Partial Purification and Properties of Citrate Synthases from the Thermoacidophilic Archaebacteria Thermoplasma acidophilum and Sulfolobus acidocaldarius

Abstract

Summary 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 K m value of 6.3 × 10 -6 M for acetyl-CoA and of 4.1 × 10 -6 M for oxaloacetate. Citrate synthase from S. acidocaldarius was purified 70-fold; its K m values were found to be 10 -5 M for acetyl-CoA and 2.6 × 10 -5 M 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.