COENZYME-F420 DEPENDENT N5, N-10-METHYLENETETRAHYDROMETHANOPTERIN DEHYDROGENASE IN METHANOL GROWN METHANOSARCINA-BARKERI

The dehydrogenation of N5,N10-methylene-tetrahydromethanopterin (CH2 = H4MPT) to N5,N10-methenyltetrahydromethanopterin (CH - H4MPT+) is an intermediate step in the oxidation of methanol to CO2 in Methanosarcina barkeri. The reaction is catalyzed by CH2 = H4MPT dehydrogenase, which was found to be specific for coenzyme F420 as electron acceptor; neither NAD, NADP nor viologen dyes could substitute for the 5-deazaflavin. The dehydrogenase was anaerobically purified almost 90-fold to apparent homogeneity in a 32% yield by anion exchange chromatography on DEAE Sepharose and Mono Q HR, and by affinity chromatography on Blue Sepharose. Sodium dodecyl sulfate/polyacrylamide gel electrophoresis revealed only one protein band with an apparent mass of 31 kDa. The apparent molecular mass of the native enzyme determined by polyacrylamide gradient gel electrophoresis was 240 kDa. The ultraviolet/visible spectrum of the purified enzyme was almost identical to that of albumin suggesting the absence of a chromophoric prosthetic group. Reciprocal plots of the enzyme activity versus the substrate concentrations were linear: the apparent K(m) for CH2 = H4MPT and for coenzyme F420 were found to be 6-mu-M and 25-mu-M, respectively. V(max) was 4,000-mu-mol min-1.mg-1 protein (k(cat) = 2,066 s-1) at pH 6 (the pH optimum) and 37-degrees-C. The Arrhenius activation energy was 40 kJ/mol. The N-terminal amino acid sequence was found to be 50% identical with that of the F420-dependent CH2 = H4MPT dehydrogenase isolated from H2/CO2 grown Methanobacterium thermoautotrophicum.