Maximal velocity inhibitors affecting

In noncompetitive inhibition (Figure 8.38), the inhibitor can combine with either the enzyme or the enzyme-substrate complex. In pure noncompetitive inhibition, the values of the dissociation constants of the inhibitor and enzyme and of the inhibitor and enzyme—substrate complex are equal (Section 8.5.1). The value of is decreased to a new value called V( i. and so the intercept on the vertical axis is increased. The new slope, which is equal to Km/ V( i. is larger by the same factor. In contrast with Vjjxix. is not affected by pure noncompetitive inhibition. The maximal velocity in the presence of a pure noncompetitive inhibitor, V ax. is given by... 

One observes, however, that the degree of inhibition is dependent on the relative proportions of S and I. In the presence of a fixed concentration of I. the inhibition decreases if the concentration of S is increased it tends towards zero at infinite substrate concentration (Fig. 11). The maximal velocity is not affected by a competitive inhibitor. 

Fig. 11. Effect of substrate concentration on the initial velocity of inhibited reactions. Maximal velocity (V), illustrated by the dashed lines, is unaffected by a competitive inhibitor, it is reduced (by 50% in this example) by a non-competitive inhibitor, is increased to by a competitive inhibitor, it is not affected by a non-...

When an inhibitor combines irreversibly with the enzyme, it prevents the subsequent binding of substrate. The fraction of enzyme combined with the inhibitor is no longer available for catalysis and cannot be regenerated by the addition of excess substrate, since the combination of the enzyme to the inhibitor is irreversible. The maximal velocity is thus reduced. The properties of the fraction of the enzyme not combined with the inhibitor are not modified and the value of Km is not affected. Such an inhibition is said to be non-competitive. 

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