18 - Lecture 18 Chemical and Enzymatic Kinetics 1 Monday To...

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Unformatted text preview: Lecture 18: Chemical and Enzymatic Kinetics 1 Monday, August 23, 2010 To understand enzymes, we most often investigate the rate at which they catalyze a chemical reaction, which is the kinetics of the reaction . The first important thing to remember about enzymes is that they only affect the rate at which a reaction occurs, not the extent to which a reaction occurs, or in other words, the final equilibrium distribution of products and substrates. Monday, August 23, 2010 The final equilibrium distribution of products and substrates is described by the equilibrium constant K eq , and therefore, the standard free energy change of the reaction, ∆G°’: For the reaction substrate (S) in equilibrium with product (P): S ⁶ P The equilibrium constant, Keq is described as: K’ eq = [ P ] / [ S ] Monday, August 23, 2010 and the relationship between free energy change and the equilibrium constant is: ∆G°’ = - RTlnK’ eq where R is the gas constant (8.3145 J K-1 mol-1 ) and T is the absolute temperature in Kelvin. A large negative value for ∆G°’ reflects a spontaneous process (substrates will convert to product). However, THIS DOES NOT SAY ANYTHING ABOUT THE RATE AT WHICH THE REACTION OCCURS! An example of this is the combustion of sugar into oxygen and water; a very favorable process, but does not occur rapidly at all under standard state conditions. Monday, August 23, 2010 Monday, August 23, 2010 The rate of reaction is related to the thermodynamics of the transition state of the reaction (the point in the reaction coordinate where the decay to the S or P state is equally probable) Monday, August 23, 2010 For a typical reaction, the free energy associated with going from substrate to the transition state is given by ∆G ‡ Monday, August 23, 2010 Transition state theory states that the rate of this reaction will be related to the amount of energy it takes to get to the transition state: ∆G ‡ = -RT ln K...
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This note was uploaded on 11/12/2010 for the course CHEN 3320 at Colorado.

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18 - Lecture 18 Chemical and Enzymatic Kinetics 1 Monday To...

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