Component of a hydro-lyase with broad substrate specificity for cis-unsaturated tricarboxylic acids. Catalyzes both the reversible dehydration of (R)-homocitrate ((R)-2-hydroxybutane-1,2,4-tricarboxylate) to produce cis-homoaconitate ((Z)-but-1-ene-1,2,4-tricarboxylate), and its hydration to homoisocitrate ((1R,2S)-1-hydroxybutane-1,2,4-tricarboxylate). Is also able to hydrate the analogous longer chain substrates cis-homo(2)-aconitate, cis-homo(3)-aconitate, and even the non-physiological cis-homo(4)-aconitate with similar efficiency. These reactions are part of the biosynthesis pathway of coenzyme B. Can also catalyze the hydration of maleate to (R)-malate, and that of cis-aconitate. Cannot catalyze the hydration of citraconate and the dehydration of (S)-homocitrate, citramalate, 2-isopropylmalate, 3-isopropylmalate, citrate or threo-DL-isocitrate. (2R)-homocitrate = (2R,3S)-homoisocitrate (2R)-homocitrate = cis-homoaconitate + H2O (2R,3S)-homoisocitrate = cis-homoaconitate + H2O cis-(homo)2aconitate + H2O = (2R,3S)-iso(homo)2citrate cis-(homo)3aconitate + H2O = (2R,3S)-iso(homo)3citrate (R)-malate = H2O + maleate cis-aconitate + H2O = D-threo-isocitrate kcat is 0.75 sec(-1) for cis-aconitate hydration reaction (PubMed:20170198). Kinetic parameters measured using the HacAB complex. Optimum pH is 9. Active from pH 8 to 10. Organic acid metabolism; 2-oxosuberate biosynthesis. Heterotetramer of 2 HacA and 2 HacB proteins. Cannot form a complex with LeuC. Belongs to the LeuD family. LeuD type 2 subfamily.