Spectroscopic Studies on the Designed Metal-Binding Sites of the 43C9 Single Chain Antibody

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.

Structural basis of an engineered dual-specific antibody: conformational diversity leads to a hypervariable loop metal-binding site

To explore dual-specificity in a small protein interface, we previously generated a 'metal switch' anti-RNase A VHH antibody using a combinatorial histidine library approach. While most metal-binding sites in proteins are found within rigid secondary structure, the engineered VHH antibody (VHH(metal)), which contained three new histidine residues, possessed metal-binding residues within the flexible hypervariable loops. Here, crystal structure analysis of the free and bound states of VHH(metal) reveals the structural determinants leading to dual-function. Most notably, CDR1 is observed in two distinct conformations when adopting the metal or RNase A bound states. Furthermore, mutagenesis studies revealed that one of the engineered residues, not located in the metal-binding pocket, contributed indirectly to metal recognition, likely through influencing CDR1 conformation. Despite these changes, VHH(metal) possesses a relatively minor energetic penalty toward binding the original antigen, RNase A (~1 kcal/mol), where the engineered gain-of-function metal-binding residues are observed to possess a mix of favorable and unfavorable contributions towards RNase A recognition. Ultimately, the conformationally distinct metal-switch interface architecture reflects the robust, library-based strategy used to produce VHH(metal). These results also suggest that even small protein interfaces, such as VHH, may be structurally and energetically forgiving in adopting novel function, while maintaining original function.

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.

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 cofactor approach to copper-dependent catalytic antibodies

A strategy for the preparation of semisynthetic copper(II)-based catalytic metalloproteins is described in which a metal-binding bis-imidazole cofactor is incorporated into the combining site of the aldolase antibody 38C2. Antibody 38C2 features a large hydrophobic-combining site pocket with a highly nucleophilic lysine residue, Lys(H93), that can be covalently modified. A comparison of several lactone and anhydride reagents shows that the latter are the most effective and general derivatizing agents for the 38C2 Lys residue. A bis-imidazole anhydride (5) was efficiently prepared from N-methyl imidazole. The 38C2-5-Cu conjugate was prepared by either (i) initial derivatization of 38C2 with 5 followed by metallation with CuCl2, or (ii) precoordination of 5 with CuCl2 followed by conjugation with 38C2. The resulting 38C2-5-Cu conjugate was an active catalyst for the hydrolysis of the coordinating picolinate ester 11, following Michaelis-Menten kinetics [kcat(11) = 2.3 min(-1) and Km(11) 2.2 mM] with a rate enhancement [kcat(11)k(uncat)(11)] of 2.1 x 10(5). Comparison of the second-order rate constants of the modified 38C2 and the Cu(II)-bis-imidazolyl complex k(6-CuCl2) gives a rate enhancement of 3.5 x 10(4) in favor of the antibody complex with an effective molarity of 76.7 M, revealing a significant catalytic benefit to the binding of the bis-imidazolyl ligand into 38C2.

Transition state docking: a probe for noncovalent catalysis in biological systems. Application to antibody-catalyzed ester hydrolysis

A strategy for pinpointing favorable noncovalent interactions between transition states and active sites of biological catalysts is described. This strategy combines high-level quantum mechanical calculations of transition state geometries with an automated docking procedure using AutoDock. By applying this methodology to antibody-catalyzed hydrolyses of aryl esters (by the 48G7, CNJ206, and 17E8 families of antibodies), varying levels of catalysis are explained in terms of specific hydrogen bonding interactions between combining site residues and transition states. Although these families of antibodies were produced in separate experiments by different researchers using related but different haptens, the mechanism of transition state stabilization appears to be highly conserved. Despite being elicited in response to anionic phosphonate haptens, the best catalysts often utilize hydrogen bond acceptors to stabilize transition states. A mutant of antibody CNJ206, designed based on this observation and predicted to be a better catalyst, is proposed. In the case of antibody 48G7, affinity maturation is shown to produce a catalyst that is highly selective for one of two enantiomeric transition states from a nonselective germline precursor.

