Involved in the catabolism of short chain fatty acids (SCFA) via the 2-methylcitrate cycle II (propionate degradation route). In vivo under anaerobic conditions, AcnD catalyzes the stereospecific dehydration of (2S,3S)-methylcitrate (2-MC) to yield the trans isomer of 2-methyl-aconitate (2-MCA). AcnD can also accept citrate and cis-aconitate, but with a lower efficiency. 2-methylisocitrate and isocitrate are not substrates. (2S,3S)-2-methylcitrate = 2-methyl-trans-aconitate + H2O citrate = D-threo-isocitrate Binds 1 [4Fe-4S] cluster per subunit. Inhibited by ferricyanide and EDTA. Organic acid metabolism; propanoate degradation. Together with PrpF, able to restore the growth of prpD mutant on propionate. Belongs to the aconitase/IPM isomerase family.