Mutations in the active site of Escherichia coli phosphofructokinase

The enzyme-catalysed transfer of a phosphoryl group from ATP is an important reaction in a wide variety of biological processes. We demonstrate here the essential function of an aspartate group in the catalysis of phosphoryl transfer by Escherichia coli phosphofructokinase, and the minor role of an arginine residue. We have used oligonucleotide-directed mutagenesis to replace two amino-acid residues which X-ray analysis has shown to be close to the transferred phosphoryl group and we have analysed the forward and back reactions of the mutant enzymes by steady-state kinetics. Changing Asp 127 to Ser reduced the turnover number by a factor of 18,000 in the forward direction and 3,100 in the back reaction, and the Michaelis constant for fructose 1,6-bisphosphate in the reverse reaction by a factor of 45. This shows that this aspartate is a key residue in the rate enhancement by the enzyme, probably acting as a base in the reaction mechanism, and that it also destabilizes the product complex. Changing Arg 171 to Ser reduced the turnover numbers by about 3.4, showing that this arginine has only a minor effect on the catalysis.