Department of Microbiology, University of Bergen, Jahnebakken 5, N-5020 Bergen, Norway.
A thermostable l-malate dehydrogenase from the hyperthermophilic sulfate-reducing archaeon Archaeoglobus fulgidus was isolated and characterized, and its gene was cloned and sequenced. The enzyme is a homodimer with a molecular mass of 70 kDa and catalyzes preferentially the reduction of oxaloacetic acid with NADH. A. fulgidus L-malate dehydrogenase was stable for 5 h at 90 degrees C, and the half-life at 101 degrees C was 80 min. Thus, A. fulgidus L-malate dehydrogenase is the most thermostable L-malate dehydrogenase characterized to date. Addition of K2HPO4 (1 M) increased the thermal stability by 40%. The primary structure shows a high similarity to L-lactate dehydrogenase from Thermotoga maritima and gram-positive bacteria, and to L-malate dehydrogenase from the archaeon Haloarcula marismortui and other L-lactate-dehydrogenase-like L-malate dehydrogenases.