Apentix/AP Biology/Unit 1: Chemistry of Life/Question
AP Biologyhardmcq1 pt

Pepsin digests protein in the stomach at pH near 2 but becomes inactive in the small intestine near pH 8. Which molecular change best accounts for the loss of activity at the higher pH?

A.Loss or gain of protons by charged side chains breaks the ionic and hydrogen bonds that hold the active site's shape
B.The sugar-phosphate backbone of pepsin is cleaved once pH rises above 7
C.A phospholipid coat around pepsin melts and exposes the substrate pocket to water
D.Disulfide bridges convert every cysteine residue into a free methionine

Explanation

Core Concept

Changing pH changes protonation states and therefore the charges of amino acid side chains. That can disrupt substrate binding, catalytic acid-base chemistry, ionic interactions, hydrogen bonding, or the protein's conformation. Some loss of activity is reversible when pH returns to normal, whereas sufficiently extreme conditions can cause more persistent denaturation; 'reversible denaturation' should not be assumed.

Correct Answer

ALoss or gain of protons by charged side chains breaks the ionic and hydrogen bonds that hold the active site's shape

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