Noncompetitive Inhibition (Molecular Biology)

1. Linear

Linear noncompetitive inhibition of an enzyme occurs whenever an inhibitory analogue of the substrate combines with a form of enzyme other than the one with which the variable substrate combines and a reversible connection exists between the points of addition of inhibitor and the variable substrate. It should be noted that reversible connections are broken either by the release of product at zero concentration or the presence of a nonvaried substrate at an essentially infinite concentration. For a two-substrate ordered kinetic mechanism (Fig. 1), the inhibition by I would be linear competitive with respect of A. It would be linear noncompetitive with respect to B, as B combines with EA, I combines with E, and a reversible connection exists between E and EA. The general form of the equation to describe noncompetitive inhibition is shown in equation (1):

where Ka denotes the Km (Michaelis constant) for substrate A, Vis the maximum velocity of the reaction, and Kis and K^ are the inhibition constants associated with the slopes and intercepts, respectively, of the double-reciprocal plot, which has the form illustrated in equation (2):

This equation shows that a plot of 1/v against 1/A, at different concentrations of I, would yield a family of straight lines that intersect at a common point to the left of the vertical ordinate. The 1/v coordinate for the crossover point isand thus, the intersection point can occur above, on, or below the abscissa, depending on whether Kis is less then, equal to, or greater than K, respectively (Fig. 2). The term noncompetitive has been applied only for the case in which the lines of a double-reciprocal plot intersect on the abscissa; the term mixed inhibition is used to describe cases in which the lines intersect either above or below the abscissa. Such a distinction is unnecessary as the position of the crossover point is simply a function of the relative values of Kis and Kii.

Figure 1. Kinetic mechanism for which I would give rise to linear competitive inhibition with respect to A and linear noncompetitive inhibition with respect to B.

Figure 2. Double-reciprocal plots for linear noncompetitive inhibition with crossover points (a) above, (b) on, and (c) below the abscissa. The respective values in arbitrary units for Kis and Kii were: (a) 0.5 and 2.0, (b) 0.5 and 0.5, and (c) 0.3 and 0.1.

The inhibition constants associated with the slopes (Kis) and intercepts (K^) of a double-reciprocal plot are calculated and interpreted as described for competitive inhibition and uncompetitive inhibition, respectively.

2. Hyperbolic

This type of inhibition would be observed for the scheme illustrated under hyperbolic competitive inhibition, when the EA and EAI complexes give rise to product at different rates. The VA term of the equation that describes this inhibition would be modified by the factor, 1+I/Kin, so as to give equation (3):

which, in reciprocal form, is described by equation (4):

Values would be determined for Kis and Kin from the variation of the slope with varying concentrations of inhibitor I and for Kin and Kid from the variation of intercept with I in the same way as described for hyperbolic competitive inhibition.