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 which regulates FliC synthesis via interaction with the ftsH transcript to decrease the intracellular levels of FtsH. The lower levels of FtsH protease activity then influence sigma-32, DnaK and ultimately FliC production. 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 do not grow on propionate unless glutamate is added, and the addition of glutamate does not restore growth to the level of wild-type. Also unable to grow on acetate and citrate and only slight improvements are observed when glutamate is added. AcnB mutant also shows an impaired binding to the surface of macrophage-like cells, is less motile and possesses fewer flagella due to a level of the flagellum protein FliC lower. 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.