J Biol Chem, Vol. 273, Issue 40, 25527-25528, October 2, 1998

MINIREVIEW PROLOGUE
Enzyme Catalytic Power Minireview Series^*

Kenneth E. Neet

From the Department of Biochemistry and Molecular Biology, Finch University of Health Sciences/The Chicago Medical School, North Chicago, Illinois 60064

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One of the great remaining unsolved questions in biological chemistry is the nature of enzyme catalysis. In the mid-twentieth century, several key questions drove biomedical research: the genetic code, the protein folding problem, and the nature of enzymatic catalysis. The code was, of course, solved, and great progress has been made with protein folding (1-3). However, enzyme catalytic efficiency is still hotly debated with new ideas or viewpoints emerging and old ideas being repackaged in an attempt to finally understand these biocatalysts. This issue of the Journal contains the first of three minireviews on "Enzyme Catalytic Power." These minireviews provide an up-to-date perspective on certain theoretical and experimental approaches and should provide the most current insights on the basis of enzyme catalysis for the general biochemist.

How does an enzyme specifically enhance the rate of a chemical reaction by 10⁸- or 10¹³- or 10¹⁸-fold? What are the general principles involved that all (or most) enzymes utilize? For a long time enzyme chemists have realized that general physical chemical principles should accommodate understanding without needing to invoke any enzymatic "secrets." The pathway of changes in chemical bonding, the kinetic order of addition of substrates, the rates of various steps, and the three-dimensional structural aspects of the protein surface are now known for many, many enzymes. However, the quantitation of very few, if any, enzymes can unequivocally be claimed to be known, i.e. what physical chemical properties have caused acceleration of the chemical reaction.

The catalytic factor, as a means to quantitate the catalytic power (rate enhancement or catalytic efficiency) of an enzyme, is simple to define as the ratio of the catalyzed rate to the uncatalyzed rate but can be difficult to estimate accurately in many instances. A few of the problems include the frequent lack of a measurable uncatalyzed rate, comparison of a pseudo-first order enzymatic reaction to a second order simple catalyst (such as OH), and the automatic exclusion of propinquity effects if an intramolecular catalyst is used for comparison. Nevertheless, chymotrypsin and triose-phosphate isomerase are estimated at 10⁸-10¹²-fold. Catalase, which is often rated as a very "fast" enzyme, does not come out well in this ranking because the spontaneous rate of breakdown of H₂O₂ is so high that the ratio of k_(catalyzed) to k_{(uncatalyzed)} is low.

Pauling (4) first suggested that an enzyme might be complementary to the transition state of a reaction and thereby speed up the reaction by binding the transition state and lowering the energy of activation along the reaction coordinate. Pauling's proposal has periodically been modified and fallen in and out of favor and focus. Nevertheless, such a concept not only can help explain enzymes quantitatively but also provides a means of judging the quality of transition state analogs (5) and, essentially, has created the field of catalytic antibodies (6). Today, at least partial complementarity to the transition state is tacitly assumed, and most theories to explain catalytic efficiency are directed toward the details of the complementarity, whether it be electrostatics, particular bonding modes, or arrangement of molecules in the active site.

Most of the theories to explain enzyme catalysis have incorporated some aspect of entropy, either implicitly or explicitly. Thus, ideas of proximity effects, the Circe effect (7), orbital steering (8), and others have included molecular ways in which the entropic portion of the activation energy is decreased by ordering or orientation of the substrate molecules or the catalytic side chains in the enzyme active site. Preorganized active sites, electrostatics, and solvent reorganization are clearly manifestations of these entropic ideas, as is the low barrier hydrogen bond, although not quite so obviously.

Central to the consideration and understanding of enzyme efficiency is the juxtaposition of theory and experimentation. Some say a good experiment is worth a thousand theories, whereas others believe that an ab initio calculation is the only real way to understand an enzyme. In reality, both theoretician and empiricist will ultimately have to agree if we are to understand catalysis. In this series of minireviews, both viewpoints are expressed. Nevertheless, as the serious reader will observe, controversy still exists among proponents of different models for enzyme catalytic power.

This three-part series starts with an article on the relatively new concept of "The Low Barrier Hydrogen Bond in Enzymatic Catalysis" by W. Wallace Cleland, Perry A. Frey, and John A. Gerlt (Universities of Wisconsin and Illinois). A low barrier hydrogen bond, in which the pK values of the proton donor and acceptor are matched in the appropriate environment, is concluded to contribute about 5 orders of magnitude to rate acceleration. Next, William R. Cannon and Stephen J. Benkovic (Pennsylvania State University) discuss "Solvation, Reorganization Energy, and Biological Catalysis." This contribution emphasizes the importance of considering the solvent and the role that the enzyme has in protecting the substrate(s) from solvent as it preorganizes and orients the chemical center for reaction. A factor as large as 10¹⁷ has been suggested for the contribution in certain enzymes. Finally, a theoretical discussion of "Electrostatic Origin of the Catalytic Power of Enzymes and the Role of Preorganized Active Sites" is presented by Arieh Warshel (University of Southern California) in which several different enzyme efficiency models are argued against, including the low barrier hydrogen bond. Warshel concludes that the lowering of the free energy of the transition state by electrostatic stabilization is largely because of a preorganized polar environment in the active site.

In 1968 a review article (9) made the following conclusion.

Until quantitative numbers for each factor can be assigned and until the products of these factors allow the enzymatic rate to be calculated from a model system, we will not have explained enzyme action. It is clear that the identification of these factors and their quantitative evaluation is one of the most important challenges of the next decade.

A few years later another treatise (7) made the following conclusion.

The extent [italics added] to which these different mechanisms (for rate acceleration) account for catalysis in particular cases remains to be established experimentally.

Three decades have now passed without complete quantitative evaluation. The convergence of theory and experiment alongside new ideas and new applications of old ideas gives renewed promise of ultimate comprehension. Progress has now led to a focus on relatively few major contributory factors. Three of these concepts are presented in this minireview series. Different enzymes will surely use different relative contributions of these principles. Readers will have to decide for themselves how to evaluate the various viewpoints and which model(s) to accept at this point in time. Future experimental efforts and refinement of theory should sort out the discrepancies and provide final answers for many enzymes.

	FOOTNOTES

^* This minireview will be reprinted in the 1998 Minireview Compendium, which will be available in December, 1998. 

To whom correspondence should be addressed: Dept. of Biochemistry and Molecular Biology, Finch University of Health Sciences/The Chicago Medical School, 3333 Green Bay Rd., N. Chicago, IL 60064. Tel.: 847-578-3220; Fax: 847-578-3240; E-mail: neetk{at}finchcms.edu.

	REFERENCES

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