Sequence Analysis of Sarcosine Oxidase and Nearby Genes Reveals Homologies with Key Enzymes of Folate One-carbon Metabolism

Abstract

Corynebacterial sarcosine oxidase, a heterotetrameric (alpha beta gamma delta) enzyme containing covalent and noncovalent FAD, catalyzes the oxidative demethylation of sarcosine to yield glycine, H2O2, and 5,10-CH2-tetrahydrofolate (H4folate) in a reaction requiring H4folate and O2. The sarcosine oxidase operon contains at least five closely packed genes encoding sarcosine oxidase subunits and serine hydroxymethyltransferase (glyA), arranged in the order glyAsoxBDAG. The operon status of a putative purU gene, found 340 nucleotides downstream from soxG, is not known. No homology with other proteins is observed for the smallest sarcosine oxidase subunits gamma and delta. The beta subunit (405 residues) contains an ADP-binding motif near its NH2 terminus, the covalent FAD attachment site (H175), and exhibits homology with the NH2-terminal half of dimethylglycine dehydrogenase (857 residues) and monomeric, bacterial sarcosine oxidases (approximately 388 residues), enzymes that contain a single covalent FAD. The alpha subunit (967 residues) contains a second ADP-binding motif within an approximately 280 residue region near the NH2 terminus that exhibits homology with subunit A from octopine and nopaline oxidases, heterodimeric enzymes that catalyze analogous oxidative cleavage reactions with N-substituted arginine derivatives. An approximately 380 residue region near the COOH terminus of alpha exhibits homology with T-protein and the COOH-terminal half of dimethylglycine dehydrogenase. These enzymes catalyze the formation of 5,10-CH2-H4folate, using different one-carbon donors. The results suggest that the alpha subunit and dimethylglycine dehydrogenase contain an NH2-terminal domain that binds noncovalent or covalent FAD, respectively, and a carboxyl-terminal H4folate-binding domain.