Catalytic Antibodies

Reflections on biocatalysis involving phosphorus

Early studies on chemical synthesis of biological molecules can be seen to progress to preparation and biological evaluation of phosphonates as analogues of biological phosphates, with emphasis on their isosteric and isopolar character. Work with such mimics progressed into structural studies with a range of nucleotide-utilising enzymes. The arrival of metal fluorides as analogues of the phosphoryl group, PO(3)(-), for transition state (TS) analysis of enzyme reactions stimulated the symbiotic deployment of (19)F NMR and protein crystallography. Characteristics of enzyme transition state analogues are reviewed for a range of reactions. From the available MF(x) species, trifluoroberyllate gives tetrahedral mimics of ground states (GS) in which phosphate is linked to carboxylate and phosphate oxyanions. Tetrafluoroaluminate is widely employed as a TS mimic, but it necessarily imposes octahedral geometry on the assembled complexes, whereas phosphoryl transfer involves trigonal bipyramidal (tbp) geometry. Trifluoromagnesate (MgF(3)(-)) provides the near-ideal solution, delivering tbp geometry and correct anionic charge. Some of the forty reported tbp structures assigned as having AlF(3)(0) cores have been redefined as trifluoromagnesate complexes. Transition state analogues for a range of kinases, mutases, and phosphatases provide a detailed description of mechanism for phosphoryl group transfer, supporting the concept of charge balance in their TS and of concerted-associative pathways for biocatalysis. Above all, superposition of GS and TS structures reveals that in associative phosphoryl transfer, the phosphorus atom migrates through a triangle of three, near-stationary, equatorial oxygens. The extension of these studies to near attack conformers further illuminates enzyme catalysis of phosphoryl transfer.

Synthesis and characterization of macrocyclic bisphosphonate dimers

Background: Attempts to optimize the synthetic yield of known macrocyclic phosphonates resulted in the discovery of two new macrocyclic bisphosphonate dimers. Methods: An attempt was carried out to optimize the yield of a known macrocyclic bisphosphonate dimer, 6, over the yield of monomers 2 and 3, using the Mitsunobu protocol in the macrocyclization step. Cyclization reactions were carried out at 0.003 M, 0.004 M, 0.005 M, 0.008 M and 0.02 M, compared to 0.002 M in our initial report of the synthesis of monomers. Results: In this attempt to optimize the production of dimer 6, two new macrocyclic diastereomers of 6, namely 28-membered bisphosphonates 9 and 10, were isolated in yields of 3% and 2%, respectively, and characterized by FTIR, LC-MS, 1 H , 1 3 C and 3 1 P NMR as well as by X-ray crystallography. Conclusions: The results described herein further illustrate the utility of the Mitsunobu macrocyclization (ring-closing) reaction toward the synthesis of macrocyclic phosphonates. The X-ray crystallographic characterization of the three bisphosphonate dimers, together with correlations to specific 1 H and 3 1 P NMR resonances allowed for the assignments of relative stereochemistries between the various dimers.

Antibody-catalyzed water-oxidation pathway

The intrinsic ability of all antibodies to generate hydrogen peroxide (H2O2) from singlet dioxygen (1O2*) via the antibody-catalyzed water-oxidation pathway (ACWOP) has triggered a rethink of the potential role of antibodies both in immune defense, inflammation, and disease. It has been shown that photochemical activation of this pathway is highly bactericidal. More recently, cholesterol oxidation by-products that may arise from the ACWOP have been discovered in vivo and are receiving a great deal of attention as possible key players in atherosclerosis and diseases of protein misfolding, such as Alzheimer's disease and Parkinson's disease.

A modelling study of a non-concerted hydrolytic cycloaddition reaction by the catalytic antibody H11

H11 is the first antibody reported to have dual activity as a non-concerted, Diels-Alderase and hydrolytic catalyst. It was previously shown to catalyse the cycloaddition of acetoxybutadiene 1a to N-alkyl maleimides 2 to afford hydroxy-substituted bicyclic adducts 3 with a 30% ee of a major isomer. To better understand this mechanism and the partial stereospecificity, a homology model of H11 was constructed and used in docking studies to evaluate potential antibody-ligand complexes. The model suggested the hydrolytic nature of H11 was due to Glu 95H acting as a catalytic base, and evaluation of the shape complementarity of the proposed antibody-ligand complexes confirmed at a semi-quantitative level the observation that the major enantiomer is produced in a 30% ee.

