Biomolecules and enzymes: mutarotation of glucose: 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: mutarotation of glucose

Sample item

A freshly prepared solution of pure alpha-D-glucose shows a specific rotation of +112 degrees, which slowly drifts to +52.7 degrees and then stays constant. Which change in the solution explains this?

AOxidation of carbon 1 of glucose to form gluconic acidBInterconversion of the alpha and beta anomers through the open-chain formCEpimerisation of glucose at carbon 4 into galactoseDIsomerisation of glucose to fructose through an enediol

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

The answer and the reasoning

B. Interconversion of the alpha and beta anomers through the open-chain form

In solution, the ring opens briefly to the aldehyde form and recloses as either anomer, so alpha and beta forms equilibrate (about one third alpha to two thirds beta). The optical rotation therefore moves to an intermediate equilibrium value. This change is called mutarotation.

Option by option

A. Oxidation of carbon 1 of glucose to form gluconic acid
Oxidation needs an oxidising agent, and none is present in a plain glucose solution.
B. Interconversion of the alpha and beta anomers through the open-chain form (correct)
Correct. Ring opening and reclosing interconvert the alpha and beta anomers until equilibrium is reached.
C. Epimerisation of glucose at carbon 4 into galactose
Epimerisation at carbon 4 needs an epimerase enzyme and does not occur spontaneously in solution.
D. Isomerisation of glucose to fructose through an enediol
Aldose-ketose isomerisation needs alkaline conditions or an isomerase and is not the cause of this drift.

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, Interconversion of the alpha and beta anomers through the open-chain form. In solution, the ring opens briefly to the aldehyde form and recloses as either anomer, so alpha and beta forms equilibrate (about one third alpha to two thirds beta).

Why is "Oxidation of carbon 1 of glucose to form gluconic acid" not the answer?

Oxidation needs an oxidising agent, and none is present in a plain glucose solution.

Why is "Epimerisation of glucose at carbon 4 into galactose" not the answer?

Epimerisation at carbon 4 needs an epimerase enzyme and does not occur spontaneously in solution.

Why is "Isomerisation of glucose to fructose through an enediol" not the answer?

Aldose-ketose isomerisation needs alkaline conditions or an isomerase and is not the cause of this drift.

What concept does this question test?

It tests Biomolecules and enzymes: mutarotation of glucose in Biochemistry. The optical rotation therefore moves to an intermediate equilibrium value. This change is called mutarotation.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. 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: mutarotation of glucose?

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.