The structure and mechanism of the C-C bond hydrolase MhpC

Bioremediation is a promising method for removing xenobiotics from the environment based on the capacity of microorganisms to transform pollutants to non-toxic products. One such catabolic pathway is that which degrades phenylpropionic acid and has been found in a number of bacteria, including Escherichia coli. The third enzyme in this pathway is MhpC, a carbon-carbon bond hydrolase which catalyses the cleavage of 2-hydroxy-6-ketonona-2,4-diene-1,9-dioic acid to succinate and 2-hydroxypenta-2,4-dienoic acid. The principle aim of this research was to use crystallographic methods to obtain structures of MhpC in complex with ligands which may shed light on the mechanism. The native structure was solved to 2.1A resolution and revealed that MhpC belonged to the α/β-hydrolase fold family with a catalytic triad of Ser110-Asp235-His263. Although MhpC exists as a dimer in solution it was found to adopt a tetrameric quaternary structure in the crystal. It was also observed that the active site serine may have been covalently modified but attempts to identify the source of this modification have at present proved futile. The structures of MhpC complexed with two substrate analogues, azelaic acid and 2,6-diketo-nona-1,9-dioic acid (DKNDA), and two product analogues, laevulinic acid and 4,6-dioxoheptanoic acid (DOHA) were obtained. The structures with azelaic acid and laevulinic acid revealed many residues within the active site cavity that could be involved in substrate binding. The structure with DOHA revealed a disparity between subunits of the MhpC dimer, which was confirmed in the DKNDA structure and indicates half-site reactivity may be a feature of the MhpC-catalysed reaction. The structure with DKNDA also revealed that this compound forms a hemi-ketal adduct with the enzyme through Ser110 and hence that the formation of an acyl-enzyme intermediate is feasible.