Tobacco etch virus protease uses this amino acid as its catalytic nucleophile and has high homology to its serine protease PA clan-mates. Four residues of this amino acid are found in the binding center of HiPIPs. SECIS elements redefine UGA codons to translate a non-standard analogue of this amino acid. Residues of this amino acid may be isomerized by PDI. Ubiquinol is bound by Q-cytochrome c oxidoreductase via the two histidines and two of this amino acid found in its 
Tobacco etch virus protease uses this amino acid as its catalytic nucleophile and has high homology to its serine protease PA clan-mates. Four residues of this amino acid are found in the binding center of HiPIPs. SECIS elements redefine UGA codons to translate a non-standard analogue of this amino acid. Residues of this amino acid may be isomerized by PDI. Ubiquinol is bound by Q-cytochrome c oxidoreductase via the two histidines and two of this amino acid found in its Rieske center. With glutamate and glycine, this amino acid forms glutathione. It coordinates with 2Fe-2S clusters in iron-sulfur proteins using its thiol group. For 10 points, name this amino acid that is distinguished from methionine by its ability to form disulfide bonds.