Practical Enzyme Kinetics

Abstract

As discussed in chapter 4, the Michaelis-Menten equation (4.10) predicts that a plot of v ₀ against S₀ is a rectangular hyperbola, i.e. one with orthogonal asymptotes, passing through the origin (figure 10.1). One limb of the hyperbola lies completely in the second quadrant in the physically meaningless region of negative substrate concentration. The arc of the curve as usually drawn is in the first quadrant but this is only a portion of the limb which extends to infinite — υ ₀ in the third quadrant. The asymptotes are given by S₀ = — K _m and υ ₀ = V. Although the S₀, υ ₀ relationship is simple, its hyperbolic nature gives rise to problems in presenting kinetic data and in calculating K _m and V. This is because it is difficult to extrapolate the hyperbola to estimate V, the asymptotic value of υ ₀. In order for υ ₀ to approach even to within 5 % of V, S₀ must be at least 19 times K _m ; apart from possible limitations in availability or solubility of the substrate, or interference with the assay method, high substrate concentrations may result in appreciable substrate inhibition (see chapter 4). As K _m is defined in terms of V an accurate estimate of V is necessary for determining K _m