Crystal structures of hevamine, a plant defence protein with chitinase and lysozyme activity, and its complex with an inhibitor.

BACKGROUND: Hevamine is a member of one of several families of plant chitinases and lysozymes that are important for plant defence against pathogenic bacteria and fungi. The enzyme can hydrolyze the linear polysaccharide chains of chitin and peptidoglycan. A full understanding of the structure/function relationships of chitinases might facilitate the production of transgenic plants with increased resistance towards a wide range of pathogens. RESULTS: The crystal structure of hevamine has been determined to a resolution of 2.2 A, and refined to an R-factor of 0.169. The enzyme possesses a (beta alpha)8-barrel fold. An inhibitor binding study shows that the substrate-binding cleft is located at the carboxy-terminal end of the beta-barrel, near the conserved Glu127. Glu127 is in a position to act as the catalytic proton donor, but no residue that might stabilize a positively charged oxocarbonium ion intermediate was found. A likely mechanism of substrate hydrolysis is by direct attack of a water molecule on the C1 atom of the scissile bond, resulting in inversion of the configuration at C1. CONCLUSIONS: The structure of hevamine shows a completely new lysozyme/chitinase fold and represents a new class of polysaccharide-hydrolyzing (beta alpha)8-barrel enzymes. Because the residues conserved in the family to which hevamine belongs are important for maintaining the structure of the (beta alpha)8-barrel, all members of the family, including fungal, bacterial and insect chitinases, are likely to share this architecture. The crystal structure obtained provides a basis for protein engineering studies in this family of chitinases.

The 1.8 A resolution structure of hevamine, a plant chitinase/lysozyme, and analysis of the conserved sequence and structure motifs of glycosyl hydrolase family 18.

The three-dimensional structure of hevamine, a plant enzyme with chitinase and lysozyme activity, has been refined at 1.8 A resolution to an R-factor of 14.9% and a free R-factor of 19.6%. The final model consists of all 273 amino acid residues and 206 ordered water molecules. Two non-proline cis-peptides were identified, involving Phe32 and Trp255, both of which are implicated in substrate binding. Other glycosyl hydrolase family 18 proteins with known three-dimensional structure are bacterial chitinase A, endo-beta-N-acetylglucosaminidase F1, endo-beta-N-acetylglucosaminidase H, and the two plant proteins concanavalin B and narbonin, which have no known enzymatic activity. All these structures contain a (beta alpha)8 barrel fold, with the two family 18 consensus regions roughly corresponding to the third and fourth barrel strands. This confirms the grouping of these proteins into family 18, which was only based on weak and local sequence similarity. The substrate specificity of the enzymes is determined by the loops following the barrel strands that form the substrate binding site. All enzymes have an aspartic acid and a glutamic acid residue in positions identical with Asp 125 and the catalytic Glu127 of hevamine. The lack of chitinase activity of concanavalin B and narbonin can be explained by the absence of one of these carboxylate groups, and by differences in the loops that form the substrate-binding cleft in hevamine.

Crystal structure of concanavalin B at 1.65 A resolution. An "inactivated" chitinase from seeds of Canavalia ensiformis.

Seeds of Canavalia ensiformis (jack bean) contain besides large amounts of canavalin and concanavalin A, a protein with a molecular mass of 33,800 which has been named concanavalin B. Although concanavalin B shares about 40% sequence identity with plant chitinases belonging to glycosyl hydrolase family 18, no chitinase activity could be detected for this protein. To resolve this incongruity concanavalin B was crystallised and its three-dimensional structure determined at 1.65 A (1 A = 0.1 nm) resolution. The structure consists of a single domain with a (beta/alpha)8 topology. A 30 amino acid residue long loop occurs between the second beta-strand of the barrel and the second alpha-helix. This extended loop is unusual for the (beta/alpha)8 topology, but appears in a similar conformation in the structures of the seed protein narbonin and several chitinases as well. Two non-proline cis-peptide bonds are present in the structure of concanavalin B: Ser34-Phe, and Trp265-Asn. This structural feature is rarely observed in proteins, but could also be identified in the three-dimensional structures of family 18 chitinases and narbonin in coincident positions. In the chitinases the aromatic residues of the non-proline cis-peptides have been proposed to have a function in the binding of the substrate. The region in concanavalin B, where in chitinases the active site is located, shows two significant differences. First, the catalytic glutamic acid is a glutamine in concanavalin B. Second, although part of the substrate binding cleft of the chitinases is present in concanavalin B, it is much shorter. From this we conclude that concanavalin B and family 18 chitinases are closely related, but that concanavalin B has lost its enzymatic function. It still may act as a carbohydrate binding protein, however.

