CRYSTAL-STRUCTURE OF CHLOROMUCONATE CYCLOISOMERASE FROM ALCALIGENES-EUTROPHUS JMP134 (PJP4) AT 3 ANGSTROM RESOLUTION

Chloromuconate cycloisomerase (E.C. 5.5.1.7) is an enzyme involved in the 2,4-dichlorophenoxyacetate degradation pathway of Alcaligenes eutr ophus JMP134 (pJP4). The crystal structure of this protein was determi ned at 3 Angstrom resolution by molecular-replacement techniques using atomic coordinates from the reported crystal structure of the homolog ous muconate cycloisomerase (E.C. 5.5.1.1) from Pseudomonas putida as the search model (42% identical positions in the sequences). Structure refinement by simulated-annealing and restrained least-squares techni ques converged at R= 0.195. In the crystals studied, space group I4, t he protein is present as two octamers per unit cell with two subunits per asymmetric unit. Each subunit consists of two globular domains, on e of which forms an alpha/beta-barrel. Comparison of this structure wi th that of muconate cycloisomerase reveals the reasons for the altered substrate specificity of chloromuconate cyclosiomerase. Marked differ ences are observed in polarity, accessibility and hydrogen-bonding pot ential in the channel leading into the active site as well as in the a ctive center itself.