Catalytic Antibodies for Organic Synthesis

Various strategies for obtaining oxidative artificial hemoproteins with a catalytic oxidative activity: from "Hemoabzymes" to "Hemozymes"?

The design of artificial hemoproteins that could lead to new biocatalysts for selective oxidation reactions using clean oxidants such as O 2 or H 2 O 2 under ecocompatible conditions constitutes a really promising challenge for a wide range of industrial applications. In vivo, such reactions are performed by heme-thiolate proteins, cytochromes P450, that catalyze the oxidation of drugs by dioxygen in the presence of electrons delivered from NADPH by cytochrome P450 reductase. Several strategies were used to design new artificial hemoproteins to mimic these enzymes, that associate synthetic metalloporphyrin derivatives to a protein that is supposed to induce a selectivity in the catalyzed reaction. A first generation of artificial hemoproteins or "hemoabzymes" was obtained by the non-covalent association of synthetic hemes such as N-methyl-mesoporphyrin IX, Fe(III) -α3β-tetra-o-carboxyphenylporphyrin or microperoxidase 8 with monoclonal antibodies raised against these cofactors. The obtained antibody-metalloporphyrin complexes displayed a peroxidase activity and some of them catalyzed the regio-selective nitration of phenols by H 2 O 2/ NO 2 and the stereo-selective oxidation of sulphides by H 2 O 2. A second generation of artificial hemoproteins or "hemozymes", was obtained by the non-covalent association of non-relevant proteins with metalloporphyrin derivatives. Several strategies were used, the most successful of which, named "host-guest" strategy involved the non-covalent incorporation of metalloporphyrin derivatives into easily affordable proteins. The artificial hemoproteins obtained were found to be able to perform efficiently the stereoselective oxidation of organic compounds such as sulphides and alkenes by H 2 O 2 and KHSO 5.

From “hemoabzymes” to “hemozymes”: towards new biocatalysts for selective oxidations

Two generations of artificial hemoproteins have been obtained: “hemoabzymes”, by non-covalent association of synthetic hemes with monoclonal antibodies raised against these cofactors and “hemozymes”, by non-covalent association of non-relevant proteins with metalloporphyrin derivatives. A review of the different strategies employed as well as their structural and catalytic properties is presented here.

Enzyme immunoassays as screening tools for catalysts and reaction discovery

This feature article summarizes the development and use of immunoassay techniques (ELISA) as screening tools for fast identification of efficient catalysts in libraries and for the discovery of new chemical reactions.

The expression of the 14D9 catalytic antibody in suspended cells of Nicotiana tabacum cultures increased by the addition of protein stabilizers and by transference from Erlenmeyer flasks to a 2-L bioreactor

The effect of two protein stabilizers (polyvinylpyrrolidone [PVP] and gelatine) on growth and 14D9 yield of Nicotiana tabacum cell suspension cultures (Ab-KDEL and sec-Ab) was analyzed. The addition of PVP at a concentration of 1.0 g L(-1) produced the highest total 14D9 yield (biomass + culture medium) in the Ab-KDEL line (4.82% total soluble protein [TSP]). With the addition of gelatine, the highest total 14D9 yield (2.48% TSP) was attained in the Ab-KDEL line at 5.0 g L(-1) gelatine. When the Ab-KDEL suspended cells were cultured in a 2-L bioreactor, the highest 14D9 yield was 8.1% TSP at a 5% w/v inoculum size, which was the best 14D9 yield so far obtained in the platforms tested (E. coli, N. tabacum leaves and seeds, N. tabacum hairy roots, and cell suspension cultures).

Artificial metalloenzymes: proteins as hosts for enantioselective catalysis

Enantioselective catalysis is one of the most efficient ways to synthesize high-added-value enantiomerically pure organic compounds. As the subtle details which govern enantioselection cannot be reliably predicted or computed, catalysis relies more and more on a combinatorial approach. Biocatalysis offers an attractive, and often complementary, alternative for the synthesis of enantiopure products. From a combinatorial perspective, the potential of directed evolution techniques in optimizing an enzyme's selectivity is unrivaled. In this review, attention is focused on the construction of artificial metalloenzymes for enantioselective catalytic applications. Such systems are shown to combine properties of both homogeneous and enzymatic kingdoms. This review also includes our recent research results and implications in the development of new semisynthetic metalloproteins for the enantioselective hydrogenation of N-protected dehydro-amino acids.