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.

Pulsed EPR spectroscopy: biological applications

Pulsed electron paramagnetic resonance (EPR) methods such as ESEEM, PELDOR, relaxation time measurements, transient EPR, high-field/high-frequency EPR, and pulsed ENDOR, have been used successfully to investigate the local structure and dynamics of paramagnetic centers in biological samples. These methods allow different contributions to the EPR spectra to be distinguished and can help unravel complicated EPR spectra consisting of overlapping resonance lines, as are often found in disordered protein samples. The basic principles, specific potentials, technical requirements, and limitations of these advanced EPR techniques will be reviewed together with recent applications to metal centers, organic radicals, and spin labels in proteins.

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.

Use of a non-rigid region in T4 lysozyme to design an adaptable metal-binding site

It is not easy to find candidate sites within a given protein where the geometry of the polypeptide chain matches that of metal-binding sites in known protein structures. By choosing a location in T4 lysozyme that is inherently flexible, it was possible to engineer a two-histidine site that binds different divalent cations. Crystallographic analysis shows that the geometry of binding of zinc is distorted tetrahedral while that of cobalt and nickel is octahedral. Insofar as spectroscopic data can be measured, they indicate that similar modes of coordination are retained in solution. The two substitutions, Thr21 --> His and Thr142 --> His, lie, respectively, on the surface of the N- and C-terminal domains on opposite sides of the active site cleft. The design takes advantage of hinge-bending motion which allows the binding site to adapt to the most favorable ligand geometry for the metal. Introduction of the two histidines increases the melting temperature of the protein by 2.0 degrees C at pH 7.4. Metal binding further increases the melting temperature, but only by a small amount (up to 1.5 degrees C). A third substitution, Gln141 --> His, which could act as a third ligand in principle, does not do so, demonstrating the difficulty in mimicking naturally occurring metal-binding sites.

From genes to protein structure and function: novel applications of computational approaches in the genomic era

The genome-sequencing projects are providing a detailed 'parts list' of life. A key to comprehending this list is understanding the function of each gene and each protein at various levels. Sequence-based methods for function prediction are inadequate because of the multifunctional nature of proteins. However, just knowing the structure of the protein is also insufficient for prediction of multiple functional sites. Structural descriptors for protein functional sites are crucial for unlocking the secrets in both the sequence and structural-genomics projects.

Structural basis for amide hydrolysis catalyzed by the 43C9 antibody

Among catalytic antibodies, the well-characterized antibody 43C9 is unique in its ability to catalyze the difficult, but desirable, reaction of selective amide hydrolysis. The crystallographic structures that we present here for the single-chain variable fragment of the 43C9 antibody, both with and without the bound product p -nitrophenol, strongly support and extend the structural and mechanistic information previously provided by a three-dimensional computational model, together with extensive biochemical, kinetics, and mutagenesis results. The structures reveal an unexpected extended beta-sheet conformation of the third complementarity determining region of the heavy chain, which may be coupled to the novel indole ring orientation of the adjacent Trp H103. This unusual conformation creates an antigen-binding site that is significantly deeper than predicted in the computational model, with a hydrophobic pocket that encloses the p -nitrophenol product. Despite these differences, the previously proposed roles for Arg L96 in transition-state stabilization and for His L91 as the nucleophile that forms a covalent acyl-antibody intermediate are fully supported by the crystallographic structures. His L91 is now centered at the bottom of the antigen-binding site with the imidazole ring poised for nucleophilic attack. His L91, Arg L96, and the bound p -nitrophenol are linked into a hydrogen-bonding network by two well-ordered water molecules. These water molecules may mimic the positions of the phosphonamidate oxygen atoms of the antigen, which in turn mimic the transition state of the reaction. This network also contains His H35, suggesting that this residue may also stabilize the transition-states. A possible proton-transfer pathway from His L91 through two tyrosine residues may assist nucleophilic attack. Although transition-state stabilization is commonly observed in esterolytic antibodies, nucleophilic attack appears to be unique to 43C9 and accounts for the unusually high catalytic activity of this antibody.