The elicitation of carboxylesterase activity in antibodies by reactive immunization with labile organophosphorus antigens: a role for flexibility

For the creation of powerfully catalytic antibodies, the technique of reactive immunization solves the problem inherent in immunization with transition-state analogs (TSAs), namely, that many interesting target reactions are multistep reactions, with multiple transition states, and thus in general, no single analog can adequately simulate all the transition states along the reaction path. In contrast, immunization with chemically reactive antigens such as phosphonylating agents, which phosphonylate B-cells during the immune response, produces antibodies that have been "trained" to recognize, bind, and stabilize all the actual transition states involved in the phosphonylation reaction. Therefore, catalytic antibodies have been selected by the immune system on the basis of their capacity to stabilize any number of transition states that occur during the target reaction. Somewhat surprisingly, phosphonolysis catalysts generated in this way commonly also catalyze esterolysis reactions. Esterolysis reactions should pass through transition states with a roughly tetrahedral disposition of ligands about a central carbon atom, while phosphonolysis reactions should pass through transition states with a roughly trigonal-bipyramidal disposition of ligands about a central phosphorus atom. These two divergent transition-state geometries suggest that the same active site should have difficulty recognizing and stabilizing both kinds of transition state. The observations thus indicate a puzzling form of "cross-reactivity" toward transition states. A possible explanation arises from evidence that at least some nucleophilic displacements at phosphorus do not pass through a trigonal-bipyramidal adduct, with a bond-formation transition state preceding it and a bond-fission transition state succeeding it. Instead a single transition state is traversed in which both bond-formation and bond-fission occur simultaneously. Such a concerted-reaction transition state should have two weak, partial bonds to phosphorus, one for formation of the nucleophile-P bond and one for fission of the P-leaving group bond. In a stepwise reaction through an intermediate, only one bond is partial and weak in each of the two transition states. The concerted-reaction transition state, with two weak bonds to phosphorus, may be more easily compressed, expanded, and otherwise distorted because of the lower force constants associated with partial bonds; particularly distortions of angles involving the two partial bonds should require relatively low energies. This may lend a high level of flexibility to phosphonolysis transition states, allowing them to be accommodated within an active site (or a range of active sites) with strong catalytic stabilization. Included among these active sites may be a majority that can also stabilize esterolysis transition states. Indeed many of the target esterolysis reactions studied to date may occur through a single concerted-reaction transition state rather than through separate transition states before and after a tetrahedral intermediate. Thus, the esterolysis transition states may also be highly flexible. Finally, flexibility present in germline antibodies may be specifically preserved in reactive immunization. The high flexibility of both kinds of ligands and of the antibody combining site may then account for the catalytic "cross-reactivity" of these antibodies.

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.

Structural diversity of antibody catalysts

The structural diversity of the immune response may be considerably restricted by the structure of the hapten used to elicit catalytic antibodies. The ligand-binding mode and the shapes of the binding pockets of hydrolytic antibodies induced to different transition-state analogs that contain an unsubstituted arylphosphonate group are very similar. Moreover, antibodies elicited against a single transition state analog evolve from a single germline gene or different precursors, depending on the nature of the hapten. Germline antibodies seem to adopt multiple conformations with antigen binding, together with somatic mutation stabilizing the conformation with optimum complementarity to antigen.

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.

Novel reactions catalysed by antibodies

New structural data on nonhydrolytic antibody catalysts gained over the past two years confirm that antibodies elicited against transition-state analogues function by differential stabilisation of the transition-state over the ground state through electrostatic, van der Waals, cation-pi and hydrogen-bonding interactions. The lack of chemical catalysis correlates with the low catalytic efficiency. Novel strategies that precisely position a key functional residue in the antibody catalyst combining site have therefore emerged, as demonstrated by crystallographic studies. Whereas antibodies with a bulky residue at position H100c of hypervariable loop H3 adopt different cavity shapes, other antibodies share a common deep combining site. This structural restriction might reflect the use of similar hydrophobic haptens to generate the antibody; novel hapten design or new immunisation strategies may, in the future, lead to more structurally diversified active sites.