Mechanistic basis for suicide inactivation of porphobilinogen synthase by 4,7-dioxosebacic acid, an inhibitor that shows dramatic species selectivity.

4,7-Dioxosebacic acid (4,7-DOSA) is an active site-directed irreversible inhibitor of porphobilinogen synthase (PBGS). PBGS catalyzes the first common step in the biosynthesis of the tetrapyrrole cofactors such as heme, vitamin B(12), and chlorophyll. 4,7-DOSA was designed as an analogue of a proposed reaction intermediate in the physiological PBGS-catalyzed condensation of two molecules of 5-aminolevulinic acid. As shown here, 4,7-DOSA exhibits time-dependent and dramatic species-specific inhibition of PBGS enzymes. IC(50) values vary from 1 microM to 2.4 mM for human, Escherichia coli, Bradyrhizobium japonicum, Pseudomonas aeruginosa, and pea enzymes. Those PBGS utilizing a catalytic Zn(2+) are more sensitive to 4,7-DOSA than those that do not. Weak inhibition of a human mutant PBGS establishes that the inactivation by 4,7-DOSA requires formation of a Schiff base to a lysine that normally forms a Schiff base intermediate to one substrate molecule. A 1.9 A resolution crystal structure of E. coli PBGS complexed with 4,7-DOSA (PDB code ) shows one dimer per asymmetric unit and reveals that the inhibitor forms two Schiff base linkages with each monomer, one to the normal Schiff base-forming Lys-246 and the other to a universally conserved "perturbing" Lys-194 (E. coli numbering). This is the first structure to show inhibitor binding at the second of two substrate-binding sites.

Structure and mechanism of activity of the cyclic phosphodiesterase of Appr>p, a product of the tRNA splicing reaction.

The crystal structure of the cyclic phosphodiesterase (CPDase) from Arabidopsis thaliana, an enzyme involved in the tRNA splicing pathway, was determined at 2.5 A resolution. CPDase hydrolyzes ADP-ribose 1",2"-cyclic phosphate (Appr>p), a product of the tRNA splicing reaction, to the monoester ADP-ribose 1"-phosphate (Appr-1"p). The 181 amino acid protein shows a novel, bilobal arrangement of two alphabeta modules. Each lobe consists of two alpha-helices on the outer side of the molecule, framing a three- or four-stranded antiparallel beta-sheet in the core of the protein. The active site is formed at the interface of the two beta-sheets in a water-filled cavity involving residues from two H-X-T/S-X motifs. This previously noticed motif participates in coordination of a sulfate ion. A solvent-exposed surface loop (residues 100-115) is very likely to play a flap-like role, opening and closing the active site. Based on the crystal structure and on recent mutagenesis studies of a homologous CPDase from Saccharomyces cerevisiae, we propose an enzymatic mechanism that employs the nucleophilic attack of a water molecule activated by one of the active site histidines.

Crystal structure of plant aspartic proteinase prophytepsin: inactivation and vacuolar targeting.

We determined at 2.3 A resolution the crystal structure of prophytepsin, a zymogen of a barley vacuolar aspartic proteinase. In addition to the classical pepsin-like bilobal main body of phytepsin, we also traced most of the propeptide, as well as an independent plant-specific domain, never before described in structural terms. The structure revealed that, in addition to the propeptide, 13 N-terminal residues of the mature phytepsin are essential for inactivation of the enzyme. Comparison of the plant-specific domain with NK-lysin indicates that these two saposin-like structures are closely related, suggesting that all saposins and saposin-like domains share a common topology. Structural analysis of prophytepsin led to the identification of a putative membrane receptor-binding site involved in Golgi-mediated transport to vacuoles.

Purification of receptor complexes of interleukin-10 stoichiometry and the importance of deglycosylation in their crystallization.