Kinetic mechanism of metallo-beta-lactamase L1 from Stenotrophomonas maltophilia

The reaction of nitrocefin with metallo-beta-lactamase L1 from Stenotrophomonas maltophilia was studied using rapid-scan and stopped-flow ultraviolet-visible (UV-vis) studies in an effort to discern the kinetic mechanism used by L1 to hydrolyze penicillins and cephalosporins. Rapid-scan and stopped-flow UV-vis studies of nitrocefin hydrolysis by L1 identified three species: (1) the substrate (nitrocefin) displayed an absorbance peak at 390 nm (epsilon = 11 500 M(-1) cm(-1)) that decreased during the reaction with a rate constant of 170 +/- 30 s(-1); (2) the product (hydrolyzed nitrocefin) displayed an absorbance peak at 485 nm (epsilon = 17 420 M(-1) cm(-1)) that increased during the reaction with rate constant of 40 +/- 1 s(-1); and (3) an intermediate displayed an absorbance peak at 665 nm (epsilon = 32 000 M(-1) cm(-1)) that increased initially with a rate constant of 190 +/- 3 s(-1) and then decreased with a rate constant of 38 +/- 2 s(-1). Single-turnover experiments demonstrated that there were no pre-steady-state bursts in the reaction of L1 with nitrocefin; moreover, the progress curves could be fit to a kinetic mechanism that includes the formation of a transient intermediate by using KINSIM and the rate constants given above. Progress curves from experiments conducted at different reaction conditions or with a different substrate could also be fit to the proposed kinetic mechanism. The evidence for the presence of an intermediate along with kinetic simulations supports a hydrolytic mechanism for L1 that involves an intermediate whose breakdown is rate-determining.

Studies of the reactivity of artificial peroxidase-like hemoproteins based on antibodies elicited against a specifically designed ortho-carboxy substituted tetraarylporphyrin

The temperature and pH dependence as well as the selectivity of the peroxidase activity of a complex associating a monoclonal antibody 13G10 with its iron(III)-alpha,alpha,alpha,beta-mesotetrakis(ortho-carboxyphenyl) porphyrin (Fe(ToCPP)) hapten have been studied and compared to those of Fe(ToCPP) alone. It first appears that the peroxidase activity of the 13G10-Fe(ToCPP) complex is remarkably thermostable and remains about 5 times higher than that of Fe(ToCPP) alone until at least 80 degrees C. Secondly, this complex is able to use not only H2O2 as oxidant but also a wide range of hydroperoxides such as alkyl, aralkyl and fatty acid hydroperoxides and catalyze their reduction 2-6-fold faster than Fe(ToCPP) alone. It is also able to catalyze the oxidation by H202 of a variety of reducing cosubstrates such as 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), o-phenylenediamine (OPD), 3,3',5,5'-tetramethylbenzidine (TMB) and 3,3'-dimethoxybenzidine 3-8-fold faster than Fe(ToCPP) alone, the bicyclic aromatic ABTS and TMB being the best reducing cosubstrates. Finally, a pH dependence study, between pH 4.6 and 7.5, of the oxidation of ABTS by H2O2 in the presence of either 13G10-Fe(ToCPP) or Fe(ToCPP) shows that Km(H2O2) values vary very similarly for both catalysts, whereas very different variations are found for the k(cat) values. With Fe(ToCPP) as catalyst the k(cat) value remains constant around 100 min(-1) whereas with the 13G10-Fe(ToCPP) complex, it increases sharply below pH 5 to reach 540 min -1 at pH 4.6. This could be due to the participation of a carboxylic acid side chain of the antibody protein, as a general acid-base catalyst, to the heterolytic cleavage of the O-O bond of H2O2 leading to the highly reactive iron(V)-oxo intermediate in the peroxidase mechanism. Accordingly, the modification of the carboxylic acid residues of antibody 13G10 by glycinamide leads to a 50% decrease of the peroxidase activity of the 13G10-Fe(ToCPP) complex.

