Enzyme kinetics: glucokinase: 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 · Enzyme kinetics: glucokinase

Sample item

A 14-year-old girl has stable mild fasting hyperglycaemia of about 7 mmol/L with HbA1c 6.4%, no ketosis and a similar finding in her mother. Heterozygous glucokinase mutation is found. Which property normally lets this enzyme act as a beta-cell glucose sensor?

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

The answer and the reasoning

B. High Km for glucose and no inhibition by glucose 6-phosphate

Glucokinase has a Km of about 10 mM and a sigmoidal response to glucose, so its activity rises in proportion to glucose across the physiological range. It is not inhibited by glucose 6-phosphate, so flux tracks glucose supply. Loss-of-function mutations (MODY 2) raise the glucose threshold for insulin release, producing mild stable hyperglycaemia that seldom needs drug treatment.

Option by option

  • A. Strong feedback inhibition by glucose 6-phosphate

    Hexokinase, not glucokinase, is inhibited by glucose 6-phosphate.

  • B. High Km for glucose and no inhibition by glucose 6-phosphate (correct)

    Correct. A high Km without product inhibition makes it track glucose concentration.

  • C. Low Km for glucose, so it is saturated at fasting glucose

    A low Km describes hexokinase, which is saturated at normal glucose and cannot sense changes.

  • D. Activation by glucagon through phosphorylation

    Glucagon does not activate glucokinase and instead reduces glycolysis in liver.

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, High Km for glucose and no inhibition by glucose 6-phosphate. Glucokinase has a Km of about 10 mM and a sigmoidal response to glucose, so its activity rises in proportion to glucose across the physiological range.

Why is "Strong feedback inhibition by glucose 6-phosphate" not the answer?

Hexokinase, not glucokinase, is inhibited by glucose 6-phosphate.

Why is "Low Km for glucose, so it is saturated at fasting glucose" not the answer?

A low Km describes hexokinase, which is saturated at normal glucose and cannot sense changes.

Why is "Activation by glucagon through phosphorylation" not the answer?

Glucagon does not activate glucokinase and instead reduces glycolysis in liver.

What concept does this question test?

It tests Enzyme kinetics: glucokinase in Biochemistry. It is not inhibited by glucose 6-phosphate, so flux tracks glucose supply. Loss-of-function mutations (MODY 2) raise the glucose threshold for insulin release, producing mild stable hyperglycaemia that seldom needs drug treatment.

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 Enzyme kinetics: glucokinase?

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.