TAILIEUCHUNG - Lecture Principles of biochemistry - Chapter 5 (part 2): Enzyme kinetics

The following will be discussed in this chapter: Define rate constant, first order reaction, second order reaction and zero order reaction; define Vmax; define Km ; Michaelis-Menton equation defines the hyperbolic curve when [S] is plotted against initial velocity; know what the initial velocity and steady state assumptions mean;. | Chapter 5 (part 2) Enzyme Kinetics Rate constant (k) measures how rapidly a rxn occurs A B + C k1 k-1 Rate (v, velocity) = (rate constant) (concentration of reactants) v= k1 [A] 1st order rxn (rate dependent on concentration of 1 reactant) v= k-1[B][C] 2nd order rxn (rate dependent on concentration of 2 reactants) Zero order rxn (rate is independent of reactant concentration) Rate constants and reaction order E + S ES E + P k1 k-1 k2 k-2 E S + E S E + P Initial Velocities [S] = 1 mM D[P]/DT = Vo1 mM [P] time Hold [E] constant [E] Glucose-6-P + ADP What does kcat mean? kcat is the 1st order rate constant describing ES E+P Also known as the turnover # because it describes the number of rxns a molecule of enzyme can catalyze per second under optimal condition. Most enzyme have kcat values between 102 and 103 s-1 For simple reactions k2 = kcat , for multistep rxns kcat = rate limiting step E + S ES E + P k1 k-1 kcat What does kcat/Km mean? It measures how the enzyme performs when S is low kcat/Km describes an enzymes preference for different substrates = specificity constant The upper limit for kcat/Km is the diffusion limit - the rate at which E and S diffuse together (108 to 109 m-1 s-1) Catalytic perfection when kcat/Km = diffusion rate More physiological than kcat Limitations of M-M Some enzyme catalyzed rxns show more complex behavior E + SESEZEP E + P With M-M can look only at rate limiting step Often more than one substrate E+S1ES1+S2ES1S2EP1P2 EP2+P1 E+P2 Must optimize one substrate then calculate kinetic parameters for the other Assumes k-2 = 0 Assume steady state conditions

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