Interleukin-10 (IL-10) is a pleiotropic immunosuppressive cytokine that has a wide range of effects in controlling inflammatory responses. Viral IL-10 (vIL-10) is a homologue of human IL-10 (hIL-10) produced by Epstein-Barr virus (EBV). Both hIL-10 and vIL-10 bind to the soluble extracellular fragment of the cytokine receptor IL-10R1 (shIL-10R1). The stoichiometry of the vIL-10 : shIL-10R1 complex has been found to be the same as hIL-10 : shIL-10R1, with two vIL-10 dimers binding to four shIL-10R1 monomers. Complexes of both hIL-10 and vIL-10 with glycosylated shIL-10R1 could not be crystallized. Controlled deglycosylation using peptide : N-glycosidase F and endo-beta-N-acetylglucosaminidase F3 resulted in the formation of crystals of both hIL-10 : shIL-10R1 and vIL-10 : shIL-10R1 complexes, indicating that the difficulty in the crystal formation was largely due to the presence of complex carbohydrate side chains. The availability of the structure of the ligand-receptor complexes should facilitate our understanding of the basis of the interaction between IL-10 and the IL-10 receptor.

Toward a universal inhibitor of retroviral proteases: comparative analysis of the interactions of LP-130 complexed with proteases from HIV-1, FIV, and EIAV.

One of the major problems encountered in antiviral therapy against AIDS is the emergence of viral variants that exhibit drug resistance. The sequences of proteases (PRs) from related retroviruses sometimes include, at structurally equivalent positions, amino acids identical to those found in drug-resistant forms of HIV-1 PR. The statine-based inhibitor LP-130 was found to be a universal, nanomolar-range inhibitor against all tested retroviral PRs. We solved the crystal structures of LP-130 in complex with retroviral PRs from HIV-1, feline immunodeficiency virus, and equine infectious anemia virus and compared the structures to determine the differences in the interactions between the inhibitor and the active-site residues of the enzymes. This comparison shows an extraordinary similarity in the binding modes of the inhibitor molecules. The only exceptions are the different conformations of naphthylalanine side chains at the P3/P3' positions, which might be responsible for the variation in the Ki values. These findings indicate that successful inhibition of different retroviral PRs by LP-130 is achieved because this compound can be accommodated without serious conformational differences, despite the variations in the type of residues forming the active-site region. Although strong, specific interactions between the ligand and the enzyme might improve the potency of the inhibitor, the absence of such interactions seems to favor the universality of the compound. Hence, the ability of potential anti-AIDS drugs to inhibit multiple retroviral PRs might indicate their likelihood of not eliciting drug resistance. These studies may also contribute to the development of a small-animal model for preclinical testing of antiviral compounds.

Crystal structures of the inactive D30N mutant of feline immunodeficiency virus protease complexed with a substrate and an inhibitor.

Crystal structures of complexes of a D30N mutant of feline immunodeficiency virus protease (FIV PR) complexed with a statine-based inhibitor (LP-149), as well as with a substrate based on a modification of this inhibitor (LP-149S), have been solved and refined at resolutions of 2.0 and 1.85 A, respectively. Both the inhibitor and the substrate are bound in the active site of the mutant protease in a similar mode, which also resembles the mode of binding of LP-149 to the native protease. The carbonyl oxygen of the scissile bond in the substrate is not hydrated and is located within the distance of a hydrogen bond to an amido nitrogen atom from one of the two asparagines in the active site of the enzyme. The nitrogen atom of the scissile bond is 3.25 A from the conserved water molecule (Wat301). A model of a tetrahedral intermediate bound to the active site of the native enzyme was built by considering the interactions observed in all three crystal structures of FIV PR. Molecular dynamics simulations of this model bound to native wild-type FIV PR were carried out, to investigate the final stages of the catalytic mechanism of aspartic proteases.

Crystal structure of Epstein-Barr virus protein BCRF1, a homolog of cellular interleukin-10.

The crystal structure of Epstein-Barr virus protein BCRF1, an analog of cellular interleukin-10 (IL-10), has been determined at the resolution of 1.9 A and refined to an R-factor 0.191. The structure of this cytokine is similar to that of human IL-10 (hIL-10), forming an intercalated dimer of two 17 kDa polypeptides related by a crystallographic 2-fold symmetry axis. BCRF1 exhibits novel conformations of the N-terminal coil and of the loop between helices A and B compared to hIL-10. These regions are likely to be involved in binding of one or more components of the IL-10 receptor system, and thus the structural differences may account for the lower binding affinity and limited spectrum of biological activities of viral IL-10, compared to hIL-10.

