Amino acids that can form nucleophile including serine, cysteine, aspartate and glutamine. Sulfur is contained in the amino acids cysteine and methionine. Some amino acids, such as cysteine and methionine contain sulfur. Mammals biosynthesize the amino acid cysteine via homocysteine. It contains inhibitors of multiple cysteine and serine protease families. Most microorganisms and plants obtain the sulfur for synthesizing methionine from the amino acid cysteine. The latter is a component of the amino acids cysteine and methionine. Peptone iron agar contains the amino acid cysteine and a chemical indicator, ferric citrate. LIM genes encode two 60 amino acid cysteine and histidine-rich LIM domains and a homeodomain. In general, zinc fingers coordinate zinc ions with a combination of cysteine and histidine residues. Disulfide bonds in proteins are formed by oxidation of the sulfhydryl group of cysteine residues. The cysteine sulfhydryl group is nucleophilic and easily oxidized. The disulfide link is readily reduced to give the corresponding thiol cysteine. Active site cysteine residues are the functional unit in cysteine protease catalytic triads. It is a derivative of the amino acid cysteine.