text: Tobacco etch virus protease uses this amino acid as its catalytic nucleophile and has high homology to its serine protease PA clan-mates.
full_question: 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.
first_sentence: Tobacco etch virus protease uses this amino acid as its catalytic nucleophile and has high homology to its serine protease PA clan-mates.
raw_answer: cysteine [or cys; or C] 