Enzymaticreactionkinetics.ppt

Section 5 Enzymatic Reaction Kinetics 5.1 The effects of substrate concentration on reaction rate Substrate saturation curve for an enzyme-catalyzed Reaction. The amount of enzyme is constant, and the Velocity of the reaction is determined at various Substrate concentration. * At low concentration, the reaction rate is increased as the increase of substrate concentration. dp/dt = k [S] * At relative high concentration, the reaction rate is increased as the increase of substrate concentration, but the increase rate is not in direct proportion * At high concentration, the reaction rate is not increased as the increase of substrate concentration. dp/dt = k [Et] Where Et is the total concentration of enzyme The relationship of reaction rate and substrate concentration can be explained by intermediate theory. E + S = ES → E + P The maximal rate means all the enzyme are saturated by substrates. k1 k3 E + S = ES → E + P k2 The enzyme (E), combines with its substrate (S) to form an enzyme-substrate complex (ES). The (ES) complex can dissociate again to form E + S, or can proceed chemically to form E and the product P. The michaelis-Menten equation: Michaelis-menten constant Km (米氏常数) Significances of Km A measure of the affinity (亲和力) of an enzyme for its substrate Km is a measure of the stability of the ES complex, being equal to the sum of the rates of breakdown of ES over its rate of formation. For many enzymes k2 is much great than k3. The value of Km depends on the relative values of k1 and k2 for formation and dissociation,respectively. A high Km indicates weak substrate binding ( k2 predominant over k1 ), a low Km indicates strong substrate binding (k1 predominant over k2 ) Km is the specific to a given enzyme. It is the featuring constant of enzyme. At given condition and substrate, an enzyme has it