Studies on the active site of deacetoxycephalosporin C synthase.

The Fe(II) and 2-oxoglutarate-dependent dioxygenase deacetoxycephalosporin C synthase (DAOCS) from Streptomyces clavuligerus was expressed at ca 25 % of total soluble protein in Escherichia coli and purified by an efficient large-scale procedure. Purified protein catalysed the conversions of penicillins N and G to deacetoxycephems. Gel filtration and light scattering studies showed that in solution monomeric apo-DAOCS is in equilibrium with a trimeric form from which it crystallizes. DAOCS was crystallized +/-Fe(II) and/or 2-oxoglutarate using the hanging drop method. Crystals diffracted to beyond 1.3 A resolution and belonged to the R3 space group (unit cell dimensions: a=b=106.4 A, c=71.2 A; alpha=beta=90 degrees, gamma=120 degrees (in the hexagonal setting)). Despite the structure revealing that Met180 is located close to the reactive oxidizing centre of DAOCS, there was no functional difference between the wild-type and selenomethionine derivatives. X-ray absorption spectroscopic studies in solution generally supported the iron co-ordination chemistry defined by the crystal structures. The Fe K-edge positions of 7121.2 and 7121.4 eV for DAOCS alone and with 2-oxoglutarate were both consistent with the presence of Fe(II). For Fe(II) in DAOCS the best fit to the Extended X-ray Absorption Fine Structure (EXAFS) associated with the Fe K-edge was found with two His imidazolate groups at 1.96 A, three nitrogen or oxygen atoms at 2.11 A and one other light atom at 2.04 A. For the Fe(II) in the DAOCS-2-oxoglutarate complex the EXAFS spectrum was successfully interpreted by backscattering from two His residues (Fe-N at 1.99 A), a bidentate O,O-co-ordinated 2-oxoglutarate with Fe-O distances of 2.08 A, another O atom at 2.08 A and one at 2.03 A. Analysis of the X-ray crystal structural data suggests a binding mode for the penicillin N substrate and possible roles for the C terminus in stabilising the enzyme and ordering the reaction mechanism.

X-ray studies reveal lanthanide binding sites at the A/B5 interface of E. coli heat labile enterotoxin.

The crystal structure determination of heat labile enterotoxin (LT) bound to two different lanthanide ions, erbium and samarium, revealed two distinct ion binding sites in the interface of the A subunit and the B pentamer of the toxin. One of the interface sites is conserved in the very similar cholera toxin sequence. These sites may be potential calcium binding sites. Erbium and samarium binding causes a change in the structure of LT: a rotation of the A1 subunit of up to two degrees relative to the B pentamer.

Heat-labile enterotoxin crystal forms with variable A/B5 orientation. Analysis of conformational flexibility.

A new native crystal form of heat-labile enterotoxin (LT) has two AB5 complexes in the asymmetric unit with different orientations of the A subunit with respect to the B pentamer. Comparison with other crystal forms of LT shows that there is considerable conformational freedom for orientating the A subunit with respect to the B pentamer. The rotations of A in different crystal forms do not follow one specific axis, but most of them share a hinge point, close to the main interaction area between A and B5. Analysis of the two high-resolution structures available shows that these rotations cause very little change in the actual interactions between A and B5.

Ribonuclease A mutant His119 Asn: the role of histidine in catalysis.