Structure–activity relationships in aminocyclopentitol glycosidase inhibitors

Aminocyclopentitol analogs of beta-D-glucose, beta-D-galactose and alpha-D-galactose bearing alkyl substituents as aglycon mimics on the amine function were prepared and tested for inhibition of various glycosidases. N-benzyl-beta-D-gluco derivatives 1-4 and N-benzyl-beta-D-galacto derivative 5 inhibited beta-galactosidase and beta-glucosidase. N-benzyl-alpha-D-galacto aminocyclopentitol 6 strongly inhibited alpha-galactosidase. The inhibitory activities observed were generally stronger compared to those of their primary amine analogs. A structure-activity relationship analysis was carried out including data from thirty-five different aminocyclopentitol glycosidase inhibitors. The strongest inhibitions reported for any enzyme were associated with a perfect stereochemical match between aminocyclopentitol and glycosidase, including the alpha- or beta-configuration of the amino-group corresponding to the enzyme's anomeric selectivity.

Hemoabzymes: towards new biocatalysts for selective oxidations

Catalytic antibodies with a metalloporphyrin cofactor or <<hemoabzymes>>, used as models for hemoproteins like peroxidases and cytochrome P450, represent a promising route to catalysts tailored for selective oxidation reactions. A brief overview of the literature shows that until now, the first strategy for obtaining such artificial hemoproteins has been to produce antiporphyrin antibodies, raised against various free-base, N-substituted Sn-, Pd- or Fe-porphyrins. Five of them exhibited, in the presence of the corresponding Fe-porphyrin cofactor, a significant peroxidase activity, with k(cat)/K(m) values of 3.7 x 10(3) - 2.9 x 10(5) M(-1) min(-1). This value remained, however, low when compared to that of peroxidases. This strategy has also led to a few models of cytochrome P450. The best of them, raised against a water-soluble tin(IV) porphyrin containing an axial alpha-naphtoxy ligand, was reported to catalyze the stereoselective oxidation of aromatic sulfides by iodosyl benzene using a Ru(II)-porphyrin cofactor. The relatively low efficiency of the porphyrin-antibody complexes is probably due, at least in part, to the fact that no proximal ligand of Fe has been induced in those antibodies. We then proposed to use, as a hapten, microperoxidase 8 (MP8), a heme octapeptide in which the imidazole side chain of histidine 18 acts as a proximal ligand of the iron atom. This led to the production of seven antibodies recognizing MP8, the best of them, 3A3, binding it with an apparent binding constant of 10(-7) M. The corresponding 3A3-MP8 complex was found to have a good peroxidase activity characterized by a k(cat)/K(m) value of 2 x 10(6) M(-1) min(-1), which constitutes the best one ever reported for an antibody-porphyrin complex. Active site topology studies suggest that the binding of MP8 occurs through interactions of its carboxylate substituents with amino acids of the antibody and that the protein brings a partial steric hindrance of the distal face of the heme of MP8. Consequently, the use of the 3A3-MP8 complexes for the selective oxidation of substrates, such as sulfides, alkanes and alkenes will be undertaken in the future.

The use of enzyme immunoassays for the detection of abzymatic activities. Application to an enantioselective thioacetal hydrolysis activity

Relying on the particularly high specificity displayed by antibodies, enzyme immunoassays have proved to be one of the most efficient tools for early detection of the catalytic activities displayed by antibodies. We took advantage of such an assay, namely the Cat-enzyme-linked immunoassay (EIA) approach developed in our laboratories, both to exhibit and characterise an antibody-catalysed thioacetal hydrolysis. Monoclonal antibody (mAb) H3-32 was thus identified to accelerate the hydrolysis reaction of thioacetal substrate (NC9) to vanillylmandelic acid (VMA), with a k(cat) of 0.148 h(-1) (k(uncat) = 6.85 x 10(-5) h(-1)), and a K(M) of 720 microM. Taking advantage of the enantiomeric discrimination between (R)- and (S)-VMA displayed by some of the anti-H3 monoclonal antibodies, we were also able to determine that (S)-VMA was preferentially formed during this abzymatic hydrolysis with a 47% enantiomeric excess. All these EIA measurements were confirmed through HPLC analyses.

Immunologically driven chemical engineering of antibodies for catalytic activity

We describe a new strategy for the preparation of catalytic antibodies based on a two-step procedure. Firstly, monoclonal antibodies are selected only if displaying the following binding features: binding both the substrate and a reactive group in such a way that the two groups are in a reactive position towards each other. Secondly, the selected monoclonal antibodies (mAbs) are chemically engineered by covalently binding the reactive group into the binding pocket of the antibody. Using previously isolated monoclonal antibodies, we have focused our studies on the control of this second step.

