H2‐forming methylenetetrahydromethanopterin dehydrogenase, a novel type of hydrogenase without iron‐sulfur clusters in methanogenic archaea

A novel hydrogenase has recently been found in methanogenic archaea. It catalyzes the reversible dehydrogenation of methylenetetrahydromethanopterin (CH2─H4MPT) to methenyltetrahydromethanopterin (CH═H4MPT+) and H2 and was therefore named H2‐forming methylenetetrahydromethanopterin dehydrogenase. The hydrogenase, which is composed of only one polypeptide with an apparent molecular mass of 43 kDa, does not mediate the reduction of viologen dyes with either H2 or CH2─H4MPT. We report here that the purified enzyme from Methanobacterium thermoautotrophicum exhibits the following other unique properties: (a) the colorless protein with a specific activity of 2000 U/mg (Vmax) did not contain iron‐sulfur clusters, nickel, or flavins; (b) the activity was not inhibited by carbon monoxide, acetylene, nitrite, cyanide, or azide; (c) the enzyme did not catalyze an isotopic exchange between 3H2 and 1H+; (d) the enzyme catalyzed the reduction of CH═H4MPT+ with 3H2 generating [methylene‐3H]CH2─H4MPT; and (e) the primary structure contained at most four conserved cysteines as revealed by a comparison of the DNA‐deduced amino acid sequence of the proteins from M. thermoautotrophicum and Methanopyrus kandleri. None of the four cysteines were closely spaced as would be indicative for a (NiFe) hydrogenase or a ferredoxintype iron‐sulfur protein. Properties of the H2‐forming methylenetetrahydromethanopterin dehydrogenase from Methanobacterium wolfei are also described indicating that the enzyme from this methanogenic archaeon is very similar to the enzyme from M. thermoautotrophicum with respect both to molecular and catalytic properties.