Biomolecules and enzymes: cation-exchange chromatography of amino 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 · Biomolecules and enzymes: cation-exchange chromatography of amino acids

Sample item

A mixture of alanine, aspartate, glutamate and lysine at pH 7 is loaded onto a cation-exchange resin that carries negatively charged groups. The column is then eluted with increasing salt concentration. Which amino acid emerges last?

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

The answer and the reasoning

A. Lysine

A cation-exchange resin binds positively charged molecules. At pH 7 lysine carries a net positive charge, so it sticks most tightly and is displaced last. Aspartate and glutamate are negatively charged and are repelled, so they run through first, and alanine is a neutral zwitterion that elutes in between.

Option by option

  • A. Lysine (correct)

    Correct. Lysine is the only amino acid with a net positive charge at pH 7, so it binds the resin most strongly.

  • B. Glutamate

    Glutamate is negatively charged at pH 7 and is repelled by the resin.

  • C. Aspartate

    Aspartate is negatively charged at pH 7 and is repelled by the resin.

  • D. Alanine

    Alanine has no net charge at pH 7 and is not retained by electrostatic attraction.

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 A, Lysine. A cation-exchange resin binds positively charged molecules.

Why is "Glutamate" not the answer?

Glutamate is negatively charged at pH 7 and is repelled by the resin.

Why is "Aspartate" not the answer?

Aspartate is negatively charged at pH 7 and is repelled by the resin.

Why is "Alanine" not the answer?

Alanine has no net charge at pH 7 and is not retained by electrostatic attraction.

What concept does this question test?

It tests Biomolecules and enzymes: cation-exchange chromatography of amino acids in Biochemistry. At pH 7 lysine carries a net positive charge, so it sticks most tightly and is displaced last. Aspartate and glutamate are negatively charged and are repelled, so they run through first, and alanine is a neutral zwitterion that elutes in between.

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 Biomolecules and enzymes: cation-exchange chromatography of amino 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.