Promiscuity of methionine salvage pathway enzymes in Methanocaldococcus jannaschii

The methionine salvage pathway (MSP) is critical for regeneration of S-adenosyl-l-methionine (SAM), a widely used cofactor involved in many essential metabolic reactions. The MSP has been completely elucidated in aerobic organisms, and found to rely on molecular oxygen. Since anaerobic organisms do not use O2, an alternative pathway(s) must be operating. We sought to evaluate whether the functions of two annotated MSP enzymes from Methanocaldococcus jannaschii, a methylthioinosine phosphorylase (MTIP) and a methylthioribose 1-phosphate isomerase (MTRI), are consistent with functioning in a modified anaerobic MSP (AnMSP). We show here that recombinant MTIP is active with six different purine nucleosides, consistent with its function as a general purine nucleoside phosphorylase for both AnMSP and purine salvage. Recombinant MTRI is active with both 5-methylthioribose 1-phosphate and 5-deoxyribose 1-phosphate as substrates, which are generated from phosphororolysis of 5'-methylthioinosine and 5'-deoxyinosine by MTIP, respectively. Together, these data suggest that MTIP and MTRI may function in a novel pathway for recycling the 5'-deoxyadenosine moiety of SAM in M. jannaschii. These enzymes may also enable biosynthesis of 6-deoxy-5-ketofructose 1-phosphate (DKFP), an essential intermediate in aromatic amino acid biosynthesis. Finally, we utilized a homocysteine auxotrophic strain of Methanosarcina acetivorans Δma1821-22Δoahs (HcyAux) to identify potential AnMSP intermediates in vivo. Growth recovery experiments of the M. acetivorans HcyAux were performed with known and proposed intermediates for the AnMSP. Only one metabolite, 2-keto-(4-methylthio)butyric acid, rescued growth of M. acetivorans HcyAux in the absence of homocysteine. This observation may indicate that AnMSP pathways substantially differ among methanogens from phylogenetically divergent genera.