PROPERTIES AND PRIMARY STRUCTURE OF A THERMOSTABLE L-MALATE DEHYDROGENASE FROM ARCHAEOGLOBUS-FULGIDUS

A thermostable L-malate dehydrogenase from the hyperthermophilic sulfa te-reducing archaeon Archaeoglobus fulgidus was isolated and character ized, and its gene was cloned and sequenced. The enzyme is a homodimer with a molecular mass of 70 kDa and catalyzes preferentially the redu ction of oxaloacetic acid with NADH. A. fulgidus L-malate dehydrogenas e 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 th ermostable L-malate dehydrogenase characterized to date. Addition of K 2HPO4 (1 M) increased the thermal stability by 40%. The primary struct ure shows a high similarity to L-lactate dehydrogenase from Thermotoga maritima and gram-positive bacteria, and to L-malate dehydrogenase fr om the archaeon Haloarcula marismortui and other L-lactate-dehydrogena se-like L-malate dehydrogenases.