Involved in the catabolism of short chain fatty acids (SCFA) via the tricarboxylic acid (TCA)(acetyl degradation route) and via the 2-methylcitrate cycle I (propionate degradation route). Catalyzes the Claisen condensation of propionyl-CoA and oxaloacetate (OAA) to yield 2-methylcitrate (2-MC) and CoA. Also catalyzes the condensation of oxaloacetate with acetyl-CoA or butyryl-CoA but with a lower specificity. H2O + oxaloacetate + propanoyl-CoA = (2S,3S)-2-methylcitrate + CoA + H(+) acetyl-CoA + H2O + oxaloacetate = citrate + CoA + H(+) kcat is 1.4 sec(-1) for citrate synthase activity with acetyl-CoA as substrate. kcat is 1.87 sec(-1) for citrate synthase activity with acetyl-CoA as substrate (at pH 8 and 30 degrees Celsius). kcat is 7.4 sec(-1) for 2-methylcitrate synthase activity with propionyl-CoA as substrate. kcat is 8.25 sec(-1) for 2-methylcitrate synthase activity with propionyl-CoA as substrate (at pH 8 and 30 degrees Celsius). Organic acid metabolism; propanoate degradation. Carbohydrate metabolism; tricarboxylic acid cycle; isocitrate from oxaloacetate: step 1/2. Homodimer at low concentrations and converted to a larger oligomers at higher concentrations. Cells lacking this gene are unable to accumulate propionyl-CoA. Belongs to the citrate synthase family.