Archaebacterial citrate synthases were partially purified from the thermoacidophiles Thermoplasma acidophilum and Sulfolobus acidocaldarius. The enzyme from T. acidophilum was purified 38-fold. It showed a Kmvalue of 6.3 × 10-6M for acetyl-CoA and of 4.1 × 10-6M for oxaloacetate. Citrate synthase from S. acidocaldarius was purified 70-fold; its Kmvalues were found to be 10-5M for acetyl-CoA and 2.6 × 10-5M for oxaloacetate. ATP, NADH and NADPH inhibited both enzymes competitively with respect to acetyl-CoA. The sensitivity of the enzymes from both archaebacteria towards NADH was low while NADPH showed stronger inhibitory effects. Both enzymes possess a molecular weight of about 70 000 as estimated by gel filtration methods. The citrate synthases from both thermoacidophilic archaebacteria resembled more closely the enzymes found in Gram-positive eubacteria and in eukaryotes rather than the enzymes from Gram-negative eubacteria. © 1985, Gustav Fischer Verlag, Stuttgart/New York. All rights reserved.
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