Bovine pancreatic ribonuclease A (RNase A) has been widely used as a convenient model for structural and functional studies. The enzyme catalyzes cleavage of phosphodiester bonds in RNA and related substrates. Three amino acid residues located at the active site of RNase A (His12, His119, and Lys41) are known to be involved in catalysis. Mutation of His119 to asparagine was generated to study the role of His119 in RNase A catalysis. The mutant enzyme has been isolated and characterized. The mutation significantly decreases the rate of the transesterification reaction and has no effect on substrate affinity of the enzyme. An analysis of the enzymatic properties of H119N RNase A suggests that the imidazole ring of His119 of the wild-type enzyme must be protonated in an enzyme-substrate productive complex. Thus our results indicate that a contribution of protonated His119 into the catalysis is not restricted to protonation of oxygen atom of the substrate leaving group and that His119 participates directly in a transition state stabilization via hydrogen bonding.

Plant lysozymes.

Structural and functional features of plant lysozymes are reviewed. All lysozymes also have chitinase activity, but not all plant chitinases are also lysozymes. However, for many chitinases it is not yet known if they also possess lysozyme activity. Enzymes with lysozyme activity occur in different, structurally unrelated, families of chitinases. Plant chitinases with lysozyme activity are basic enzymes with high isoionic points. Their lysozyme activities have a shart pH optimum around pH 4.5-5.0, while they show chitinase activities in a much broader pH range. High lysozyme activities are observed at low ionic strength values (0.05). The X-ray structure of a lysozyme/chitinase from latex of the rubber tree, Hevea brasiliensis, is presented. This enzyme is also known under the name hevamine. It belongs to the family 18 or h-type chitinases (also called class III chitinases). The structure consists of an alpha/beta barrel fold, which has not been found in other chitinase or lysozyme structures. A glutamic acid residue may be catalytically active in the substrate-binding cleft of the enzyme. Other plant lysozymes are homologous with the family 19 or b-type chitinases (class I, II and IV). The X-ray structure of barley chitinase, a representative of this family with negligible lysozyme activity, has a similar folding as found in animal and phage lysozymes.

Multiple isomorphous replacement on merohedral twins: structure determination of deacetoxycephalosporin C synthase.

Merohedral twinning is a packing anomaly that seriously impairs the determination of macromolecular crystal structures. Crystals of deacetoxycephalosporin C synthase (DAOCS), an enzyme involved in the expansion of the penicillin nucleus to form the core structure of the cephalosporin antibiotics, were found to be merohedrally twinned by many diagnostic criteria. Here, the structure determination of DAOCS from twinned crystals based on a combination of isomorphous replacement and the use of a multiple-wavelength diffraction data set is described.

Stereochemistry of chitin hydrolysis by a plant chitinase/lysozyme and X-ray structure of a complex with allosamidin: evidence for substrate assisted catalysis.

The plant enzyme hevamine has both chitinase and lysozyme activity. HPLC analysis of the products of the hydrolysis of chitopentaose shows that hevamine acts with retention of the configuration, despite the absence of a nucleophilic or stabilizing carboxylate. To analyze the stabilization of a putative oxocarbonium ion intermediate, the X-ray structure of hevamine complexed with the inhibitor allosamidin was determined at 1.85 A resolution. This structure supports the role of Glu127 as a proton donor. The allosamizoline group binds in the center of the active site, mimicking a reaction intermediate in which a positive charge at C1 is stabilized intramolecularly by the carbonyl oxygen of the N-acetyl group at C2.

Localization and analysis of nonpolar regions in onconase.

A detailed analysis of the composition and properties of hydrophobic nuclei and microclusters has been carried out for onconase. Two main hydrophobic nuclei in the onconase structure were detected. Their composition and shape were found to be very similar to those of RNase A, in accordance with the predictions made. The nuclei in onconase are more compact, the side-chain atoms of residues included in the nuclei in onconase form more contacts with the environment than in RNase A. The hydrophobic nuclei should be considered as individual structural units along with elements of the secondary structure. Differences in composition and conformation of exposed loops between onconase and RNase A were found. The additional hydrophobic clusters attached to the nuclei in onconase might be involved in the fixation of an appropriate conformation of site(s) for manifestation of the biological activity of onconase. A comparison of amphibian representatives of the RNase A superfamily was also made. The results obtained suggest that the availability of nonpolar residues in established key positions of amino acid sequences determines the characteristic fold of homologous proteins and the structure of the active site cleft.