tailieunhanh - Harper’s Illustrated Biochemistry - Part 2

Nếu ΔG0 là một số âm, Keq sẽ lớn hơn so với sự thống nhất và tập trung của các sản phẩm ở trạng thái cân bằng sẽ vượt quá của các chất nền. Nếu ΔG0 là tích cực, Keq sẽ được ít hơn sự đoàn kết và sự hình thành của chất nền sẽ được ưa chuộng. | ENZYMES KINETICS 61 For the reaction A B P Q Keq P Q A B 4 and for reaction 5 A A P 5 K eq P A 2 6 AG0 may be calculated from equation 3 if the concentrations of substrates and products present at equilibrium are known. If AG0 is a negative number K will be greater than unity and the concentration of products at equilibrium will exceed that of substrates. If AG0 is positive Keq will be less than unity and the formation of substrates will be favored. Notice that since AG0 is a function exclusively of the initial and final states of the reacting species it can provide information only about the direction and equilibrium state of the reaction. AG0 is independent of the mechanism of the reaction and therefore provides no information concerning rates of reactions. Conse-quently and as explained below although a reaction may have a large negative AG0 or AG0 it may nevertheless take place at a negligible rate. THE RATES OF REACTIONS ARE DETERMINED BY THEIR ACTIVATION ENERGY Reactions Proceed via Transition States The concept of the transition state is fundamental to understanding the chemical and thermodynamic basis of catalysis. Equation 7 depicts a displacement reaction in which an entering group E displaces a leaving group L attached initially to R. E R - L E - R L 7 Midway through the displacement the bond between R and L has weakened but has not yet been completely severed and the new bond between E and R is as yet incompletely formed. This transient intermediate in which neither free substrate nor product exists is termed the transition state E R L. Dotted lines represent the partial bonds that are undergoing formation and rupture. Reaction 7 can be thought of as consisting of two partial reactions the first corresponding to the formation F and the second to the subsequent decay D of the transition state intermediate. As for all reactions characteristic changes in free energy AGF and AGD are associated with each partial reaction. E R - L E-R-L AGf 8 E-R L AGd