I is important lo stress that whether the AG for a reaction is larger, smaller, or the same as ag depends on the concentrations of the reactants. The criterion of spontaneity for a reaction is△G,not△G
ENZYMES CANNOT ALTER REACTION EQUILIBRIA An enzyme is a catalyst, and consequently it cannot alter the equilibrium of a chemical reaction. This means that an enzyme accelerates the for- ward and reverse reaction by precisely the same factor. Consider the inter- conversion of A and B. Suppose that in the absence of enzyme the fo ward rate constant(kp)is 10-4s-l and the reverse rate constant (kr)is 10-6s-I. The equilibrium constant k is given by the ratio of these rate constants 10-4s-1 A B 10-6s-1 b k 10-4 K F =100 [A]kR10-6
The equilibrium concentration of B is 100 times that of A, whether or not enzyme is present. However, it would take more than an hour to ap- proach this equilibrium without enzyme, whereas equilibrium would be attained within a second in the presence of a suitable enzyme Enzymes accelerate the attainment of equilibria but do not shill their position
ENZYMES ACCELERATE REACTIONS BY STABILIZING TRANSITION STATES A chemical reaction of substrate s to form product P go oes throug transition state SF that has a higher free energy than either S or P Substrate Transition Product state The transition state is the most seldom occupied species along the reac- tion pathway because it has the highest free energy. The Gibbs ree energy of activation, symbolized by AG", is equal to the difference in free energy between the transition state and the substrate
△G=Gst-Gs The reaction rate V is proportional to the concentration of st which depends on A Gf because it is in equilibrium with S [S]= [S]e- AG/RT k7 小s]="[S SI △G/RT In these equations, k is boltzmann's constant and h is Plancks constant. The value of kT/h at 25C is 6. x 1012 s-1