THE PYRUVATE-DEHYDROGENASE COMPLEX FROM THERMOPHILIC ORGANISMS - THERMAL-STABILITY AND RE-ASSOCIATION FROM THE ENZYME COMPONENTS

Examples of pyruvate dehydrogenase complexes, and of its probable prec ursors, the pyruvate ferredoxin oxidoreductases, both isolated from th ermophilic organisms, are described. The pyruvate ferredoxin oxidoredu ctases are mostly characterized from thermophilic archaea like Sulfolo bus solfataricus and Pyrococcus furiosus. They retain their catalytic activity up to 60 and 90 degrees C, respectively. Characteristic for t he thermophilic nature is a biphasic temperature behavior, reflecting a more stable low temperature and a metastable high temperature form. Another feature is the strong binding of the cofactor thiamin diphosph ate. Detailed analysis of thermostable pyruvate dehydrogenase complexe s so far only exist for the enzymes from Bacillus stearothermophilus a nd Thermus flavus. In most respects, especially in the structural feat ures, the enzyme complex from B. stearothermophilus resembles its meso philic counterparts and only an elevated temperature maximum for the c atalytic activity reveals the thermophilic nature. In contrast to this , the more thermostable enzyme complex from T. flavus shows a quite di stinct behavior. One single protein chain (M-r = 100 kDa) instead of a n alpha(2)beta(2) aggregate was found for the pyruvate dehydrogenase ( El) subunits of this enzyme complex. Its catalytic activity is control led by allosteric regulation, while the enzyme complex from B. stearot hermophilus shows no such regulation. Reversible phosphorylation as a regulatory principle of pyruvate dehydrogenase complexes from higher o rganisms does not take place in the thermophilic enzyme complexes. The overall activity of the enzyme complex from B. stearothermophilus rem ains stable at 60 degrees C for 50 min while that from T. flavus is ac tive up to 83 degrees C. Thermophilic pyruvate dehydrogenase complexes do not spontaneously renature from their separated enzyme components. However, chaperonins from Thermus thermophilus stimulate the reactiva tion of the enzyme complex from T. flavus. (C) 1998 Elsevier Science B .V. All rights reserved.