Purification, properties and primary structure of H-2-forming N-5,N(10)methylenetetrahydromethanopterin dehydrogenase from Methanococcus thermolithotrophicus

H-2-Forming N-5,N-10-methylenetetrahydromethanopterin dehydrogenase (Hmd) is a novel type of hydrogenase found in methanogenic Achaea that contains neither nickel nor iron-sulfur clusters. The enzyme has previously been characterized from Methanobacterium thermoautotrophicum and from Methanopyrus kandleri. We report here on the purification and properties of the enzyme from Methanococcus thermolithotrophicus. The hmd gene was cloned and sequenced. The results indicate that the enzyme from Me. thermolithotrophicus is functionally and structurally closely related to the H-2-forming methylene tetrahydromethanopterin dehydrogenase from Mh. thermoautotrophicum and Mp. kandleri. From amino acid sequence comparisons of the three enzymes, a phylogenetic tree was deduced that shows branching orders similar to those derived from sequence comparisons of the 16S rRNA of the orders Methanococcales, Methanobacteriales, and Methanopyrales.