STRUCTURES AND FUNCTIONS OF 4 ANABOLIC 2-OXOACID OXIDOREDUCTASES IN METHANOBACTERIUM-THERMOAUTOTROPHICUM

Methanobacterium thermoautotrophicum (strain Marburg), which grows aut otrophically on H-2 and CO2, was found to contain 2-oxoisovalerate oxi doreductase (Vor) and indolepyruvate oxidoreductase (Ior) besides pyru vate oxidoreductase (For) and 2-oxoglutarate oxidoreductase (Kor). So far, Vor and Ior have only been detected in peptide-utilizing hyperthe rmophilic Archaea. The four 2-oxoacid oxidoreductases were purified an d characterized with respect to their subunit composition, N-terminal amino acid sequences, and catalytic properties. For and Kor were compo sed of four different subunits, Vor was composed of three different su bunits, and Ior of two different subunits. Comparisons of the N-termin al amino acid sequences revealed that the four enzymes are structurall y related to each other and to the respective enzymes from Pyrococcus and Thermococcus sp. Vor from M. thermoautotrophicum differed from Vor from Pyrococcus furiosus in being composed of only three instead of f our different subunits. Evidence is presented that in the autotrophic methanogen the four 2-oxoacid oxidoreductases have anabolic functions, Vor and Ior being involved in the biosynthesis of amino acids from fa tty acids taken up from the growth medium, as shown by C-14-labelling studies.