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Tất cả các protomers trong enzyme phải hoặc là R hoặc T conformations nhà nước hỗn hợp không được phép. Các trạng thái R và T của enzyme ở trạng thái cân bằng với nhau. Do đó, một hằng số cân bằng (L) có thể được viết cho quá trình chuyển đổi RT (L = [T] / [R]). 4. Mối quan hệ ràng buộc của một phối tử cụ thể phụ thuộc vào cấu tạo của các enzyme | 110 MULTISITEAND COOPERATIVE ENZYMES T xo I k J S It kT TS DO S It kT TS2 00 L _ Ro S It kR L _ m RS S It kR L . . . S Is RS2 Figure 8.5. Diagrammatic representation of the concerted transition model for a two-site cooperative enzyme. 3. All protomers within the enzyme must be in either the R or T state mixed conformations are not allowed. The R and T states of the enzyme are in equilibrium with each other. Thus an equilibrium constant L can be written for the R T transition L T R . 4. The binding affinity of a specific ligand depends on the conformation of the enzyme R or T and not on neighboring site occupancy. Based on equilibrium arguments a general expression for the velocity of a cooperative enzyme-catalyzed reaction can be derived. The equilibrium macroscopic KT KR and microscopic kT kR dissociation constants for the different enzyme-substrate species present in a two-protomer enzyme are Kr 1kR RS ỊR RS R 2 R RS L J Kr kR RS S R S 2 R S 2 Kr 2kR Y RS2 R R RS2 KR kR 1 kr T S T S 2 T S 8-17 T 2T TS Kt kT _ - _ TS S _ T S 2 T S 2 Kt 2kT TS2 77 -7 T 1 TS2 kT kT A useful parameter sometimes reported in kinetic studies is the nonexclusive binding coefficient c . This coefficient is defined as the ratio of the intrinsic enzyme - substrate dissociation constants for the enzyme in the R CONCERTED TRANSITION OR SYMMETRY MODEL 111 and T states kR c kT 8.18 A lower value of the nonexclusive binding coefficient is associated with a higher cooperativity and therefore sigmoidicity of the velocity curves. A lower value of this coefficient implies a decreased affinity of the T state for substrate relative to the R state. If the enzyme in the T state does not bind substrate kT to c 0. To simplify the mathematical treatment further assumptions have to be made see Fig. 8.6 1. Substrate can only bind to the R state of the protomer substrate does not bind to the T state of the protomer c 0 . 2. The R state of the protomer is catalytically active and the T state is catalytically .
