title: [Substrate specificity of cysteine lyase].
abstract: Substrate specificity is studied of cysteine lyase, a phosphopyridoxal-dependent enzyme belonging to the subgroup of beta-replacing lyases. This enzyme has a narrow specificity for the amino substrate; its only primary substrate is L-cysteine. Cysteine lyase has a broad specificity for the cosubstrate (replacing agent), catalysing the synthesis of L-cysteic acid from L-cysteine and sulfite ion or cystein thioesters (in the presence of some thiols). Enzyme is incapable to use alpha-phenyl- and alpha-methylcysteine as substrates. It is found that enzyme catalyses the exchange of alpha-H atoms of the aminoacid substrate cysteine with 3H2O. It does not catalyse alpha-hydrogenexchange in close structural analogues of substrate: L-alanine, D-serine, treonine, allo-threonine and 3-phosphoserine. L-Serine inhibited the synthesis of S-hydroxyethylcystein from cysteine and beta-mercaptoethanol (Ki of L-serine is 0,8-10(-2) M), participating at the first stage of reaction: the formation of a pyridoxylidenic derivative, which does not undergo the further alpha,beta-elimination of beta-replacement reactions.
mesh_terms: D000818:Animals; D013437:Carbon-Sulfur Lyases; D002642:Chick Embryo; D003544:Cysteic Acid / Q000096:biosynthesis; D003545:Cysteine / Q000031:analogs & derivatives / Q000378:metabolism; D004530:Egg Yolk; D005260:Female; D008190:Lyases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D011732:Pyridoxal Phosphate; D012694:Serine / Q000494:pharmacology; D013329:Structure-Activity Relationship
chemical_list: D012694:Serine; D011732:Pyridoxal Phosphate; D003544:Cysteic Acid; D008190:Lyases; D013437:Carbon-Sulfur Lyases; C019971:cysteine lyase; D003545:Cysteine
vernacular_title: Substratnaia Spetsifichnost Tsisteniliazy