Enzymes: allosteric regulation of aspartate transcarbamoylase: 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 · Enzymes: allosteric regulation of aspartate transcarbamoylase

Sample item

Aspartate transcarbamoylase of E. coli gives a sigmoidal curve when reaction velocity is plotted against aspartate concentration. When CTP is added to the assay, the curve moves and the maximal velocity at saturating aspartate is unchanged. Which interpretation is correct?

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

The answer and the reasoning

B. The curve shifts to the right because CTP stabilises the low-affinity T state of the enzyme

Aspartate transcarbamoylase catalyses the committed step of bacterial pyrimidine synthesis and has catalytic and regulatory subunits. The sigmoid curve reflects cooperative T-to-R transition. CTP, the end product, binds the regulatory subunits (heterotropic negative effector), stabilises the T state and shifts the curve to the right (K-type effect) without changing Vmax, while ATP shifts it to the left.

Option by option

  • A. The curve becomes hyperbolic because CTP competes with aspartate for the active site of the enzyme

    Binding is at a separate regulatory site and the curve stays sigmoidal; a competitive inhibitor would give a hyperbolic enzyme a higher apparent Km but would not act like this.

  • B. The curve shifts to the right because CTP stabilises the low-affinity T state of the enzyme (correct)

    CTP is a heterotropic negative effector that favours the T state, so more substrate is needed to reach half-maximal velocity.

  • C. The curve shifts to the left because CTP stabilises the high-affinity R state and increases cooperativity

    A leftward shift with greater R-state stability is the effect of ATP, the positive effector, and not of CTP.

  • D. The curve flattens and Vmax falls because CTP binds covalently to the catalytic subunits

    Allosteric effectors bind non-covalently and reversibly, and Vmax is unchanged at saturating substrate in this K-type system.

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, The curve shifts to the right because CTP stabilises the low-affinity T state of the enzyme. Aspartate transcarbamoylase catalyses the committed step of bacterial pyrimidine synthesis and has catalytic and regulatory subunits.

Why is "The curve becomes hyperbolic because CTP competes with aspartate for…" not the answer?

Binding is at a separate regulatory site and the curve stays sigmoidal; a competitive inhibitor would give a hyperbolic enzyme a higher apparent Km but would not act like this.

Why is "The curve shifts to the left because CTP stabilises the high-affinity…" not the answer?

A leftward shift with greater R-state stability is the effect of ATP, the positive effector, and not of CTP.

Why is "The curve flattens and Vmax falls because CTP binds covalently to the…" not the answer?

Allosteric effectors bind non-covalently and reversibly, and Vmax is unchanged at saturating substrate in this K-type system.

What concept does this question test?

It tests Enzymes: allosteric regulation of aspartate transcarbamoylase in Biochemistry. The sigmoid curve reflects cooperative T-to-R transition. CTP, the end product, binds the regulatory subunits (heterotropic negative effector), stabilises the T state and shifts the curve to the right (K-type effect) without changing Vmax, while ATP shifts it to the left.

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 Enzymes: allosteric regulation of aspartate transcarbamoylase?

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.