Crystal structure of human interleukin-10 at 1.6 A resolution and a model of a complex with its soluble receptor.

The crystal structure of human interleukin-10 (IL-10) was refined at 1.6 A resolution against X-ray diffraction data collected at 100 K with the use of synchrotron radiation. Although similar to the IL-10 structure determined previously at room temperature, this low-temperature IL-10 structure contains, in addition, four N-terminal residues, three sulfate anions, and 175 extra water molecules. Whereas the main-chain conformation is preserved, about 30% of the side chains, most of them on the protein surface, assume different conformations. A computer model of a complex of IL-10 with its two soluble receptors was generated based on the topological similarity of IL-10 to interferon-gamma. The contact region between the cytokine and each receptor shows excellent complementarity of polar and hydrophobic interactions, suggesting that the model is generally correct and should be useful in guiding mutagenesis experiments.

Structure of equine infectious anemia virus proteinase complexed with an inhibitor.

Equine infectious anemia virus (EIAV), the causative agent of infectious anemia in horses, is a member of the lentiviral family. The virus-encoded proteinase (PR) processes viral polyproteins into functional molecules during replication and it also cleaves viral nucleocapsid protein during infection. The X-ray structure of a complex of the 154G mutant of EIAV PR with the inhibitor HBY-793 was solved at 1.8 A resolution and refined to a crystallographic R-factor of 0.136. The molecule is a dimer in which the monomers are related by a crystallographic twofold axis. Although both the enzyme and the inhibitor are symmetric, the interactions between the central part of the inhibitor and the active site aspartates are asymmetric, and the inhibitor and the two flaps are partially disordered. The overall fold of EIAV PR is very similar to that of other retroviral proteinases. However, a novel feature of the EIAV PR structure is the appearance of the second alpha-helix in the monomer in a position predicted by the structural template for the family of aspartic proteinases. The parts of the EIAV PR with the highest resemblance to human immunodeficiency virus type 1 PR include the substrate-binding sites; thus, the differences in the specificity of both enzymes have to be explained by enzyme-ligand interactions at the periphery of the active site as well.

Crystal structure of interleukin-10 reveals the functional dimer with an unexpected topological similarity to interferon gamma.

BACKGROUND: Interleukin (IL)-10 is a cytokine that inhibits production of other regulatory factors, including interferon gamma (IFN-gamma) and IL-2. A dimer of IL-10 is present in solution and is presumed to participate in receptor binding, but the nature of the dimer has not been previously reported. An atomic model is necessary to interpret biological activity of IL-10 and to design mutants with agonistic or antagonistic properties. RESULTS: The X-ray crystal structure of a recombinant form of human IL-10 has been solved at 1.8 A resolution and refined to a crystallographic R-factor of 0.156. The molecule is a tight dimer made of two interpenetrating subunits, forming a V-shaped structure. Each half of the structure consists of a six alpha-helices, four originating from one subunit and two from the other. Four of the helices form a classical 'up-up-down-down' bundle observed in all other helical cytokines. The overall topology of the helices bears close resemblance to IFN gamma, although the similarity is less striking when examined in greater detail. CONCLUSIONS: The topological similarity of IL-10 to IFN gamma was totally unexpected, and may be a reflection of the close relationship between the biological effects of these two cytokines. The structure of IL-10 provides insights into the possible modes of conversion of the dimer into monomers, and of putative sites of receptor interactions. The good level of refinement and high resolution of this structure show that the internal disorder often associated with other helical cytokines is not an essential feature of this class of proteins.

Structure of an inhibitor complex of the proteinase from feline immunodeficiency virus.

The crystal structure of a recombinant form of the proteinase encoded by the feline immunodeficiency virus (FIV PR) has been solved at 2 A resolution and refined to an R-factor of 0.148. The refined structure includes a peptidomimetic, statine-based inhibitor, LP-149, which is an even more potent inhibitor of HIV PR. Kinetic parameters were obtained for the cleavage of five substrates by FIV PR, and inhibition constants were measured for four inhibitors. The structure of FIV PR resembles other related retroviral enzymes although few inhibitors of HIV PR are capable of inhibiting FIV PR. The structure of FIV PR will enhance our knowledge of this class of enzymes, and will direct testing of new proteinase inhibitors in a feline animal model.

