Practice question

Chemically selenocysteine represents cysteine analogue where chalcogen sulfur replaced by heavier selenium, generating selenol side chain -SeH instead of thiol -SH. Selenium larger radius, higher polarizability, pKa approximately 5.2 versus cysteine 8.3, rendering selenolate anion predominant at physiological pH, conferring superior nucleophilicity, leaving group ability, and resistance to irreversible overoxidation forming selenenic acid reversible. This explains enhanced catalytic rate of glutathione peroxidases, thioredoxin reductases, and deiodinases. Selenium originates from dietary trace element via selenophosphate pathway, not post-translational sulfur substitution. Similarity permits substitution maintaining tertiary fold but dramatically alters redox potentials and enzymatic rate acceleration crucial for antioxidant defense.