A modular assembly strategy for improving the substrate specificity of small catalytic peptides

In contrast to large proteins, small peptide catalysts typically display limited specificity for small molecule substrates. This is presumably a result of the limited opportunities small peptides have to fold in a manner that provides for the formation of an isolated reaction vessel that effectively binds and sequesters substrates from bulk solvent while at the same time catalyzing their transformation. For the preparation of small peptide catalysts that possess improved substrate specificity, we have developed a modular assembly strategy that involves appending phage display-derived substrate binding-domain modules to catalytically active peptide domains. We demonstrate the potential of this strategy with the construction of a small 35-amino acid residue aldolase peptide with improved substrate specificity. The advantages of this approach are that it reduces the demand on the functionalization of the catalytic site and it is modular, therefore making its adaptation to a variety of specificities rapid. The modular assembly strategy studied here may present advantages over exhaustive searches of large random-sequence peptide libraries for peptides with singular function.

Coordination chemistry of iron(III)-porphyrin-antibody complexes

An artificial peroxidase-like hemoprotein has been obtained by associating a monoclonal antibody, 13G10, and its iron(III)-alpha,alpha,alpha,beta-meso-tetrakis(ortho-carboxyphenyl)porphyrin [Fe(ToCPP)] hapten. In this antibody, about two-thirds of the porphyrin moiety is inserted in the binding site, its ortho-COOH substituents being recognized by amino-acids of the protein, and a carboxylic acid side chain of the protein acts as a general acid base catalyst in the heterolytic cleavage of the O-O bond of H2O2, but no amino-acid residue is acting as an axial ligand of the iron. We here show that the iron of 13G10-Fe(ToCPP) is able to bind, like that of free Fe(ToCPP), two small ligands such as CN-, but only one imidazole ligand, in contrast to to the iron(III) of Fe(ToCPP) that binds two. This phenomenon is general for a series of monosubstituted imidazoles, the 2- and 4-alkyl-substituted imidazoles being the best ligands, in agreement with the hydrophobic character of the antibody binding site. Complexes of antibody 13G10 with less hindered iron(III)-tetraarylporphyrins bearing only one [Fe(MoCPP)] or two meso-[ortho-carboxyphenyl] substituents [Fe(DoCPP)] also bind only one imidazole. Finally, peroxidase activity studies show that imidazole inhibits the peroxidase activity of 13G10-Fe(ToCPP) whereas it increases that of 13G10-Fe(DoCPP). This could be interpreted by the binding of the imidazole ligand on the iron atom which probably occurs in the case of 13G10-Fe(ToCPP) on the less hindered face of the porphyrin, close to the catalytic COOH residue, whereas in the case of 13G10-Fe(DoCPP) it can occur on the other face of the porphyrin. The 13G10-Fe(DoCPP)-imidazole complex thus constitutes a nice artificial peroxidase-like hemoprotein, with the axial imidazole ligand of the iron mimicking the proximal histidine of peroxidases and a COOH side chain of the antibody acting as a general acid-base catalyst like the distal histidine of peroxidases does.

Pivalase catalytic antibodies: towards abzymatic activation of prodrugs

Screening of monoclonal-antibody libraries generated against the tert-butyl phosphonate hapten 2 and the chloromethyl phosphonate hapten 3 with pivaloyloxymethyl-umbelliferone 1 as a fluorogenic substrate led to the isolation of eleven catalytic antibodies with rate accelerations around kcat/ kuncat = 10(3). The antibodies are not inhibited by the product and accept different acyloxymethyl derivatives of acidic phenols as substrates. The highest activity was found for the bulky, chemically less-reactive pivaloyloxymethyl group: there is no activity with acetoxymethyl or acetyl esters. This difference might reflect the preference of the immune system for hydrophobic interactions in binding and catalysis. Pivalase catalytic antibodies might be useful for activating orally available pivaloyloxymethyl prodrugs.

Canonical binding arrays as molecular recognition elements in the immune system: tetrahedral anions and the ester hydrolysis transition state

The structures, obtained by X-ray crystallography, of the binding sites of catalytic antibodies raised to bind different phosphonates are compared. Although the amino acid sequences differ, all exhibit a tetrahedral array of hydrogen bond donors (a 'canonical binding array') complementary to the tetrahedral anion, which represents a 'transition state epitope' for the basic hydrolysis of esters and amides. Antibodies for phosphates, arsonates, and sulfonates are found also to possess the tetrahedral anion canonical binding array.

