In biology, sulfur frequently occurs in the form of cysteine, an amino acid that fulfills a wide range of different functions in proteins including disulfide formation, metal-binding, electron donation, hydrolysis, and redox-catalysis. The 'redox-chameleon' sulfur appears in several oxidation states in vitro. each of them exhibiting specific reactivity, redox-activity, and metal-binding properties. While cysteine-peptidases rely on reduced cysteine to catalyze hydrolytic reactions, many redox-enzymes use distinctively different cysteine redox-couples for exchange. electron, atom, and radical transfer reactions, Although cysteine and cystine can still be considered as the most abundant forms of cysteine in vivo, other modifications such as cysteine acids and sulfur-centered radicals are becoming increasingly important in biochemical research. As such, the biochemistry of sulfur remains a source of continuous investigation and excitement. (C) 2002 Elsevier Science (USA). All rights reserved.