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 apo form of AcnB functions as a RNA-binding regulatory protein. During oxidative stress inactive AcnB apo-enzyme without iron sulfur clusters binds the acnB mRNA 3' UTRs (untranslated regions), stabilizes acnB mRNA and increases AcnB synthesis, thus mediating a post-transcriptional positive autoregulatory switch. AcnB also decreases the stability of the sodA transcript. citrate = D-threo-isocitrate (2S,3R)-3-hydroxybutane-1,2,3-tricarboxylate = 2-methyl-cis-aconitate + H2O Binds 1 [4Fe-4S] cluster per subunit. Optimum pH is 7.4. Organic acid metabolism; propanoate degradation. Carbohydrate metabolism; tricarboxylic acid cycle; isocitrate from oxaloacetate: step 2/2. Monomer. AcnB can also form a homodimer. The monomer-homodimer transition is dependent on iron availability and the carboxymethylation of C-273 inhibits the dimer formation. Cells lacking this gene are more sensitive to peroxide stress. The acnAB double mutant does not grow on unsupplemented glucose minimal medium and does not respond under aerobic conditions to glutamate. The acnAB double mutant retains a low but significant aconitase activity. AcnB is sensitive to oxidation in vivo. Belongs to the aconitase/IPM isomerase family.