Biomolecules and enzymes: heparin and antithrombin: 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.

Try it

Biochemistry · Biomolecules and enzymes: heparin and antithrombin

Sample item

A patient on unfractionated heparin has a prolonged aPTT. The drug acts by binding, through a specific sulfated pentasaccharide sequence, to which plasma protein and enhancing its inhibition of clotting factors?

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

The answer and the reasoning

B. Antithrombin

Heparin is a highly sulfated glycosaminoglycan with the greatest negative charge density of any biological macromolecule. Its pentasaccharide sequence binds antithrombin and causes a conformational change that speeds inactivation of thrombin and factor Xa by about a thousand-fold.

Option by option

  • A. Plasminogen

    Plasminogen is the precursor of plasmin in fibrinolysis; heparin does not bind it to produce anticoagulation.

  • B. Antithrombin (correct)

    Correct. The heparin pentasaccharide activates antithrombin, which then inhibits thrombin and factor Xa.

  • C. Alpha-2 antiplasmin

    Alpha-2 antiplasmin inhibits plasmin, so enhancing it would reduce fibrinolysis rather than clotting.

  • D. Protein C

    Protein C is a vitamin K-dependent anticoagulant activated by thrombin-thrombomodulin, and heparin does not act through it.

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.

More Biochemistry questions

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is B, Antithrombin. Heparin is a highly sulfated glycosaminoglycan with the greatest negative charge density of any biological macromolecule.

Why is "Plasminogen" not the answer?

Plasminogen is the precursor of plasmin in fibrinolysis; heparin does not bind it to produce anticoagulation.

Why is "Alpha-2 antiplasmin" not the answer?

Alpha-2 antiplasmin inhibits plasmin, so enhancing it would reduce fibrinolysis rather than clotting.

Why is "Protein C" not the answer?

Protein C is a vitamin K-dependent anticoagulant activated by thrombin-thrombomodulin, and heparin does not act through it.

What concept does this question test?

It tests Biomolecules and enzymes: heparin and antithrombin in Biochemistry. Its pentasaccharide sequence binds antithrombin and causes a conformational change that speeds inactivation of thrombin and factor Xa by about a thousand-fold.

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: heparin and antithrombin?

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.