. Biophysical science. Biophysics. 408 Thermodynamics of Enzyme Reactions /22 : 3. 3. Absolute Rate Theory The proportionality constant a in Equation 2 of the collision theory is a sort of "correction factor" to make theory and experiment agree. In the case of gaseous reactions, a is often very small, having values in some reactions as low as 10"10. A somewhat different thermodynamic analysis called absolute rate theory has been outstandingly successful in predicting these small values of a for gaseous reactions. Its application to reactions in liquids is considerably more tenuo
. Biophysical science. Biophysics. 408 Thermodynamics of Enzyme Reactions /22 : 3. 3. Absolute Rate Theory The proportionality constant a in Equation 2 of the collision theory is a sort of "correction factor" to make theory and experiment agree. In the case of gaseous reactions, a is often very small, having values in some reactions as low as 10"10. A somewhat different thermodynamic analysis called absolute rate theory has been outstandingly successful in predicting these small values of a for gaseous reactions. Its application to reactions in liquids is considerably more tenuous, although the theory is widely accepted. In order to describe absolute rate theory, it is convenient to again use the potential energy diagram of the form found in Figures 1 and 2. Now, three separate regions must be distin- guished as shown in Figure 4. The abscissa does not have to be regarded as simply a distance apart. It is called the reaction coordinate and will, in general, have the dimension of length. When the reactants are far out on the reaction coordinate, they are considered as separate mole- cules A and B. Above the highest part of the potential barrier, they are considered as an activated complex^! ⢠Finally, in the region of the potential well there is a single molecular species C. This method of analysis is an approximation method because the region in which the activated complex exists is arbitrary. The reasoning employed is very similar to that used to develop Michaelis-Menten kinetics in Chapter 17. The complexes introduced in that chapter and here both control reaction rates. However, the intermediate complex of enzyme kinetics stays in existence for a much longer time, and its rate of breakdown cannot be determined on a priori grounds. The rate of breakdown of the activated complex A â B*, on the contrary, is always 1 d[A-B*] RT Figure 4. The absolute rate theory. The absolute rate theory postulates an activated complex A-B*, which must be in eq
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