Practice question

Genetic code expansion beyond canonical twenty residues includes selenocysteine as unique amino acid incorporated into active sites of antioxidant oxidoreductases such as glutathione peroxidases, thioredoxin reductases, and formate dehydrogenases. Encoded by in-frame UGA codon recoded via specialized machinery, selenocysteine possesses selenol group with lower pKa ~5.2 and higher nucleophilicity than cysteine enhancing catalytic efficiency. Officially designated twenty-first proteinogenic amino acid, it occurs sparsely across all domains of life. Its discovery overturned dogma of immutable codon assignments, illustrating context-dependent recoding influenced by mRNA structure, selenium availability, and dedicated elongation factors enabling selenoprotein diversity.