Amino acids: isoelectric point: 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 · Amino acids: isoelectric point

Sample item

Lysine has pKa values of 2.2 (alpha carboxyl), 9.0 (alpha amino) and 10.5 (side chain amino). What is its isoelectric point?

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

The answer and the reasoning

D. 9.75

The isoelectric point is the average of the two pKa values that bracket the zwitterion with zero net charge. For lysine the net charge changes from +1 to 0 at pKa 9.0 and from 0 to -1 at pKa 10.5, so pI = (9.0 + 10.5) / 2 = 9.75. Basic amino acids therefore have high pI values and move to the cathode at neutral pH.

Option by option

  • A. 5.6

    (2.2 + 9.0) / 2 = 5.6 would apply to a neutral amino acid, not a basic one.

  • B. 7.4

    7.4 is plasma pH, not the average of any bracketing pair.

  • C. 6.35

    (2.2 + 10.5) / 2 = 6.35 averages the wrong pair of pKa values.

  • D. 9.75 (correct)

    Correct. (9.0 + 10.5) / 2 = 9.75.

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 D, 9.75. The isoelectric point is the average of the two pKa values that bracket the zwitterion with zero net charge.

Why is "5.6" not the answer?

(2.2 + 9.0) / 2 = 5.6 would apply to a neutral amino acid, not a basic one.

Why is "7.4" not the answer?

7.4 is plasma pH, not the average of any bracketing pair.

Why is "6.35" not the answer?

(2.2 + 10.5) / 2 = 6.35 averages the wrong pair of pKa values.

What concept does this question test?

It tests Amino acids: isoelectric point in Biochemistry. For lysine the net charge changes from +1 to 0 at pKa 9.0 and from 0 to -1 at pKa 10.5, so pI = (9.0 + 10.5) / 2 = 9.75. Basic amino acids therefore have high pI values and move to the cathode at neutral pH.

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 Amino acids: isoelectric point?

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.