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.

Structural evidence for a programmed general base in the active site of a catalytic antibody.

The crystal structure of the complex of a catalytic antibody with its cationic hapten at 1.9-A resolution demonstrates that the hapten amidinium group is stabilized through an ionic pair interaction with the carboxylate of a combining-site residue. The location of this carboxylate allows it to act as a general base in an allylic rearrangement. When compared with structures of other antibody complexes in which the positive moiety of the hapten is stabilized mostly by cation-pi interactions, this structure shows that the amidinium moiety is a useful candidate to elicit a carboxylate in an antibody combining site at a predetermined location with respect to the hapten. More generally, this structure highlights the advantage of a bidentate hapten for the programmed positioning of a chemically reactive residue in an antibody through charge complementarity to the hapten.

Structures of escherichia coli CMP kinase alone and in complex with CDP: a new fold of the nucleoside monophosphate binding domain and insights into cytosine nucleotide specificity.

BACKGROUND: . Nucleoside monophosphate kinases (NMP kinases) catalyze the reversible transfer of a phosphoryl group from a nucleoside triphosphate to a nucleoside monophosphate. Among them, cytidine monophosphate kinase from Escherichia coli has a striking particularity: it is specific for CMP, whereas in eukaryotes a unique UMP/CMP kinase phosphorylates both CMP and UMP with similar efficiency. RESULTS: . The crystal structure of the CMP kinase apoenzyme from E. coli was solved by single isomorphous replacement and refined at 1.75 A resolution. The structure of the enzyme in complex with CDP was determined at 2.0 A resolution. Like other NMP kinases, the protein contains a central parallel beta sheet, the strands of which are connected by alpha helices. The enzyme differs from other NMP kinases in the presence of a 40-residue insert situated in the NMP-binding (NMPbind) domain. This insert contains two domains: one comprising a three-stranded antiparallel beta sheet, the other comprising two alpha helices. CONCLUSIONS: . Two features of the CMP kinase from E. coli have no equivalent in other NMP kinases of known structure. Firstly, the large NMPbind insert undergoes a CDP-induced rearrangement: its beta-sheet domain moves away from the substrate, whereas its helical domain comes closer to it in a motion likely to improve the protection of the active site. Secondly, residues involved in CDP recognition are conserved in CMP kinases and have no counterpart in other NMP kinases. The structures presented here are the first of a new family of NMP kinases specific for CMP.

Mechanism of inactivation of a catalytic antibody by p-nitrophenyl esters.

Antibody CNJ206 catalyses the hydrolysis of p-nitrophenyl esters with significant rate enhancement; however, after a few cycles, 90% of the catalytic activity of CNJ206 is irreversibly lost. This report investigates the properties of the inactivated Fab (fragment antigen binding). After inactivation, the residual esterase activity of CNJ206 is similar to that of the catalytic antibody inhibited by the transition-state analogue (TSA) used to elicit it; the affinity of CNJ206 for the TSA is also dramatically lowered. Here we propose a simple scheme that accounts for the steady-state kinetics of inactivation. The following lines of evidence, when taken together, suggest that stable acylated tyrosine side chains within or close to the Fab combining site are involved in the inactivation process: isoelectric focusing and matrix-assisted-laser-desorption-ionisation-time-of-flight (MALDI-TOF) mass spectrometry show that incubation with substrate results in several acylated Fab species; inactivation is stable at pH 8, is reversed by mild hydroxylamine treatment and follows the same kinetics as inhibition of binding, which is slowed down by the presence of the TSA hapten. Analysis of the Fab-TSA X-ray structure shows that three tyrosine residues are potential candidates for the inactivation of CNJ206 by its substrates, Tyr L96 being the most likely one; this also suggests that site-directed mutation of one or more of these residues might prevent substrate inactivation and significantly improve catalysis.

Structural convergence in the active sites of a family of catalytic antibodies.

