Discovery of a new type of methylenetetrahydrofolate reductase family that couples with tetrahydrofolate-dependent demethylases

Methylenetetrahydrofolate reductase (MTHFR) is a key enzyme in one-carbon (1C) metabolism, catalyzing the reduction of 5,10-methylenetetrahydrofolate to 5-methyltetrahydrofolate. Interestingly, Sphingobium lignivorans SYK-6, a model bacterium for the catabolism of lignin-derived aromatic compounds, has a unique MTHFR (S6MTHFR) that has been proposed to catalyze the reverse reaction of typical MTHFRs—namely, the oxidation of 5-methyltetrahydrofolate. However, no direct evidence for this has been obtained. Here, we demonstrated that S6MTHFR catalyzes the oxidation of 5-methyltetrahydrofolate and elucidated the molecular mechanism underlying the unique enzymatic properties of S6MTHFR based on its crystal structure. Furthermore, a database search revealed that a group of bacteria, including S. lignivorans SYK-6, utilize tetrahydrofolate-dependent demethylases to produce 5-methyltetrahydrofolate, which in turn is oxidized by an S6MTHFR-type enzyme. We propose that the combination of a demethylase with an S6MTHFR-type enzyme forms a new type of 1C metabolism that may regulate methionine biosynthesis.