H-2-FORMING N5,N10-METHYLENETETRAHYDROMETHANOPTERIN DEHYDROGENASE FROM METHANOBACTERIUM-THERMOAUTOTROPHICUM - STUDIES OF THE CATALYTIC MECHANISM OF H-2 FORMATION USING HYDROGEN ISOTOPES

H-2-forming N5,N-10-methylenetetrahydromethanopterin dehydrogenase is a novel hydrogenase found in most methanogenic archaea. It catalyzes the reversible conversion of N5,N-10-methylenetetrahydromethanopterin (CH2 = H4MPT) to N5,N-10-methenyltetrahydromethanopterin (CH = H4MPT) and dihydrogen; CH2 = HMPT + H+ half arrow right over half arrow left CH = HMPT+ + H-2; DELTAG-degrees' = + 5.5 kJ/mol. In the following investigation, the formation of H-2, HD and D2 was studied in experiments in which either the methylene group of CH2 = H4MPT or water were deuterium labelled. In the case of CD2 = H4MPT and H2O, the dihydrogen formed immediately after the start of the reaction was composed of approximately 50% HD and 50% of H-2 at all pH tested. In the case of CH2 = HMPT and D2O, the dihydrogen generated was composed of approximately 50% HD and 50% D, at pD 5.7 and of approximately 85% HD and 15% D2 at pD 7.0. Evidence is presented that the enzyme catalyzes a CH = H4MPT+-dependent isotopic exchange between HD and H2O and between HD and D2O, yielding H-2 and D2, respectively. A catalytic mechanism aimed to explain these findings is discussed.