Involved in the catabolism of short chain fatty acids (SCFA) via the tricarboxylic acid (TCA)(acetyl degradation route) and the 2-methylcitrate cycle I (propionate degradation route). Catalyzes the reversible isomerization of citrate to isocitrate via cis-aconitate. Also catalyzes the hydration of 2-methyl-cis-aconitate to yield (2R,3S)-2-methylisocitrate. The (2S,3S)-2-methylcitrate (2-MC) is a very poor substrate (PubMed:11294638). The apo form of AcnA functions as a RNA-binding regulatory protein (By similarity). citrate = D-threo-isocitrate (2S,3R)-3-hydroxybutane-1,2,3-tricarboxylate = 2-methyl-cis-aconitate + H2O Binds 1 [4Fe-4S] cluster per subunit. Carbohydrate metabolism; tricarboxylic acid cycle; isocitrate from oxaloacetate: step 2/2. Organic acid metabolism; propanoate degradation. Monomer. Cells lacking this gene show no phenotype during growth on propionate, acetate, succinate, glycerol and glucose. The acnAB double mutant does not grow on propionate even when supplemented with glutamate and is unable to respire propionate under anaerobic growth conditions. Belongs to the aconitase/IPM isomerase family.