Emerging awareness of the critical roles of S-phosphocysteine and selenophosphate in biological systems.

S-Phosphocysteine residues in proteins are formed as key intermediates in certain enzyme-catalyzed reactions. In phosphoenolpyruvate (PEP)-dependent carbohydrate transport processes, the phosphoryl group of PEP is transferred sequentially to histidine residues of two cytoplasmic proteins, Enzyme I and HPr, common to all of the PEP-dependent carbohydrate transport systems. The phosphoryl group of HPr then is transferred to an essential histidine and an essential cysteine residue located in the cytoplasmic domains of certain substrate-specific, membrane-bound proteins, the actual transporters. Both the N-phosphohistidine and the S-phosphocysteine residues in these Enzyme II domains are transient catalytic intermediates in the final steps that lead to phosphorylation of the bound carbohydrate substrate. In a different type of biological system, numerous proteins involved in signal transduction pathways are phosphorylated on specific tyrosine residues by various kinases and the effects of these modifications are modulated by a family of protein tyrosine phosphate-specific phosphatases. The mechanism of action of these phosphatases involves the transfer of the phosphoryl group from a phosphotyrosine residue in the protein substrate to an essential ionized cysteine residue in the phosphatase polypeptide forming an S-phosphocysteine residue. Reaction of the latter with water and release of orthophosphate complete the phosphatase-catalyzed reaction. Selenophosphate, formed by phosphorylation of a selenol, is a key selenium donor compound in prokaryotes. This extremely oxygen-labile compound is synthesized from ATP and selenide by selenophosphate synthetase. This novel reactive selenium compound is the selenium donor for selenocysteyl-tRNA biosynthesis and for conversion of 2-thiouridine residues in tRNAs to 2-selenouridine.(ABSTRACT TRUNCATED AT 250 WORDS)