N⁵,N¹⁰‐Methylenetetrahydromethanopterin dehydrogenase from Methanobacterium thermoautotrophicum has hydrogenase activity

N 5 , N 10 ‐Methylenetetrahydromethanopterin dehydrogenase from Methanobacterium thermoautotrophicum (strain Marburg) was purified under anaerobic conditions to apparent homogeneity and a specific activity of approximately 750 μol/min/mg protein. Polyacrylamide gel electrophoresis under native and denaturing conditions revealed that the enzyme is composed of only one polypeptide with an apparent molecular mass of 43 kDa. The purified enzyme catalyzed the dehydrogenation of N 5 , N 10 ‐methylenetetrahydromethanopterin (CH 2 =H 4 MPT) (apparent K m ≡20 μM) to N 5 , N 10 ‐methenyltetrahydromethanopterin (CH≡H 4 MPT) in the absence of any added electron acceptors. One mol of H 2 was generated per mol CH≡H 4 MPT formed, indicating that protons served as electron acceptor. Coenzyme F 420 , NAD, NADP and viologen dyes were not reduced by CH 2 =H 4 MPT. The dehydrogenase also catalyzed the reverse reaction, the reduction of CH≡H 4 MPT to CH 2 =H 4 MPT with H 2 . The data indicate that CH 2 =H 4 MPT dehydrogenase from M. thermoautotrophicum is a novel type of hydrogenase.