Molecular lesion affecting the ADP-combining site in a mutant isozyme of erythrocyte pyruvate kinase

Erythrocyte pyruvate kinase (PK) from a patient with PK deficiency was characterized according to internationally standardized procedures. In addition to low activity, the mutant isozyme displayed impaired kinetics specifically affecting the ADP-combining site:Km (ADP) was 3-5 times greater than normal when determined at three different concentrations of phosphoenolpyruvate (P-enolpyruvate). Maximum reaction velocities were not achieved until ADP was 10 times the concentration normally required for control PK. Substrate inhibition by high concentrations of ADP and competitive inhibition by ATP were markedly diminished. Other nucleoside diphosphates normally capable of replacing ADP in the PK reaction were less effective with the mutant isozyme than with PK from controls or from subjects with other forms of PK deficiency, and Michaelis--Menten constants for several of these (UDP, GDP, CDP) were significantly elevated. Whereas all previously known PK kinetic defects have involved the substrate P-enolpyruvate, the half-saturation constant K0.5s (P-enolpyruvate) for this mutant isozyme was normal, as was its response to fructose-1,6-bisphosphate activation.