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1. A theoretical appraisal of the alternative pathway mechanism for a two-substrate enzyme shows that this mechanism is capable of giving rise to apparent substrate inhibition or substrate activation (Dalziel, 1958). It has now been shown that these phenomena may occur simultaneously in the following ways. With certain relationships between the kinetic parameters and the constant concentration of one substrate, A, the plot of initial rate, v, against the concentration of the other substrate, B, may show substrate ;activation' at low concentrations of B and substrate ;inhibition' at high concentrations of B. In other circumstances the plot of v against [B], with [A] constant, may be sigmoid (substrate activation), whereas the plot of v against [A], with [B] constant, may pass through a maximum (substrate inhibition). 2. Kinetic data for phosphofructokinase are of the latter type and it is suggested that the mechanism of this enzyme may involve a kinetically preferred pathway. It is emphasized that the phenomena of substrate inhibition and activation need not necessarily involve more than one binding site for each substrate on the enzyme molecule, nor more than one monomer per molecule.

The interpretation of non-hyperbolic rate curves for two-substrate enzymes. A possible mechanism for phosphofructokinase