title: Identification and biochemical characterization of serine hydroxymethyl transferase in the hydrogenosome of Trichomonas vaginalis.
abstract: Serine hydroxymethyl transferase (SHMT) is a pyridoxal phosphate (PLP)-dependent enzyme that catalyzes the reversible conversion of serine and tetrahydrofolate to glycine and methylenetetrahydrofolate. We have identified a single gene encoding SHMT in the genome of Trichomonas vaginalis, an amitochondriate, deep-branching unicellular protist. The protein possesses a putative N-terminal hydrogenosomal presequence and was shown to localize to hydrogensomes by immunofluorescence analysis, providing evidence of amino acid metabolism in this unusual organelle. In contrast to the tetrameric SHMT that exists in the mammalian host, we found that the T. vaginalis SHMT is a homodimer, as found in prokaryotes. All examined SHMT contain an 8-amino-acid conserved sequence, VTTTTHKT, containing the active-site lysyl residue (Lys 251 in TvSHMT) that forms an internal aldimine with PLP. We mutated this Lys residue to Arg and Gln and examined structural and catalytic properties of the wild-type and mutant enzymes in comparison to that reported for the mammalian protein. The oligomeric structure of the mutant K251R and K251Q TvSHMT was not affected, in contrast to that observed for comparable mutations in the mammalian enzyme. Likewise, contrary to that observed for mammalian SHMT, the catalytic activity of K251R TvSHMT was unaffected in the presence of PLP. The K251Q TvSHMT, however, was found to be inactive. These studies indicate that the active site of the parasite enzyme is distinct from its prokaryotic and eukaryotic counterparts and identify TvSHMT as a potential drug target.
authors.affiliation: Department of Microbiology, Immunology, and Molecular Genetics, University of California, Los Angeles, 609 Charles E. Young Drive East, Los Angeles, CA 90095-1489, USA.
authors.forename: Mandira
authors.initials: M
authors.lastname: Mukherjee
authors.forename: Stuart A
authors.initials: SA
authors.lastname: Sievers
authors.forename: Mark T
authors.initials: MT
authors.lastname: Brown
authors.forename: Patricia J
authors.initials: PJ
authors.lastname: Johnson
mesh_terms: D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D000818:Animals; D019281:Dimerization; D017125:Genes, Protozoan; D012696:Glycine Hydroxymethyltransferase / Q000235:genetics / Q000378:metabolism*; D007700:Kinetics; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D015388:Organelles / Q000201:enzymology; D020836:Protein Structure, Quaternary; D015800:Protozoan Proteins / Q000235:genetics / Q000378:metabolism*; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D014246:Trichomonas vaginalis / Q000201:enzymology* / Q000235:genetics
chemical_list: D015800:Protozoan Proteins; D011994:Recombinant Proteins; D012696:Glycine Hydroxymethyltransferase