Bacterial selenoenzymes and mechanisms of action

Abstract

A review, with refs. There are two types of bacterial selenoenzymes known at present: those that contain selenium in the form of selenocysteine residues in the polypeptide chains and those that instead contain a catalytically essential selenium in a dissociable cofactor form. Among the latter are the molybdopterin contg. enzymes, nicotinic acid hydroxylase, xanthine dehydrogenase and the recently discovered purine hydroxylase. The crystal structure of a related enzyme, carbon monoxide dehydrogenase, has been detd. This enzyme, unlike the other three, is present in various aerobic bacteria. Escherichia coli formate dehydrogenase is a selenium-dependent enzyme that contains selenocysteine, molybdenum, a molybdopterin-guanine dinucleotide and an iron-sulfur center. The crystal structures of the oxidized and reduced enzymes have been reported. This enzyme differs from classical molybdenum hydroxylases in that oxygen from water is not incorporated in the product. Clostridial glycine reductase is a selenocysteine-dependent enzyme that reduces glycine to acetylphosphate and ammonia. The reaction mechanism of this enzyme involves the cleavage of a carbon-nitrogen bond of the Schiff base deriv. of the glycine substrate and represents a new role of the ionized selenol group of the selenocysteine residue in metab. Selenium-carboxymethylselenocysteine is formed as the enzyme-bound intermediate on the selenoprotein A subunit of the enzyme complex in this reaction. Reductive cleavage of the selenoether, transfer to a cysteine residue of another subunit forming an acetylthioester and reaction with phosphate results in the formation of acetylphosphate. [on SciFinder(R)]