis.
C. Glutamate decarboxylase, in GABA formation
Glutamate decarboxylase needs PLP in brain, a small pool compared with the muscle store.
D. Aminolevulinate synthase, in heme synthesis
Aminolevulinate synthase needs PLP, but it is a small hepatic and erythroid pool, not the main store.

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.

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is B, Glycogen phosphorylase, participating in phosphorolysis. A large share of body pyridoxal phosphate is bound to glycogen phosphorylase in muscle.

Why is "Cystathionine synthase, in transsulfuration" not the answer?

Cystathionine synthase needs PLP, but it is a minor liver pool compared with muscle phosphorylase.

Why is "Glutamate decarboxylase, in GABA formation" not the answer?

Glutamate decarboxylase needs PLP in brain, a small pool compared with the muscle store.

Why is "Aminolevulinate synthase, in heme synthesis" not the answer?

Aminolevulinate synthase needs PLP, but it is a small hepatic and erythroid pool, not the main store.

What concept does this question test?

It tests Carbohydrate metabolism: vitamin B6 in glycogen phosphorylase in Biochemistry. Here the phosphate group, not the aldehyde, participates in acid-base catalysis of the phosphorolytic cleavage of glycogen. This pool is not readily lost in short-term deficiency.

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 Carbohydrate metabolism: vitamin B6 in glycogen phosphorylase?

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.