Citrate synthase from Cyanidioschyzon merolae exhibits high oxaloacetate and acetyl-CoA catalytic efficiency

Key messageThis study demonstrated that CmCS4, a citrate synthase from Cyanidioschyzon merolae mitochondria, exhibits oxaloacetate and acetyl-CoA catalytic efficiency, reflecting the abundance of carbon flow of the TCA cycle for ATP production. Citrate synthase (CS) catalyzes the reaction that produces citrate and CoA from oxaloacetate and acetyl-CoA in the tricarboxylic acid (TCA) cycle. All TCA cycle enzymes are localized to the mitochondria in the model organism, the red alga Cyanidioschyzon merolae. The biochemical properties of CS have been studied in some eukaryotes, but the biochemical properties of CS in algae, including C. merolae, have not been studied. We then performed the biochemical analysis of CS from C. merolae mitochondria (CmCS4). The results showed that the k(cat)/K-m of CmCS4 for oxaloacetate and acetyl-CoA were higher than those of the cyanobacteria, such as Synechocystis sp. PCC 6803, Microcystis aeruginosa PCC 7806 and Anabaena sp. PCC 7120. Monovalent and divalent cations inhibited CmCS4, and in the presence of KCl, the K-m of CmCS4 for oxaloacetate and acetyl-CoA was higher in the presence of MgCl2, the K-m of CmCS4 for oxaloacetate and acetyl-CoA was higher and k(cat) lower. However, in the presence of KCl and MgCl2, the k(cat)/K-m of CmCS4 was higher than those of the three cyanobacteria species. The high catalytic efficiency of CmCS4 for oxaloacetate and acetyl-CoA may be a factor in the increased carbon flow into the TCA cycle in C. merolae.