Self-Association of Human Erythrocyte Phosphofructokinase. Kinetic Behaviour in Dependence on Enzyme Concentration and Mode of Association

Abstract

The kinetic hehavior of human erythocyte phosphofructokinase (EC 2.7.1.11) was analyzed over a relative wide range of enzyme concentration (0.02-1.7 .mu.g/ml). The kinetic cooperativity, which becomes apparent when the enzymic reaction rate is plotted vs. the fructose 6-phosphate concentration, decreases with increasing enzyme concentration. Simultaneously, a decrease of the half-saturation concentration for fructose 6-phosphate [S]0.5 is observed. Maximum velocity passes through a maximum at increasing enzyme concentrations. Sets of curves represinting specific enzymic activity of phosphofructokinase vs. enzyme concentration obtained at various fixed concentrations of fructose 6-phosphate and ATP are analyzed. The shapes of these curves are interpreted in terms of an association model of human erythrocyte phosphofructokinase, in which an inactive dimer (Mr 190,000) and active multimers of the dimeric form are involved. The sigmoidal shape of the plots of the enzymic reaction rate vs. fructose 6-phosphate concentration is partially caused by a displacement of the equilibrium between different states of association of phosphofructokinase to multimers by this substrate. The inhibition of the enzyme by high concentration of ATP may be partially caused by a shift of this equilibrium to the state of the inactive dimer.