Catalyzes both citrate generation and citrate cleavage. Part of a reversible tricarboxylic acid (TCA) cycle that can fix carbon dioxide autotrophically and may represent an ancestral mode of the conventional reductive TCA (rTCA) cycle. The direction is controlled by the available carbon source(s). acetyl-CoA + H2O + oxaloacetate = citrate + CoA + H(+) kcat is 0.931 sec(-1) with citrate as substrate. kcat is 1.34 sec(-1) with CoA as substrate. kcat is 250 sec(-1) with acetyl-CoA as substrate. Carbohydrate metabolism; tricarboxylic acid cycle; isocitrate from oxaloacetate: step 1/2. Belongs to the citrate synthase family.