The x-ray structures of three esterase-like catalytic antibodies identified by screening for catalytic activity the entire hybridoma repertoire, elicited in response to a phosphonate transition state analog (TSA) hapten, were analyzed. The high resolution structures account for catalysis by transition state stabilization, and in all three antibodies a tyrosine residue participates in the oxyanion hole. Despite significant conformational differences in their combining sites, the three antibodies, which are the most efficient among those elicited, achieve catalysis in essentially the same mode, suggesting that evolution for binding to a single TSA followed by screening for catalysis lead to antibodies with structural convergence.

Similarities of hydrolytic antibodies revealed by their X-ray structures: a review.

Numerous antibodies have been programmed to catalyse the hydrolysis of esters as well as other acyl transfer reactions. They were raised against stable analogues that model the structure of the tetrahedral transition states of these reactions. The three-dimensional structures of four hydrolytic antibodies complexed to their respective phosphonate transition state analogues (TSAs) reveal a similar orientation of hapten relative to the antibody. Analysis of the four combining sites suggests that residues binding the phosphonate TSA stabilise the oxyanion intermediate of the reaction and play a preponderant role in catalysis. Comparison of catalytic antibodies selected from the same hybridoma fusion indicates a high similarity of the motifs that catalyse the hydrolysis of a given substrate.

Crystal structure of the complex of a catalytic antibody Fab fragment with a transition state analog: structural similarities in esterase-like catalytic antibodies.

The x-ray structure of the complex of a catalytic antibody Fab fragment with a phosphonate transition-state analog has been determined. The antibody (CNJ206) catalyzes the hydrolysis of p-nitrophenyl esters with significant rate enhancement and substrate specificity. Comparison of this structure with that of the uncomplexed Fab fragment suggests hapten-induced conformational changes: the shape of the combining site changes from a shallow groove in the uncomplexed Fab to a deep pocket where the hapten is buried. Three hydrogen-bond donors appear to stabilize the charged phosphonate group of the hapten: two NH groups of the heavy (H) chain complementarity-determining region 3 (H3 CDR) polypeptide chain and the side-chain of histidine-H35 in the H chain (His-H35) in the H1 CDR. The combining site shows striking structural similarities to that of antibody 17E8, which also has esterase activity. Both catalytic antibody ("abzyme") structures suggest that oxyanion stabilization plays a significant role in their rate acceleration. Additional catalytic groups that improve efficiency are not necessarily induced by the eliciting hapten; these groups may occur because of the variability in the combining sites of different monoclonal antibodies that bind to the same hapten.

Refined three-dimensional structure of the Fab fragment of a murine IgGl,lambda antibody.

We report the cDNA sequence determination and the crystal structure of the Fab fragment of a murine IgG1,lambda antibody (HC19), specific for an influenza virus hemagglutinin. The HC19 Fab-fragment structure has been refined; the crystallographic R-factor is 19.5% at 2.3 A resolution. We have compared the conformation of HC19 complementarity determining regions (CDRs) with those of CDR loops of Fab structures available from the Protein Data Bank. These loops were chosen based on the identity of key residues, following the canonical-structure approach; four CDRs have a main-chain conformation very similar to the canonical structure that had been identified. HC19 L1 CDR adopts a conformation clearly distinct from all L1 CDRs that belong to a chain of a different class or origin; this is determined by the nature of a few residues at positions in the sequence different from those of key residues in other light chains. This canonical structure should be representative of most murine lambda-class light chains, as inferred from the very high sequence homologies of these polypeptides.

Crystal structure of a catalytic antibody Fab with esterase-like activity.

