-~-------------------------------------N8NSANDVIEWS----------------_N_A_TU_R_E_V-O_L_._32_5_22_J_A_N_U_A_RY__ 19~87 Enzyme catalysis kinetics and is subject to competitive inhibition. Although the detailed chemical mechanisms of these catalytic Antibodies with some bite antibodies have yet to be determined, it is reasonable to suppose that the ester or David E. Hansen carbamate is strained towards a tetra­ hedral geometry on binding, thus facilitat­ THE spectacular specificities and rate William Jencks', more directly, stated in ing the attack of the hydroxide ion. Fur­ enhancements of enzymes are without 1969 thermore, the Scripps group has already equal among all known catalysts. It is no If complementarity between active site and found that their antibody can act via both wonder then that the design of new en­ transition state contributes significantly to nucleophilic and general base catalysis, zymes has long been a goal of biochemists. enzyme catalysis, it should be possible to syn­ depending on the substrate used. Now two independent groups, one led by thesize an enzyme by constructing a binding The fact that this approach succeeded at Alfonso Tramontano and Richard Lerner site. One way to do this is to prepare an anti­ all gives great hope to those attempting to of the Scripps Clinic and Research Foun­ body to a haptenic group which resembles the isolate even more efficient antibody cat­ dation', and the other by Peter Schultz of transition state of a given reaction. The com­ alysts by this and by other strategies. In the University of California at Berkeley\ bining sites of such antibodies should be com­ addition, it may be possible to modify have taken steps towards this goal by plementary to the transition state and should cause an acceleration by forcing bound sub­ existing catalytic antibodies. Because the demonstrating that antibodies can have strate to resemble transition state. crystal structures of monoclonal anti­ catalytic activity. bodies containing their bound haptens can These accomplishments stem from With the potential for more than 10 12 dif­ be determined - and in fact such a crystal almost a century of research on enzyme ferent antibody molecules, one may structure had already been solved for the catalysis that began with the legendary speculate that every antibody catalyst antibody used by the Berkeley group - chemist Emil Fischer. In 1902, Fischer' imaginable is obtainable from the existing site-directed mutagenesis can be intel­ stated in his Nobel Prize lecture repertoire of antibodies (just as every ligently used to change particular amino The examination of synthetic glucosides has antibody needed to bind all antigens acids so as to generate enhanced spec­ shown that the action of the enzymes depends seems to be accessible). The challenge is ificity or catalytic function . In addition, to a large extent on the geometrical structure of therefore in selecting an antibody with the both groups are planning to attach co­ the molecule to be attacked, that the two must desired catalytic function from this huge factors and metal ions to the antibodies in match like a lock and key ... To equal Nature repertoire. However, as pointed out by attempts to increase catalytic efficiency here, the same means have to be applied, and I Jencks, if in a stable molecule one could further. One immediate goal is the crea­ therefore foresee the day when physiological model the transition state for a reaction, tion of sequence-specific proteases, which chemistry will not only make extensive use of such a selection process might be at hand. would have many important biochemical natural enzymes as agents, but when it will also applications. Overall, the possibilities prepare synthetic [enzymes] for its purposes. Although several early attempts to isolate antibodies with catalytic activity appear almost limitless, and as Schultz's 6 Since then there have been revolution­ failed -" the transition state analogue group concludes, "these approaches may ary advances in our understanding of strategy has now succeeded. Both the enable us to tailor-make catalysts for use enzyme catalysis and of protein structure, Scripps group and the Berkeley group as tools in biology, chemical synthesis and but it is a measure of the immense com­ used tetrahedral phosphorus compounds medicine". plexity of enzymes that a completely syn­ to mimic the presumed transition state in Given the fact that antibody molecules thetic system has yet to be prepared. the hydrolysis of a carboxylate ester. can act as catalysts, it will now be possible Present thinking on the basis of enzyme Tramontano et al. generated monoclonal to address several theoretical questions catalysis began in the 1940s when Linus antibodies against four phosphonate about enzyme catalysis. For example, Pauling suggested that Fischer's Lock and esters and demonstrated that some of enzymes are flexible molecules that may key analogy must be modified: an enzyme these antibodies have catalytic activity assume different conformations during a active site should not match the substrate, (these workers distressingly call these catalytic cycle, but the contribution of it should rather be complementary to the catalytic antibodies 'abzymes'). The these motions to catalysis remains un­ activated complex (or transition state) of values of the first-order rate constant k,,, certain. Antibodies, on the other hand, the reaction being catalysed. Much recent at pH 8 for two ester substrates with the although not static molecules, seem to work on enzyme mechanisms has suppor­ monoclonal antibody 604 are 1.62 min-' have only one important functional con­ ted Pauling's view, and one may con­ and 0.48 min -'. The corresponding Km formation . It will be of great interest to see vincingly argue that most of the principles values, the concentrations of substrate if antibodies with catalytic efficiencies of enzyme catalysis are now well under­ required for half-maximal velocity, are equal to those of enzymes can be stood (even if we do not necessarily see 1.90 11M and 0.62 11M. obtained, or if, ultimately, fundamental how these principles are manifested). Schultz et al. tested the mouse mono­ differences between antibodies and Although a concomitant understanding clonal antibody MOPC167, which is enzymes will limit this approach. 0 of protein folding and structure has not so known to bind nitrophenyl phosphoryl­ 1. Tramontano, A., Janda, K. D. & Lerner, R.A. Science 234, far been achieved, a strategy for generat­ choline, as a catalyst for the hydrolysis of 1566 - 1570 (1986) . ing new enzymes that depends on an an analogous carbamate and measured a 2. Pollack , S.J ., Jacobs. J.W. & Schultz. P.G. Science 234. 1570 - 1573 ( \986). understanding of catalysis, but not on the k,,, of 0.4 min-' and a Kmof 208 11M at pH 3. Nobel Lec:tures Chemistry J9()J-1921 (Elsevier, New York, ability to design polypeptides that will fold 7. These rates correspond to accelerations 1966). 4. Pauling, L. Am. Sciefll. 36,51 - 58 (1948). properly, was first alluded to by Pauling' of approximately 1,OOO-fold over the 5. Jencks, W. Catalysis ill Chemistry and Enzymology in 1948 uncatalysed rates, but are approximately (McGraw Hill. New York. 1969). 5,OOO-fold slower than, for example, 6. Slobin , 1..1. 8iochemisrry 5. 2836-2844 (1%6). An enzyme has a structure closely similar to 7. Raso , V. & Slollar. B.D. lliochemisr,y 14. 581-591; 591- the rate for a good ester substrate of 599 (1975). that found for antibodies, but with one impor­ a-chymotrypsin. 8. Kohen. F .• Kim.J.B .• Lindler. H.R., Eshhar, Z. & Green. tant difference. Namely that the surface con­ B. FFllS I.err. 111 ,427- 431 (1980). figuration of the enzyme is not so closely com­ Both research groups note that their plementary to its specific substrate as is that of antibodies have many of the same charac­ David E. Hansen is in the Department of Chem­ an antibody to its antigen, but instead is com­ teristics as enzymes: each shows good istry at Amherst College, Amherst, Mass­ plementary to the activated complex. substrate specificity, exhibits saturation achusetts 01002, USA. .