2 MALATE-DEHYDROGENASES IN METHANOBACTERIUM-THERMOAUTOTROPHICUM

Methanobacterium thermoautotrophicum (strain Marburg) was found to con tain two malate dehydrogenases, which were partially purified and char acterized. One was specific for NAD(+) and catalyzed the dehydrogenati on of malate at approximately one-third of the rate of oxalacetate red uction, and the other could equally well use NAD(+) and NADP(+) as coe nzyme and catalyzed essentially only the reduction of oxalacetate. Via the N-terminal amino acid sequences, the encoding genes were identifi ed in the genome of M. thermoautotrophicum (strain Delta H). Compariso n of the deduced amino acid sequences revealed that the two malate deh ydrogenases are phylogenetically only distantly related. The NAD(+)-sp ecific malate dehydrogenase showed high sequence similarity to L-malat e dehydrogenase from Methanothermus fervidus, and the NAD(P)(+)-using malate dehyrogenase showed high sequence similarity to L-lactate dehyd rogenase from Thermotoga maritima and L-malate dehydrogenase from Baci llus subtilis. A function of the two malate dehydrogenases in NADPH:NA D(+) transhydrogenation is discussed.