Biosynthesis of bacterial glycogen. Mutagenesis of a catalytic site residue of ADP-glucose pyrophosphorylase from Escherichia coli

Abstract

Site-directed mutagenesis was used to explore the role of Lys-195 in ADP-glucose pyrophosphorylase from Escherichia coli. This residue, which is conserved in every bacterial and plant source sequenced to date, was originally identified as a potential catalytic site residue by covalent modification studies. Mutation of Lys-195 to glutamine produces an enzyme whose Km for glucose 1-phosphate is 600-fold greater than that measured for the wild-type enzyme. The effect on glucose 1-phosphate is very specific since kinetic constants measured for ATP, Mg2+, and the allosteric activator, fructose 1,6-bisphosphate, are unchanged relative to those measured for the wild-type enzyme. Furthermore, the catalytic rate constant, Kcat, for the glutamine mutant is similar to that of the wild-type enzyme. Taken together, the results suggest a role for Lys-195 in binding of glucose 1-phosphate and exclude its role as a participant in the rate-determining step(s) in the catalytic reaction mechanism. To further study the effect of charge, shape, size, and hydrophobicity of the amino acid residue at position 195, a series of mutants were prepared including arginine, histidine, isoleucine, and glutamic acid. In every case, the kinetic constants measured for ATP, Mg2+, and fructose 1,6-bisphosphate were similar to wild-type constants, reinforcing the notion that this residue is responsible for a highly localized effect at the glucose 1-phosphate-binding site and also suggesting that the protein can accommodate a wide range of substitutions at this position without losing its global folding properties. Thermal stability measurements corroborate this finding. The mutations did, however, produce a range of glucose 1-phosphate Km values from 100- to 10,000-fold greater than wild-type, which indicate that both size and charge properties of lysine are essential for proper binding of glucose 1-phosphate at the catalytic site. AMP binding was also affected by the nature of the mutation at position 195. A model for glucose 1-phosphate, ATP, and AMP binding is presented.