Transition complementarity

The kinetic nature of the glass transition should be clear from the last chapter, where we first identified this transition by a change in the mechanical properties of a sample in very rapid deformations. In that chapter we concluded that molecular motion could simply not keep up with these high-frequency deformations. The complementarity between time and temperature enters the picture in this way. At lower temperatures the motion of molecules becomes more sluggish and equivalent effects on mechanical properties are produced by cooling as by frequency variations. We shall return to an examination of this time-temperature equivalency in Sec. 4.10. First, however, it will be profitable to consider the possibility of a thermodynamic description of the transition which occurs at Tg. 

In the following sections the effect of pressure on different types of electron-transfer processes is discussed systematically. Some of our work in this area was reviewed as part of a special symposium devoted to the complementarity of various experimental techniques in the study of electron-transfer reactions (124). Swaddle and Tregloan recently reviewed electrode reactions of metal complexes in solution at high pressure (125). The main emphasis in this section is on some of the most recent work that we have been involved in, dealing with long-distance electron-transfer processes involving cytochrome c. However, by way of introduction, a short discussion on the effect of pressure on self-exchange (symmetrical) and nonsymmetrical electron-transfer reactions between transition metal complexes that have been reported in the literature, is presented. 

Obviously, strong binding of the substrate to the catalyst may distort the structure of S towards that of TS, thereby making reaction easier. However, such distortion simply reflects the complementarity of the catalyst and the transition state (Fersht, 1985). From a purely thermodynamic point of view, the formation of a strong S-catalyst complex lowers free energy by an additional amount that must be overcome in the process of activation of the kc process (3) (Fig. 2). Living organisms and their enzymes have evolved so... 

Fig. 3. Binding site of 2,3-bisphosphoglycerate (DPG) between the two /3 chains in human deoxyhemoglobin. On transition to the oxy struaure, the a-amino groups move apart and the EF segments close up, so that the complementarity of the binding site to DPG is lost (103).

The concept of catalytic antibodies was suggested succinctly by Jencks. If complementarity between the active site and the transition state contributes significantly to enzymatic catalysis, it should be possible to synthesize an enzyme by constructing such an active site. One way to do this is to prepare an antibody to a haptenic group which resembles the transition state of a given reaction. The combining sites of such antibodies should be complementary to the transition state and should cause an acceleration by forcing bound substrates to resemble the transition state. ... 

In terms of enzyme catalysis, the following factors are likely to influence the magnitude of the rate enhancement in enzymatic processes (a) proximity and orientation effects (b) electrostatic complementarity of the enzyme s active site with respect to the reactant s stabilized transition state configuration (c) enzyme-bound metal ions that serve as template, that alter pK s of catalytic groups, that facilitate nucleophilic attack, and that have... 

A new model for enzyme catalysis that challenges the long-standing concept of transition state complementarity as the sole source of enzymatic catalytic efficacy. This shifting model states that (a) enzymes evolved to bind substrates (b) enzyme-substrate complexes have evolved to bind transition states and (c) stronger interactions of substrate with the enzyme facilitate rapid conversion to product. This model questions the concept that strong interactions of enzyme and substrate reduce catalytic efficiency. 

It was mentioned above (see Sect. 2) that for centrosymmetic molecules, such as all-trans polyenes and a,c<>-disubstituted frans-polyenes, there is a complementarity between the selection rules for IP- and 2P-allowed transitions. Transitions from the groimd state (which is of gerade type) to a B state are visible in the IPA spectriun, while transitions from the ground state to an Ag state appear in the 2PA spectrum. For this reason, 2P spectroscopy has... 

The role of transition state complementarity in enzyme catalysis is further explored in Box 6-3. 

The transition state of a reaction is difficult to study because it is so short-lived. To understand enzymatic catalysis, however, we must dissect the interaction between the enzyme and this ephemeral moment in the course of a reaction. Complementarity between an enzyme and the transition state is virtually a requirement for catalysis, because the energy hill upon which the transition state sits is what the enzyme must lower if catalysis is to occur. How can we obtain evidence for enzyme-transition state complementarity Fortunately, we have a variety of approaches, old and new, to address this problem, each providing compelling evidence in support of this general principle of enzyme action. 

The idea of complementarity in enzyme - substrate interactions was introduced by E. Fischer with his famous lock and key analogy.7 In modem terminology this would represent enzyme-substrate complementarity. The currently favored concept of enzyme-transition state complementarity was introduced by Haldane1 and elaborated by L. Pauling.2... 

Enzyme complementarity to transition state implies that kcatfKM is at a maximum... 

B Experimental evidence for the utilization of binding energy in catalysis and enzyme transition state complementarity... 

The effects of enzyme-transition state complementarity on the binding of transition state analogues may be masked by extraneous binding artifacts. Chapter... 

---