N5,N10-methylenetetrahydromethanopterin reductase from Methanocaldococcus jannaschii also serves as a methylglyoxal reductase

In Methanocaldococcus jannaschii, methylglyoxal (MG) is required for aromatic amino acid biosynthesis. Previously, the reduction of MG to lactaldehyde in Methanocaldococcus jannaschii cell extracts using either NADPH or F420H2 was demonstrated; however, the enzyme responsible was not identified. Using NADPH as the reductant, the unknown enzyme was purified from cell extracts of Methanocaldococcus jannaschii and determined to be the F-420-dependent N-5,N-10-methylenetetrahydromethanopterin reductase (Mer). Here, we report that the recombinantly overexpressed Mer is able to use NADPH and MG (K-M of 1.6 and 1.0 mM, respectively) to produce lactaldehyde. Additionally, Mer does not catalyze the reduction of MG to lactaldehyde in the presence of reduced Fo, the precursor of F-420.