Principles of Antibody Catalysis

Departments of Chemistry and Molecular Biology The Scripps Research Institute 10666 North Torrey Pines Road La Jolla, California 92037 USA 

Why is 1F7 10 -times iess active than chorismate mutase The antibody molecule s distinctive architecture does not appear to impose intrinsic structural limitations on catalysis. Indeed, comparison of the active sites of the antibody and the monofunctional enzyme from Bacillus subtilis [26] suggests that the differences between them are more a matter of degree than of kind. Upon complex formation, the hapten is buried to a similar extent in both proteins, and similar types of interactions are available for orienting the flexible substrate correctly for reaction. Furthermore, the enzyme and the antibody seem to promote the rearrangement of chorismate via the same concerted transition state as the uncatalyzed reaction. Other formal mechanistic possibilities, such as a two-step heterolytic process assisted by an enzymic nucleophile, can be ruled out by the lack of appropriate functional groups in the respective active sites [25,26]. 

Unlike 1F7, chorismate mutase appears to have more extensive interactions with the hapten s alcohol moiety than with the tertiary carboxylate [26], but this is unlikely to account for the observed differences in rate. In conjunction with the overall shape complementarity of the protein pocket, recognition of either the C-1 carboxylate or the C-4 alcohol of chorismate should be sufficient for correctly positioning the cyclohexadiene ring for reaction. Consistent with this idea, antibody 11F1-2E9, which was elicited in the same way as 1F7, has remarkably high catalytic activity [21]. Detailed structural data for 11F1-2E9 are eagerly awaited. 

Controlling chemical reactivity and selectivity with antibodies 

The importance of substrate destabilization is illustrated by the antibody-catalyzed decarboxylation of 3-carboxybenzisoxazoles (5) to give salicylo-nitriles (7) [34]. This reaction is extraordinarily sensitive to its solvent microenvironment, with rate enhancements up to 10 -fold observed upon transfer of the reactant from aqueous buffCT to aprotic dipolar solvents [35]. De solvation of the negatively charged carboxylate group greatly destabilizes the substrate, while the charge delocalized transition state (6) may be stabilized by dispersion interactions with solvent. Similar factors are believed 

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Hydrolysis of activated aryl esters, in particular nitrophenyl esters, has been used extensively to demonstrate the principles of antibody catalysis. This type of substrate is advantageous for model studies because formation of the tetrahedral intermediate is rate limiting, so that the issue of guiding decomposition of this intermediate towards hydrolysis does not need to be addressed in design. Nevertheless, hydrolyses of non-activated esters are also catalyzed by antibodies, the most notable example being that of R- and S-selective esterolyses of 2 by antibodies raised against hapten 1 (Scheme 1) [13]. Another anti-1 antibody was shown to promote enantioselective acylation of alcohols such as (S)-4 using vinyl ester 3 as acyl donor [14]. 

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