Glycolysis: arsenate poisoning: 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 · Glycolysis: arsenate poisoning

Sample item

A farm worker accidentally ingests a pesticide containing sodium arsenate. Red cell glycolysis is studied in vitro. Arsenate substitutes for inorganic phosphate in the glyceraldehyde-3-phosphate dehydrogenase reaction. What is the net effect on ATP produced by glycolysis from one glucose molecule?

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

The answer and the reasoning

B. Zero net ATP, because the arseno-compound hydrolyses spontaneously and bypasses phosphoglycerate kinase

Arsenate forms 1-arseno-3-phosphoglycerate, which is unstable and hydrolyses non-enzymatically to 3-phosphoglycerate. The step that normally yields ATP through phosphoglycerate kinase is bypassed, so the two ATP made in the pay-off phase are lost. Two ATP are still spent in the investment phase, so the net yield is zero; this is critical in red cells, which rely solely on glycolysis.

Option by option

  • A. Zero net ATP, because pyruvate kinase is directly inhibited by arsenate

    The block is at the phosphoglycerate kinase step, and arsenate does not directly inhibit pyruvate kinase.

  • B. Zero net ATP, because the arseno-compound hydrolyses spontaneously and bypasses phosphoglycerate kinase (correct)

    Correct. Bypass of phosphoglycerate kinase cancels the pay-off ATP, leaving no net gain.

  • C. Two net ATP, since hexokinase and phosphofructokinase steps are unaffected

    Two ATP is the normal net yield and ignores the loss of the pay-off from the 1,3-bisphosphoglycerate step.

  • D. Four net ATP, because substrate-level phosphorylation is increased

    Substrate-level phosphorylation is bypassed, not increased, so there cannot be extra ATP.

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, Zero net ATP, because the arseno-compound hydrolyses spontaneously and bypasses phosphoglycerate kinase. Arsenate forms 1-arseno-3-phosphoglycerate, which is unstable and hydrolyses non-enzymatically to 3-phosphoglycerate.

Why is "Zero net ATP, because pyruvate kinase is directly inhibited by arsena…" not the answer?

The block is at the phosphoglycerate kinase step, and arsenate does not directly inhibit pyruvate kinase.

Why is "Two net ATP, since hexokinase and phosphofructokinase steps are unaff…" not the answer?

Two ATP is the normal net yield and ignores the loss of the pay-off from the 1,3-bisphosphoglycerate step.

Why is "Four net ATP, because substrate-level phosphorylation is increased" not the answer?

Substrate-level phosphorylation is bypassed, not increased, so there cannot be extra ATP.

What concept does this question test?

It tests Glycolysis: arsenate poisoning in Biochemistry. The step that normally yields ATP through phosphoglycerate kinase is bypassed, so the two ATP made in the pay-off phase are lost. Two ATP are still spent in the investment phase, so the net yield is zero; this is critical in red cells, which rely solely on glycolysis.

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 Glycolysis: arsenate poisoning?

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.