Phosphoenolpyruvate carboxylase (PEPC): Mutational analysis of a flexible loop and a putative binding site for an allosteric activator, glucose 6-phosphate (G6P)

Abstract

The crystal structure of E.coli PEPC and Zea mays C4-form PEPC have been resolved recently, and we found an untraceable loop structure from K702 to G708 in E.coli numbering (from K762 to G768 in Zea mays numbering). This loop structure has flexibility and mobilty even in crystal state and participate in forming the active site (Kai et al. (1999) Proc. Natl. Acad. Sci. USA, 96, 823-828). Five mutants of this loop in Zea mays PEPC have been constructed. In the double mutation P765R/G766R, Vmax increased and extent of activation by an allosteric activator, G6P, increased. However, the Km value for Mg2+ increased. The two mutations P765R and P765G/G766R decreased the extent of activation by G6P. P765G mutant decreased the activity of the bicarbonate dependent PEP hydrolysis reaction and increased the extent of activation by G6P. In the insertional mutation of Arg residue between P765 and G766, Vmax increased and the extent of activation by G6P increased. These results indicated that this loop has multiple functions. From the studies of three-dimentional structure of Zea mays PEPC, we found the putative G6P binding site (R183, R184, R231 and R372). We prepared four mutations (R183Q, R183Q, R183Q/R184Q, and R231Q) and found that R183 and R184 are essential for G6P activation. During the course of these experiments, we obtained the data suggesting the presence of two affinity states for HCO3-in wild type PEPC.