Selenocysteine is the 21st proteinogenic amino acid. Selenocysteine is a cysteine analogue with a selenium-containing selenol group in place of the sulfur-containing thiol group. It is present in several enzymes. Selenocysteine was discovered by biochemist Theresa Stadtman, wife of Earl R. Stadtman, at the National Institutes of Health. Selenocysteine has a structure similar to that of cysteine, but with an atom of selenium taking the place of the usual sulfur, forming a selenol group which is deprotonated at physiological pH. Proteins that contain one or more selenocysteine residues are called selenoproteins and those with catalytic activities which depend on selenocysteine's biochemical activity are called selenoenzymes. The structurally characterized selenoenzymes have been found to employ catalytic triad structures that influence the nucleophilicity of the active site selenocysteine.
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