Site-directed mutagenesis of aspartic acid 372 at the ATP binding site of yeast phosphoglycerate kinase: over-expression and characterization of the mutant enzyme.

A new phosphoglycerate kinase over-expression vector, pYE-PGK, has been constructed which greatly facilitates the insertion and removal of mutant enzyme genes by cleavage at newly introduced BamHI sites. This vector has been used to prepare mutant protein in appreciable (100 mg) quantities for use in kinetic, crystallographic and NMR experiments. Aspartate 372 is an invariant amino acid residue in genes known to code for a functionally active PGK. The function of this acidic residue appears to be to help desolvate the magnesium ion complexed with either ADP or ATP when this substrate binds to the enzyme. Both crystallographic and nuclear magnetic resonance experiments show that the replacement of the residue with asparagine has only minimal effects on the overall structure. The substitution of the charged carboxyl group with that of the neutral amide affects the binding of the nucleotide substrate as predicted but not, as might have been expected, the binding of 3-phosphoglycerate. The overall velocity of the enzymic reaction (Vmax) is reduced 10-fold by the substitution of aspartic acid 372 by an asparagine residue (D372N). This reduction in Vmax is considerably less than one would expect from its known position within the structure of the enzyme. This result therefore poses questions about our understanding of charged groups at the active centres of enzymes and of the reason for their apparent conservation.