Apentix/AP Biology/Unit 1: Chemistry of Life/Question
AP Biologymediummcq1 pt

In sickle cell disease one amino acid of hemoglobin's beta chain, a surface glutamic acid, is replaced by valine. How does this single substitution deform red blood cells?

A.Valine's nonpolar side chain creates a sticky hydrophobic patch on the surface that clumps hemoglobin molecules into fibers
B.The substitution destroys the heme group, so the chains can no longer bind oxygen at all
C.Glutamic acid was needed to join the four subunits with peptide bonds, which now fail to form
D.The change scrambles the sugar-phosphate backbone that organizes the hemoglobin tetramer

Explanation

Core Concept

Surface glutamate normally presents a negative charge that interacts favorably with water. Replacing it with valine exposes a nonpolar hydrocarbon patch instead. Under low oxygen these patches slot into hydrophobic pockets on neighboring hemoglobin tetramers, driving the long fiber aggregates that push red blood cells into their sickle shape. Oxygen binding persists, subunits are never linked by peptide bonds, and hemoglobin contains no sugar-phosphate backbone.

Correct Answer

AValine's nonpolar side chain creates a sticky hydrophobic patch on the surface that clumps hemoglobin molecules into fibers

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