A comparison of flexible and constrained haptens in eliciting antibody catalysts for paraoxon hydrolysis

Strategies for the selection of catalytic antibodies against organophosphorus nerve agents

Among the strategies aimed at biocompatible means for organophosphorus nerve agents neutralization, immunoglobulins have attracted attention in the 1990's and 2000's both for their ability to immobilize the toxicants, but also for their ability to be turned into enzymatically active antibodies known as catalytic antibodies or abzymes (antibodies--enzymes). We will present here a critical review of the successive strategies used for the selection of these nerve agent-hydrolyzing abzymes, based on hapten design, namely antibodies raised against a wide variety of transition state analogs, and eventually the strategies based on anti-idiotypic antibodies and reactibodies.

Catalytic antibodies: balancing between Dr. Jekyll and Mr. Hyde

The immunoglobulin molecule is a perfect template for the de novo generation of biocatalytic functions. Catalytic antibodies, or abzymes, obtained by the structural mimicking of enzyme active sites have been shown to catalyze numerous chemical reactions. Natural enzyme analogs for some of these reactions have not yet been found or possibly do not exist at all. Nowadays, the dramatic breakthrough in antibody engineering and expression technologies has promoted a considerable expansion of immunoglobulin's medical applications and is offering abzymes a unique chance to become a promising source of high-precision "catalytic vaccines." At the same time, the discovery of natural abzymes on the background of autoimmune disease revealed their beneficial and pathogenic roles in the disease progression. Thus, the conflicting Dr. Jekyll and Mr. Hyde protective and destructive essences of catalytic antibodies should be carefully considered in the development of therapeutic abzyme applications.

Technological advancements for the detection of and protection against biological and chemical warfare agents

There is a growing need for technological advancements to combat agents of chemical and biological warfare, particularly in the context of the deliberate use of a chemical and/or biological warfare agent by a terrorist organization. In this tutorial review, we describe methods that have been developed both for the specific detection of biological and chemical warfare agents in a field setting, as well as potential therapeutic approaches for treating exposure to these toxic species. In particular, nerve agents are described as a typical chemical warfare agent, and the two potent biothreat agents, anthrax and botulinum neurotoxin, are used as illustrative examples of potent weapons for which countermeasures are urgently needed.

Catalysis of the Photo-Fries Reaction: Antibody-Mediated Stabilization of High Energy States

A conformationally constrained hapten is presented that is capable of catalyzing the first antibody-mediated photo-Fries rearrangement. In this reaction, absorption of light energy by a diphenyl ether substrate results in homolytic C-O bond cleavage followed by recombination to yield biphenyl-derived products. The most proficient antibody studied converts 4-phenoxyaniline 15 into 2-hydroxy-5-aminobiphenyl 16 under high-intensity irradiation at a rate of 8.6 microM/min. These results support a recent hypothesis stating that immunization with conformationally constrained haptens provides higher titers for the acquisition of simple binding antibodies; however, in this case, conformational constraint does not ensure the development of more efficient catalysts. Using the obtained antibodies, the presence of products resulting from escape of free radicals from the solvent cage can be suppressed, altering the excited state energy surface such that free radicals are funneled into the formation of the desired biphenyl product. However, studies also show the inactivation of the antibodies as a result of photodecay of the biphenyl product. Using an isocyanate scavenging resin, the photodecay product could be removed and the inactivation of the antibody drastically reduced. Furthermore, despite the observed photodecay, turnover of the antibody was present; this represents the first case in which true turnover of a photochemical reaction using a catalytic antibody could be observed.

Turnover-based in vitro selection and evolution of biocatalysts from a fully synthetic antibody library

This report describes the selection of highly efficient antibody catalysts by combining chemical selection from a synthetic library with directed in vitro protein evolution. Evolution started from a naive antibody library displayed on phage made from fully synthetic, antibody-encoding genes (the Human Combinatorial Antibody Library; HuCAL-scFv). HuCAL-scFv was screened by direct selection for catalytic antibodies exhibiting phosphatase turnover. The substrate used was an aryl phosphate, which is spontaneously transformed into an electrophilic trapping reagent after cleavage. Chemical selection identified an efficient biocatalyst that then served as a template for error-prone PCR (epPCR) to generate randomized repertoires that were subjected to further selection cycles. The resulting superior catalysts displayed cumulative mutations throughout the protein sequence; the ten-fold improvement of their catalytic proficiencies (>10(10) M(-1)) resulted from increased kcat values, thus demonstrating direct selection for turnover. The strategy described here makes the search for new catalysts independent of the immune system and the antibody framework.

Multiple reactive immunization towards the hydrolysis of organophosphorus nerve agents: hapten design and synthesis

We designed and synthesized a series of haptens to elicit catalytic antibodies with phosphatase activity against nerve agents. The design is based on the novel concept of multiple reactive immunization which aims to afford two or more catalytic residues within the antibody's binding cleft. The haptens showed the desired reactivity in vitro and were submitted for immunization.

Cyclic five-membered phosphinate esters as transition state analogues for obtaining phosphohydrolase antibodies

Two novel cyclic five-membered phosphinate esters (6 and 7) were synthesized and used as transition state analogues (TSAs) for raising phosphohydrolase antibodies. The key step in the syntheses was a McCormack reaction between (2-chloroethyl)dichlorophosphite and 1,4-diphenyl-1,3-butadiene to form isomeric cyclic phosphinyl chlorides which upon further elaboration yielded the TSAs. X-ray crystal structures of two cyclic phosphinate TSA precursors (14a and 14b) show highly compressed CPC bond angles of 94°-97° indicating that the CPC bond angles in 6 and 7 are significantly distorted towards the ideal trigonal bipyramidal equatorial-apical transition state angle of 90° formed during phosphate ester hydrolysis. Antibodies were raised to 6 and 7 using standard hybridoma technology. One antibody, Jel 541, an IgM class antibody, was obtained that was capable of catalyzing the hydrolysis of phosphonate substrate 9 and, to a much lesser extent, phosphate substrate 8. This antibody bound TSA 6 approximately 100 times more tightly than TSA 7 as determined by solid phase radio immune assays. The activity of the antibody-catalyzed reaction was completely inhibited by the presence of TSA6. Although the relative instability and poor solubility of Jel 541 made it impossible to perform a detailed kinetic analysis of the antibody-catalyzed reactions, these results demonstrate that phosphohydrolase antibodies can indeed be obtained using cyclic five membered phosphinates as haptens.Key words: transition state analogues, phosphate ester hydrolysis, catalytic antibodies.