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Let Se = x. H = +1, O = -2. Equation: 2(+1) + x + 3(-2) = 0, 2 + x - 6 = 0, x = +4.
Ref: NCERT Class 11 Chemistry > Chapter 7: Redox Reactions > Topic: Classical and Electronic Concept of Redox and Oxidation Number
The +4 oxidation state is most stable for S in group 16, forming SO₂ (gaseous, strong S=O bonds), more stable than O₂, SeO₂ (polymeric), or TeO₂ (solid).
Ref: NCERT Class 11 Chemistry > Chapter 3: Classification of Elements and Periodicity in Properties > Topic: Modern Periodic Law and Present Form - Long Form Periodic Table
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
Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 3: Selenocysteine – Selenium vs Sulfur Chemistry
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 cod
Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 6: Selenocysteine – 21st Amino Acid
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