Enzyme Memory

Abstract

A theory is presented that associates burst (or lag) kinetics with the respective concentrations of enzyme initial states X 1 and X 6 and with the cooperation of a mnemonical enzyme. The theory predicts that for an enzyme with a negative cooperation, decreasing the initial concentration of X 1 (or increasing that of X 6 ) tends to increase the induction time. This increase may correspond to a reversal of a burst into a lag. Similarly, if the enzyme has a positive cooperation, decreasing the initial concentration of X 6 (or increasing that of X 1 ) increases the induction time. The first case above is expected to apply to wheat germ hexokinase LI, X 1 being the form that binds glucose preferentially, and X 6 the one that binds glucose 6‐phosphate. By changing solely the respective concentrations of the two initial forms, one may expect to modify the pre‐steady‐state phase but not the steady‐state kinetics of the reaction. By jumping the temperature of the enzyme solution from 4°C to 30°C and letting the trans‐conformation equilibrium relax for various periods of time before mixing enzyme with the substrates, one can analyse the effect of the relative concentrations of X 1 and X 6 on the induction time. One can estimate in that way one of the rate constants of the transconformation between the two free enzyme forms. The shorter the incubation time at 30°C then the smaller is the negative induction time (in absolute values). Another possibility of controlling the ratio between the two initial concentrations of the enzyme, is to pre‐mix hexokinase with glucose 6‐phosphate and to arrange that glucose‐6‐phosphate concentration, after mixing enzyme and substrates, is held constant whatever the pre‐mixing conditions.When wheat germ hexokinase LI is pre‐mixed 30 min at 30°C with glucose 6‐phosphate before the reaction starts, the burst does not disappear. If, on the other hand, pre‐mixing is effected at 4°C the burst is reversed into a lag. This result is taken to mean that the equilibrium constant between the two free enzyme forms (the ‘circle’ and the ‘rhombus’) is strongly dependent on temperature.A direct study of the effect of glucose 6‐phosphate on the conformational equilibrium of wheat germ hexokinase, gives support to this interpretation. If hexokinase is mixed at 4°C with glucose 6‐phosphate a slow increase in fluorescence of tryptophanyl residues is observed, which indicates that the ‘rhombus’ conformation accumulates under these conditions. On the other hand, at 30°C, glucose 6‐phosphate does not produce any significant change in the fluorescence of the protein. As expected, these results imply that the equilibrium between the two free enzymes species is freely reversible at 4°C and nearly irreversible at 30°C.The equations derived from the mnemonical model allow fitting or simulation of the experimental results.