Methane-producing archaea and methylotrophic bacteria use tetrahydromethanopterin (HMPT) and/or tetrahydrofolate (HF) as coenzymes in one-carbon (C1) transfer pathways. The α-proteobacterium AM1 contains a dihydromethanopterin reductase (DmrA) and two annotated dihydrofolate reductases (DfrA and DfrB). DmrA has been shown to catalyze the final step of HMPT biosynthesis; however， the functions of DfrA and DfrB have not been examined biochemically. Moreover， sequence alignment (BLAST) searches have recognized scores of proteins that share up to 99% identity with DmrA but are annotated as diacylglycerol kinases (DAGK). In this work， we used bioinformatics and enzyme assays to provide insight into the phylo... More
Methane-producing archaea and methylotrophic bacteria use tetrahydromethanopterin (HMPT) and/or tetrahydrofolate (HF) as coenzymes in one-carbon (C1) transfer pathways. The α-proteobacterium AM1 contains a dihydromethanopterin reductase (DmrA) and two annotated dihydrofolate reductases (DfrA and DfrB). DmrA has been shown to catalyze the final step of HMPT biosynthesis; however， the functions of DfrA and DfrB have not been examined biochemically. Moreover， sequence alignment (BLAST) searches have recognized scores of proteins that share up to 99% identity with DmrA but are annotated as diacylglycerol kinases (DAGK). In this work， we used bioinformatics and enzyme assays to provide insight into the phylogeny and substrate specificity of selected Dfr and DmrA homologs. In a phylogenetic tree， DmrA and homologs annotated as DAGKs grouped together in one clade. Purified histidine-tagged versions of the annotated DAGKs from and (respectively， sharing 69 and 84% identity with DmrA) showed only low activity in phosphorylating 1，2-dihexanoyl--glycerol when compared with a commercial DAGK from However， the annotated DAGKs successfully reduced a dihydromethanopterin analog (dihydrosarcinapterin， HSPT) with kinetic values similar to those determined for AM1 DmrA. DfrA and DfrB showed little or no ability to reduce HSPT under the conditions studied; however， both catalyzed the NADPH-dependent reduction of dihydrofolate. These results provide the first evidence that DfrA and DfrB function as authentic dihydrofolate reductases， while DAGKs with greater than 69% identity to DmrA may be misannotated and are likely to function in HMPT biosynthesis.