Hemoabzymes. Different strategies for obtaining artificial hemoproteins based on antibodies

Besides existing models of chemical or biotechnological origin for hemoproteins like peroxidases and cytochromes P450, catalytic antibodies (Abs) with a metalloporphyrin cofactor represent a promising alternative 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 Abs, raised against various free-base, N-substituted, Sn-, Pd-, or Fe-porphyrins. Four of them exhibited, in the presence of the corresponding Fe-porphyrin cofactor, a significant peroxidase activity, with kcat/K(m) values of 10(2) to 5 x 10(3)/M/s. This value remained low when compared to that of peroxidases, probably because neither a proximal ligand of the Fe, nor amino acid residues participating in the catalysis of the heterolytic cleavage of the O-O bond of H2O2, have been induced in those Abs. This strategy has been shown to be insufficient for the elaboration of effective models of cytochromes P450, because only one set of Abs, raised against meso-tetrakis(para-carboxyvinylphenyl)porphyrin, was reported to catalyze the nonstereoselective oxidation of styrene by iodosyl benzene using a Mn-porphyrin cofactor, and attempts to generate Abs having binding sites for both the substrate and the metalloporphyrin cofactor, using as a hapten a porphyrin covalently linked to the substrate, were not successful. A second strategy is then proposed, which involves the chemical labeling of antisubstrate Abs with a metalloporphyrin. As an example, preliminary results are presented on the covalent linkage of an Fe-porphyrin to an antiestradiol Ab, in order to obtain semisynthetic catalytic Abs able to catalyze the selective oxidation of steroids.

The de novo design of a rubredoxin-like Fe site

A redox center similar to that of rubredoxin was designed into the 56 amino acid immunoglobulin binding B1 domain of Streptococcals protein G. The redox center in rubredoxin contains an iron ion tetrahedrally coordinated by four cysteine residues, [Fe(S-Cys)4](-1),(-2). The design criteria for the target site included taking backbone movements into account, tetrahedral metal-binding, and maintaining the structure and stability of the wild-type protein. The optical absorption spectrum of the Co(II) complex of the metal-binding variant is characteristic of tetrahedral chelation by four cysteine residues. Circular dichroism and nuclear magnetic resonance measurements reveal that the metal-free and Cd(II)-bound forms of the variant are folded correctly and are stable. The Fe(III) complex of the metal-binding mutant reproduces the optical and the electron paramagnetic resonance spectra of oxidized rubredoxin. This demonstrates that the engineered protein chelates Fe(III) in a tetrahedral array, and the resulting center is similar to that of oxidized rubredoxin.

Engineering metal-binding sites in proteins

Metal-binding sites have been engineered into both de novo designed and naturally occurring proteins. Although the redesign of existing metal-binding sites in naturally occurring proteins still offers the most promise for a successful design, the more challenging goal of engineering metal-binding sites in de novo designed proteins and peptides is being achieved with increasing frequency. Creating new metal-binding sites in naturally occurring proteins combines the strength of both approaches. Currently, all three approaches are being used effectively in elucidating the structure and function of naturally occurring metalloproteins.

Artificial peroxidase-like hemoproteins based on antibodies constructed from a specifically designed ortho-carboxy substituted tetraarylporphyrin hapten and exhibiting a high affinity for iron-porphyrins

