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Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Enzyme Kinetics
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Enzymes
• Enzymes endow cells with the remarkable capacity to exert kinetic control over thermodynamic potentiality
• Enzymes are the agents of metabolic function
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Catalytic Power
• Enzymes can accelerate reactions as much as 1016 over uncatalyzed rates!
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Specificity
• Enzymes selectively recognize proper substrates over other molecules
• Enzymes produce products in very high yields - often much greater than 95%
• Specificity is controlled by structure - the unique fit of substrate with enzyme controls the selectivity for substrate and the product yield
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
The Transition State
Understand the difference between G and G‡
• The overall free energy change for a reaction is related to the equilibrium constant
• The free energy of activation for a reaction is related to the rate constant
• It is extremely important to appreciate this distinction!
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
What Enzymes Do....
• Enzymes accelerate reactions by lowering the free energy of activation by binding the transition state of the reaction better than the substrate
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Enzyme Kinetics
Several terms to know!
• rate or velocity
• rate constant
• rate law
• order of a reaction
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
The Michaelis-Menten Equation
You should be able to derive this! • Louis Michaelis and Maude Menten's theory
• It assumes the formation of an enzyme-substrate complex
• It assumes that the ES complex is in rapid equilibrium with free enzyme
• Breakdown of ES to form products is assumed to be slower than 1) formation of ES and 2) breakdown of ES to re-form E and S
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
The dual nature of the Michaelis-Menten equation
Combination of 0-order and 1st-order kinetics
• The Michaelis-Menten equation describes a rectangular hyperbolic dependence of v on S!
• When S is low, the equation for rate is 1st order in S
• When S is high, the equation for rate is 0-order in S
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Understanding Km
The "kinetic activator constant"
• Km is a constant
• Km is a constant derived from rate constants
• Km is, under true Michaelis-Menten conditions, an estimate of the dissociation constant of E from S
• Small Km means tight binding; high Km means weak binding
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Understanding Vmax
The theoretical maximal velocity
• Vmax is a constant
• Vmax is the theoretical maximal rate of the reaction - but it is NEVER achieved in reality
• To reach Vmax would require that ALL enzyme molecules are tightly bound with substrate
• Vmax is asymptotically approached as substrate is increased
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
The turnover number
A measure of catalytic activity
• kcat, the turnover number, is the number of substrate molecules converted to product per enzyme molecule per unit of time, when E is saturated with substrate.
• If the M-M model fits, k2 = kcat = Vmax/Et
• Values of kcat range from less than 1/sec to many millions per sec
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
The catalytic efficiencyName for kcat/Km
• An estimate of "how perfect" the enzyme is
• kcat/Km is an apparent second-order rate constant
• It measures how the enzyme performs when S is low
• The upper limit for kcat/Km is the diffusion limit - the rate at which E and S diffuse together
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Enzyme Inhibitors
Reversible versus Irreversible
• Reversible inhibitors interact with an enzyme via noncovalent associations
• Irreversible inhibitors interact with an enzyme via covalent associations
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Classes of Inhibition
• Competitive inhibition - inhibitor (I) binds only to E, not to ES
• Noncompetitive inhibition - inhibitor (I) binds either to E and/or to ES
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Competitive inhibition
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Non-competitive inhibition