Involved in the catabolism of short chain fatty acids (SCFA) via the tricarboxylic acid (TCA)(acetyl degradation route) and probably via the 2-methylcitrate cycle I (propionate degradation route). Catalyzes the reversible isomerization of citrate to isocitrate via cis-aconitate (PubMed:8892815). Could catalyze the hydration of 2-methyl-cis-aconitate to yield (2R,3S)-2-methylisocitrate. 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. Higher under aerobic conditions than under anaerobic conditions. Cells lacking this gene grow more slowly, but also reach a lower cell density than the wild-type. The ability of the mutant to establish an effective root nodule symbiosis with soybean plants is not affected. Belongs to the aconitase/IPM isomerase family.