Electrostatic Enzyme

Specificity for a particular charged substrate can be engineered into an enzyme by replacement of residues within the enzyme-active site to achieve electrostatic complementarity between the enzyme and substrate (75). Protein engineering, when coupled with detailed stmctural information, is a powerful technique that can be used to alter the catalytic activity of an enzyme in a predictable fashion. 

Destabilization of the ES complex can involve structural strain, desolvation, or electrostatic effects. Destabilization by strain or distortion is usually just a consequence of the fact (noted previously) that the enzyme is designed to bind the transition state more strongly than the substrate. When the substrate binds, the imperfect nature of the fit results in distortion or strain in the substrate, the enzyme, or both. This means that the amino acid residues that make up the active site are oriented to coordinate the transition-state structure precisely, but will interact with the substrate or product less effectively. 

Theoretical Studies of Enzymic Reactions Dielectric, Electrostatic and Steric Stabilizations of the Carbonium Ion in the Reaction of Lysozyme A. Warshel and M. Levitt Journal of Molecular Biology 103 (1976) 227-249... 

The second part of lanosterol biosynthesis is catalyzed by oxidosqualene lanosterol cyclase and occurs as shown in Figure 27.14. Squalene is folded by the enzyme into a conformation that aligns the various double bonds for undergoing a cascade of successive intramolecular electrophilic additions, followed by a series of hydride and methyl migrations. Except for the initial epoxide protonation/cyclization, the process is probably stepwise and appears to involve discrete carbocation intermediates that are stabilized by electrostatic interactions with electron-rich aromatic amino acids in the enzyme. 

After the somewhat tedious parametrization procedure presented above you are basically an expert in the basic chemistry of the reaction and the questions about the enzyme effect are formally straightforward. Now we only want to know how the enzyme changes the energetics of the solution EVB surface. Within the PDLD approximation we only need to evaluate the change in electrostatic energy associated with moving the different resonance structures from water to the protein-active site. 

The approach taken above estimates the effect of the metal by simply considering its electrostatic effect (subjected, of course, to the correct steric constraint as dictated by the metal van der Waals parameters). To examine the validity of this approach for other systems let s consider the reaction of the enzyme carbonic anhydrase, whose active site is shown in Fig. 8.6. The reaction of this enzyme involves the hydration of C02, which can be described as (Ref. 5)... 

With the valence bond structures of the exercise, we can try to estimate the effect of the enzyme just in terms of the change in the activation-free energy, correlating A A g with the change in the electrostatic energy of if/2 and i/r3 upon transfer from water to the enzyme-active site. To do this we must first analyze the energetics of the reaction in solution and this is the subject of the next exercise. 

As discussed and demonstrated in the previous chapters, the catalytic effect of several classes of enzymes can be attributed to electrostatic stabilization of the transition state by the surrounding active site. Apparently, enzymes can create microenvironments which complement by their electrostatic potential the changes in charges during the corresponding reactions. This provides a simple and effective way of reducing the activation energies in enzymatic reactions. 

As discussed in the early sections it seems that there are very few effective ways to stabilize the transition state and electrostatic energy appears to be the most effective one. In fact, it is quite likely that any enzymatic reaction which is characterized by a significant rate acceleration (a large AAgf +p) will involve a complimentarity between the electrostatic potential of the enzyme-active site and the change in charges during the reaction (Ref. 10). This point may be examined by the reader in any system he likes to study. 

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