In order to get catalytic antibodies modelling peroxidases BALB/c mice have been immunized with iron(III)-alpha,alpha,alpha,beta-mesotetrakis-orthocarboxypheny l-porphyrin (Fe-(ToCPP))-KLH conjugates. Monoclonal antibodies have been produced by the hybridoma technology. Three antibodies, 2 IgG1 and 1 IgG2a, were found to bind both Fe(ToCPP) and the free base ToCPPH2 with similar binding constants. None of those antibodies was found to bind tetraphenylporphyrin. Those results suggest that the recognition of Fe(ToCPP) by the antibodies was mainly due to the binding of the carboxylate groups to some amino acid residues of the protein. True Kd values of 2.9 x 10(-9) M and 5.5 x 10(-9) M have been determined for the two IgG1-Fe(ToCPP) complexes. Those values are the best ones ever reported for iron-porphyrin-antibody complexes. UV-vis. studies have shown that the two IgG1-Fe(ToCPP) complexes were high-spin hexacoordinate iron(III) complexes, with no amino acid residue binding the iron, whereas the IgG2a-Fe(ToCPP) complex was a low-spin hexacoordinate iron(III) complex with two strong ligands binding the iron atom. Both IgG1-Fe(ToCPP) complexes were found to catalyze the oxidation of 2,2'-azinobis (3-ethylbenzothiazoline-6-sulfonic acid (ABTS) 5-fold more efficiently than Fe(ToCPP) alone whereas the binding of IgG2a to this iron-porphyrin had no effect on its catalytic activity. kcat values of 100 min(-1) and 63 min(-1) and kcat/Km values of 105 M(-1) s(-1) and 119 M(-1) s(-1) have been found respectively for the two IgG1-Fe(ToCPP) complexes.

Characterization of the metal-binding sites of the beta-lactamase from Bacteroides fragilis

In an effort to better understand the structure and function of the metallo-beta-lactamase from Bacteroides fragilis, spectroscopic and metal-binding studies were performed on the native, metal-substituted, and mutant forms of the enzyme. Atomic absorption studies demonstrate that the native B. fragilis enzyme tightly binds 2 mol of Zn(II) and, along with mutagenesis studies, that the presence of both metal ions is required for full catalytic activity. EPR spectroscopy was used to confirm that the Co(II)-substituted beta-lactamase binds 2 mol of Co(II) per mole of enzyme, that the two Co(II)'s are highspin and probably uncoupled, with apparent g values of 6.5, 4.2, and 2.0, and that the coordination number of the Co(II) is 5 or 6. This number of ligands for the Co(II)-substituted enzyme is confirmed by UV-Vis spectra, which demonstrate the presence of very weak d-d transitions between 550 and 650 nm (epsilon approximately 30 M-1.cm-1) and an intense feature at 320 nm (epsilon approximately 1570 M-1.cm-1). The latter is assigned to a cysteine sulfur to Co(II) ligand-to-metal charge transfer band, and this assignment is confirmed by the disappearance of this band in the UV-Vis spectrum of a Co(II)-substituted C168S mutant. H NMR studies on the Co(II)-substituted enzyme suggest the presence of three histidine ligands bound to Co(II). Taken together, these studies support the sequence comparison study of Rasmussen et al., in which there is a catalytic metal-binding site with three histidines and one cysteine (C168). The remaining ligands are postulated to be water molecules involved in catalysis. Mutagenesis studies, in combination with activity assays and metal-binding studies, have been used to identify Asp61, Asp90, Asp152, and Asp183 as possible ligands to the second metal-binding site, with Asp90 and Asp152 having a pronounced effect on kcat. These results are discussed in light of the recent crystal structure of the metallo-beta-lactamase from B. cereus.

Metalloprotein design

The rational design of novel proteins offers a new method of studying structure and function, and makes possible the construction of new biomaterials. The richness of metal chemistry, the relative ease of creating stable complexes, and the remarkable degree of subtle, highly specific control of reactivity imposed by the protein matrix upon the metal center make metalloprotein design a very fruitful area for the exploration and application of design techniques. So far, most designs have concentrated on the exploration of simple metal-chelation properties. Even so, this has led to the development of new methods for protein stabilization and affinity purification, of metal biosensors, of novel strategies for control of protein activity, and of model systems for the exploration of fundamental principles of molecular recognition.