Sickle-cell hemoglobin arises from a Glu→Val substitution at one position in the beta chain. Analyze how this single change alters higher-order structure and leads to altered red blood cell morphology.

• A. The mutation changes the primary sequence but is rapidly corrected post-translationally so there are no structural or cellular consequences.
• B. The substitution is buried, causing only minor local changes to alpha helices and never affecting quaternary assembly, so it cannot plausibly change red blood cell shape or oxygen transport.
• C. Glu→Val always creates a disulfide bond that stiffens the tertiary structure and increases oxygen affinity but does not affect cell morphology.
• D. The Glu→Val substitution replaces a negatively charged, hydrophilic residue with a nonpolar residue on the surface of hemoglobin; the exposed hydrophobic patch promotes abnormal intermolecular interactions between hemoglobin tetramers (quaternary/aggregation), forming fibers that distort red blood cells into a sickle shape and impair function.

Answer: (D)