BACKGROUND: Antibodies with catalytic properties can be prepared by eliciting an antibody response against 'transition state analog' haptens. The specificity, rate and number of reaction cycles observed with these antibodies more closely resemble the properties of enzymes than any of the many other known enzyme-mimicking systems. RESULTS: We have determined to 3 A resolution the first X-ray structure of a catalytic antibody Fab. This antibody catalyzes the hydrolysis of a p-nitrophenyl ester. In conjunction with binding studies in solution, this structure of the uncomplexed site suggests a model for transition state fixation where two tyrosines mimic the oxyanion binding hole of serine proteases. A comparison with the structures of known Fabs specific for low molecular weight haptens reveals that this catalytic antibody has an unusually long groove at its combining site. CONCLUSION: Since transition state analogs contain elements of the desired product, product inhibition is a severe problem in antibody catalysis. The observation of a long groove at the combining site may relate to the ability of this catalytic antibody to achieve multiple cycles of reaction.

Possible involvement of Lys603 from Escherichia coli glucosamine-6-phosphate synthase in the binding of its substrate fructose 6-phosphate.

Pyridoxal 5'-phosphate is a competitive inhibitor of glucosamine-6-phosphate synthase with respect to the substrate fructose 6-phosphate. Irreversible inactivation of pyridoxal-5'-phosphate-treated enzyme with [14C]-cyanide resulted in covalent incorporation of close to 1 mol pyridoxal 5'-phosphate/mol enzyme subunit. The enzyme-pyridoxal-5'-phosphate complex could also be inactivated by reduction with NaBH3CN. Sequence analysis of the unique radioactively labelled tryptic peptide, resulting from inactivation with [3H]NaBH3CN, identified the C-terminal nonapeptide encompassing the modified Lys603. The presence of fructose 6-phosphate protected this residue from pyridoxylation. Direct evidence that a lysine residue is involved in the binding of the substrate as a Schiff base came from the isolation at 4 degrees C of a enzyme-fructose-6-phosphate complex in a 1:1 molar ratio. Treatment of the enzyme-[14C]fructose-6-phosphate complex with NaBH3CN revealed one site of modification in the tryptic peptide map. In contrast, trapping the same complex with potassium cyanide resulted in the isolation of several radiolabelled peptides containing lysines which could potentially bind fructose 6-phosphate. However, since the radioactivity was not specifically associated with the lysine residues, it is suggested that these 14C-labelled peptides resulted from the decomposition of an unstable alpha,alpha'-dihydroxyaminonitrile adduct rather than from a lack of specificity of fructose 6-phosphate fixation. Lys603 is then the candidate of choice for fructose 6-phosphate binding since it lies at or near the active site as demonstrated by the trapping experiments with pyridoxal 5'-phosphate described above, and among the lysines which belong to the sugar-binding domain this is the only one conserved between the three members of the purF, glutamine-dependent, amidotransferase subfamily which include the glucosamine-6-phosphate synthase from Escherichia coli, Saccharomyces cerevisiae and the Rhizobium nodulation protein NodM.

Monofunctional chorismate mutase from Bacillus subtilis: purification of the protein, molecular cloning of the gene, and overexpression of the gene product in Escherichia coli.

The monofunctional chorismate mutase from Bacillus subtilis has been purified 2200-fold to homogeneity. The enzyme is a homodimer of subunit Mr = 14,500 and is the smallest natural chorismate mutase that has been characterized. The purified enzyme follows Michaelis-Menten kinetics with a Km of 100 microM and a kcat of 50 s-1, carries no other associated enzymic activities, and is unaffected by any of the aromatic amino acids. The N-terminal amino acid sequence of the protein has been determined, and this information has been used to construct a precise oligonucleotide probe for the gene by means of in vitro DNA amplification from total chromosomal DNA by the polymerase chain reaction. The cloned aroH gene encodes a protein of 127 amino acid residues and is expressed in Escherichia coli. The cloned gene product is indistinguishable from that purified from Bacillus. The aroH coding region was directly subcloned into a phagemid expression vector by means of the polymerase chain reaction. The resulting construct, with the aroH gene positioned behind efficient transcription and translation initiation sequences of E. coli, results in the production of the monofunctional mutase at levels of 30-35% of the soluble cell protein in E. coli transformants. Chorismate mutases comprise a set of functionally related proteins that show little sequence similarity to each other. This diversity stands in contrast to other chorismate-utilizing enzymes.