Catalytic Antibody Route to the Naturally Occurring Epothilones: Total Synthesis of Epothilones A-F

Naturally occurring epothilones have been synthesized starting from enantiomerically pure aldol compounds 9-11, which were obtained by antibody catalysis. Aldolase antibody 38C2 catalyzed the resolution of (+/-)-9 by enantioselective retro-aldol reaction to afford 9 in 90% ee at 50 % conversion. Compounds 10 and 11 were obtained in more than 99% ee at 50% conversion by resolution of their racemic mixtures using newly developed aldolase antibodies 84G3, 85H6 or 93F3. Compounds 9, 10 and 11 were resolved in multigram quantities and then converted to the epothilones by metathesis processes, which were catalyzed by Grubbs' catalysts.

Novel methods for biocatalyst screening

There have been a number of recent advances in catalysis assays applicable for screening biocatalyst libraries in high-throughput format. These include instrumental assays such as high-performance liquid chromatography, mass spectrometry, capillary electrophoresis and IR-thermography, reagent-based assays producing spectroscopic signals (UV/VIS or fluorescence) in response to reaction progress, and assays based on fluorogenic or chromogenic substrates. These fluorogenic substrates enable the assaying of a variety of enzymes in enantioselective and stereoselective manner, including alcohol dehydrogenases, aldolases, lipases, amidases, epoxide hydrolases and phosphatases.

Fluorogenic polypropionate fragments for detecting stereoselective aldolases

A series of fluorogenic polypropionate fragments has been prepared. These undergo retroaldolization to an intermediate aldehyde that liberates the fluorescent product umbelliferone by a secondary beta-elimination reaction. leading to a >20-fold increase in fluorescence (lambda(em) = 460 +/- 20 nm, lambdaex = 360 +/- 20 nm). By applying the principle of microscopic reversibility to the reversible aldol reaction, we can use these substrates to detect stereoselective aldolases. Test substrates are available to probe the classical cases of syn- and anti-selective aldolization (11a-d), Cram/ anti-Cram-selective aldolization (10a-d), and double stereoselective aldolization (3a-h). The selectivity of aldolase antibody 38C2 for these substrates is demonstrated as an example. The assay is suitable for high-throughput screening for catalysis in microtiter plates, and therefore provides a convenient tool for the isolation of new stereoselective aldolases from catalyst libraries.

Anticalins versus antibodies: made-to-order binding proteins for small molecules

Engineering proteins to bind small molecules presents a challenge as daunting as drug discovery, for both hinge upon our understanding of receptor-ligand molecular recognition. However, powerful techniques from combinatorial molecular biology can be used to rapidly select artificial receptors. While traditionally researchers have relied upon antibody technologies as a source of new binding proteins, the lipocalin scaffold has recently emerged as an adaptable receptor for small molecule binding. 'Anticalins', engineered lipocalin variants, offer some advantages over traditional antibody technology and illuminate features of molecular recognition between receptors and small molecule ligands.

Toward antibody-catalyzed hydrolysis of organophosphorus poisons

We report here our preliminary results on the use of catalytic antibodies as an approach to neutralizing organophosphorus chemical weapons. A first-generation hapten, methyl-alpha-hydroxyphosphinate Ha, was designed to mimic the approach of an incoming water molecule for the hydrolysis of exceedingly toxic methylphosphonothioate VX (1a). A moderate protective activity was first observed on polyclonal antibodies raised against Ha. The results were further confirmed by using a mAb PAR 15 raised against phenyl-alpha-hydroxyphosphinate Hb, which catalyzes the hydrolysis of PhX (1b), a less toxic phenylphosphonothioate analog of VX with a rate constant of 0.36 M(-1) x min(-1) at pH 7.4 and 25 degrees C, which corresponds to a catalytic proficiency of 14,400 M(-1) toward the rate constant for the uncatalyzed hydrolysis of 1b. This is a demonstration on the organophosphorus poisons themselves that mAbs can catalytically hydrolyze nerve agents, and a significant step toward the production of therapeutically active abzymes to treat poisoning by warfare agents.

Sets of aldolase antibodies with antipodal reactivities. Formal synthesis of epothilone E by large-scale antibody-catalyzed resolution of thiazole aldol

[formula: see text] Three monoclonal aldolase antibodies, generated against a beta-diketone hapten by reactive immunization, catalyzed rapid and highly enantioselective retro-aldol reactions of ent-8a-k, providing optically pure 8a-k by kinetic resolution. Compounds (+/-)-8a, (+/-)-8g, and (+/-)-8k have been resolved in multigram quantities using 0.003, 0.005, and 0.0004 mol% antibody catalysts, respectively. Resolved compounds 8a-k are useful synthons for the construction of epothilones A-E (2-6) and their analogues. Here, a formal synthesis of epothilone E, 6 has been achieved starting from compound 8g.