CATALYTIC METAL-ION BINDING IN ENOLASE - THE CRYSTAL-STRUCTURE OF AN ENOLASE-MN2+-PHOSPHONOACETOHYDROXAMATE COMPLEX AT 2.4-ANGSTROM RESOLUTION

Enolase, a glycolytic enzyme that catalyzes the dehydration of 2-phospho-D-glycerate (PGA) to form phosphoenolpyruvate (PEP), requires two divalent metal ions per active site for activity. The first metal ion, traditionally referred to as ''conformational'', binds in a high-affinity site I. The second metal ion, ''catalytic'', binds in site II only in the presence of a substrate or substrate analogue and with much lower affinity for the physiological cofactor Mg2+. While the high-affinity site has been well characterized, the position of the lower affinity site has not been established so far. Here, we report the structure of the quaternary complex between enolase, the transition-state analogue phosphonoacetohydroxamate (PhAH), and two Mn2+ ions. The structure has been refined by using 16 561 reflections with F/sigma a(F) greater than or equal to 3 to an R = 0.165 with average deviations of bond lengths and bond angles from ideal values of 0.013 Angstrom and 3.1 degrees, respectively. The ''catalytic'' metal ion is coordinated to two oxygen atoms of the phosphono moiety of PhAH and to the carbonyl oxygen of Gly37. Most likely, disordered water molecules complement its coordination sphere. The interaction with the site II metal ion must stabilize negative charge on the phosphate group and produce electron withdrawal from carbon 2 of the substrate, facilitating proton abstraction from carbon 2, the rate-limiting step in the catalytic process. The Gly37 residue is located in the flexible loop Ser36-His43, which assumes an ''open'' conformation in the absence of substrate and a ''closed'' conformation in the presence of a substrate. The metal ion binding in site II must stabilize the ''closed'' conformation and the substrate/product binding. Thus the inhibitory effect of higher Mg2+ concentrations on enolase activity may be explained by the sequential reaction mechanism in which the site II metal ion must leave before the product is released from the enzyme.