Giudici-Orticoni M T, Buc J, Ricard J
Centre de Biochimie et de Biologie moléculaire du C.N.R.S., Marseille, France.
Eur J Biochem. 1990 Dec 12;194(2):475-81. doi: 10.1111/j.1432-1033.1990.tb15641.x.
The principles of structural kinetics allow one to define the thermodynamic conditions that are sufficient to generate a certain type of kinetic behavior. If subunits are loosely coupled, that is if no quaternary constraint exists between them, the kinetic behavior of the polymeric enzyme is qualitatively defined by the behavior of an ideal dimer. The nature and the extent of the kinetic cooperativity are defined by the energy of interaction, delta G rho, between two subunits. This energy of interaction is that of an ideal dimer relative to that of the A2 and B2 states. This thermodynamic formulation of a given type of cooperativity holds whatever the degree of polymerization of the enzyme. Under these conditions of loose coupling between subunits, positive kinetic cooperativity cannot be associated with any sigmoidicity of the rate curve. The range of energy coupling where positive kinetic cooperativity must, of necessity, be observed becomes more and more narrow as the number of subunit interactions is increased. This range, however, is independent of the number of subunits. The same situation is not observed for negative cooperativity which appears to be independent of both the number of subunits and the number of subunit interactions. If the subunits are tightly coupled, that is if quaternary constraints exist between them, three thermodynamic parameters, delta G' rho, delta G lambda, delta G mu, are required to define the nature of kinetic cooperativity. delta G' rho is the free energy of an ideal strained dimer relative to that of strained A2 and B2 states. delta G lambda and delta G mu represent the difference of strain energies between conformations A and B and B and B relative to that existing between conformations A and A. One may determine in the parametric space (delta G' rho, delta G lambda, delta G mu) the boundaries between the sufficient conditions that generate a certain type of cooperativity and the lack of these conditions. The kinetic parameters of the rate equation are not all independent. A number of constraint conditions exist between them which depend upon the subunit design of the polymeric enzyme. The existence of these constraint conditions may be diagnostic of a certain type of subunit interactions.
结构动力学原理使人们能够定义足以产生特定类型动力学行为的热力学条件。如果亚基之间是松散耦合的，也就是说如果它们之间不存在四级约束，那么聚合酶的动力学行为在性质上就由理想二聚体的行为来定义。动力学协同性的性质和程度由两个亚基之间的相互作用能ΔGρ来定义。这种相互作用能是理想二聚体相对于A2和B2状态的相互作用能。无论酶的聚合程度如何，给定类型协同性的这种热力学表述都是成立的。在亚基之间这种松散耦合的条件下，正动力学协同性与速率曲线的任何S形无关。随着亚基相互作用数量的增加，必然会观察到正动力学协同性的能量耦合范围变得越来越窄。然而，这个范围与亚基的数量无关。对于负协同性，情况则不同，它似乎与亚基的数量和亚基相互作用的数量都无关。如果亚基是紧密耦合的，也就是说如果它们之间存在四级约束，那么就需要三个热力学参数ΔG'ρ、ΔGλ、ΔGμ来定义动力学协同性的性质。ΔG'ρ是理想应变二聚体相对于应变A2和B2状态的自由能。ΔGλ和ΔGμ分别表示构象A与B以及B与B之间的应变能差相对于构象A与A之间存在的应变能差。人们可以在参数空间（ΔG'ρ，ΔGλ，ΔGμ）中确定产生特定类型协同性的充分条件与缺乏这些条件之间的界限。速率方程的动力学参数并非全部独立。它们之间存在一些约束条件，这些条件取决于聚合酶的亚基设计。这些约束条件的存在可能有助于诊断特定类型的亚基相互作用。
