Lipid metabolism: conjugation of bile acids: 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 · Lipid metabolism: conjugation of bile acids

Sample item

Primary bile acids are conjugated in the liver before secretion. With which amino acids are they conjugated, and why does this help?

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

The answer and the reasoning

B. Glycine or taurine, which lowers the pKa so they stay ionised at intestinal pH

Cholic and chenodeoxycholic acids are conjugated with glycine or taurine. The conjugates have a lower pKa, so they remain ionised and water-soluble at the pH of the small intestine, which makes them better detergents for micelle formation. Bacterial deconjugation in the gut is followed by reabsorption through the enterohepatic circulation.

Option by option

  • A. Cysteine or methionine, which makes them insoluble micelles

    Cysteine and methionine are not conjugating amino acids, and conjugation improves solubility.

  • B. Glycine or taurine, which lowers the pKa so they stay ionised at intestinal pH (correct)

    Correct. Conjugation with glycine or taurine makes them stronger, fully ionised detergents at intestinal pH.

  • C. Alanine or serine, which allows reabsorption in the stomach

    Alanine and serine are not used for conjugation, and conjugated bile acids are not absorbed in the stomach.

  • D. Glutamine or asparagine, which raises the pKa so they stay un-ionised

    Glutamine and asparagine are not the conjugating amino acids, and conjugation lowers rather than raises the pKa.

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, Glycine or taurine, which lowers the pKa so they stay ionised at intestinal pH. Cholic and chenodeoxycholic acids are conjugated with glycine or taurine.

Why is "Cysteine or methionine, which makes them insoluble micelles" not the answer?

Cysteine and methionine are not conjugating amino acids, and conjugation improves solubility.

Why is "Alanine or serine, which allows reabsorption in the stomach" not the answer?

Alanine and serine are not used for conjugation, and conjugated bile acids are not absorbed in the stomach.

Why is "Glutamine or asparagine, which raises the pKa so they stay un-ionised" not the answer?

Glutamine and asparagine are not the conjugating amino acids, and conjugation lowers rather than raises the pKa.

What concept does this question test?

It tests Lipid metabolism: conjugation of bile acids in Biochemistry. The conjugates have a lower pKa, so they remain ionised and water-soluble at the pH of the small intestine, which makes them better detergents for micelle formation. Bacterial deconjugation in the gut is followed by reabsorption through the enterohepatic circulation.

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 Lipid metabolism: conjugation of bile acids?

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.