A model of the complex between interleukin-4 and its receptors.

A three-dimensional model of interleukin-4 (IL-4) bound to one molecule each of the high- and low-affinity receptors (IL-4R and IL-2R gamma) was built, using the crystal structure of the complex of human growth hormone (HGH) with its receptor (HGHR) as a starting model. The modeling of IL-4 with its receptors was based on the conservation of the sequences and on the predicted structural organization for cytokine receptors, and assuming that the binding mode of the ligands would be similar. Analysis of the interface between IL-4 and both receptor molecules was carried out to reveal which residues are important for complex formation. The modeling procedures showed that there were no major problems in maintaining a reasonable fit of IL-4 with the two receptor molecules, in a manner analogous to the complex of HGH-HGHR. Many of the residues that appear by modeling to be important for binding between IL-4 and the receptors have been previously implicated in that role by different methods. A striking motif of aromatic and positively charged residues on the surface of the C-terminal domains of the receptors is highly conserved in the structure of HGH-HGHR and in the models of IL-4 complexed with its receptors.

Structure of a single-chain antibody variable domain (Fv) fragment complexed with a carbohydrate antigen at 1.7-A resolution.

We describe here the 1.7-A resolution structure of a single-chain antibody variable domain (scFv) molecule, based on the carbohydrate-binding antibody Se155-4, complexed with the trisaccharide ligand alpha-D-Gal(1-->2)[alpha-D-Abe(1-->3)]alpha-D-Manp1-->OMe, where Abe is abequose. The scFv expressed in Escherichia coli has the variable region light chain to heavy chain polarity with the domains connected by a 19-residue linker. Although the linker is partially disordered in the crystal, the packing of the molecules suggests a monomeric state of the scFv. The carbohydrate adopts a different conformation about the Man-Gal linkage than was observed previously in the Fab-trisaccharide complex. Instead of a direct hydrogen bond between O2Abe and O2Gal, these two atoms are bridged by a water molecule in the present complex.

Stoichiometry of the complex of human interleukin-4 with its receptor.

A large number of cytokines have been shown to possess a four-helix bundle structure with a unique up-up-down-down connectivity. The receptors for this family of cytokines have been shown to be homologous as well, each possessing two tandem repeats of a fibronectin type III-like domain. The crystal structure of human growth hormone bound to the soluble portion of its receptor has served as the only experimentally-determined example of the interaction between the four-helix bundle cytokines and their receptors: two identical receptor subunits bind to different epitopes on the same growth hormone ligand. We have conducted a series of experiments to determine if this structural paradigm is true for interleukin-4 and interleukin-4 receptor. Native polyacrylamide gel electrophoresis and gel filtration chromatography reveal that interleukin-4 forms a tight 1:1 complex with the system.

Solution structure of a trisaccharide-antibody complex: comparison of NMR measurements with a crystal structure.

NMR and crystallography have been used to study antigen conformational changes that occur in a trisaccharide-Fab complex in solution and in the solid state. NOE buildup rates from transferred NOE experiments show that the antigenic determinant of a Salmonella lipopolysaccharide, represented by the trisaccharide methyl glycoside alpha-D-Galp(1-->2 [alpha-D-Abep(1-->3)]- alpha-D-Manp1-->OMe (1), undergoes a protein-induced conformational shift about the Gal-->Man glycosidic linkage when it is bound by a monoclonal antibody in aqueous solution. The same trisaccharide was crystallized with Fab, and a solved structure at 2.1-A resolution revealed that the conformation of the trisaccharide ligand was similar to that seen in a dodesaccharide-Fab complex [Cygler et al. (1991) Science 253, 442-445), where the Gal-Man linkage also experienced a similar conformational shift. Distance constraints derived from the TRNOE buildup curves are consistent with two bound trisaccharide conformations, one of which correlates with the ligand conformation of the crystalline Fab-trisaccharide complex. In this bound conformation, short interatomic distances between Abe O-2 and Gal O-2 permit an oligosaccharide intramolecular hydrogen bond. Despite its relatively low energy, a preponderance of this conformer could not be detected in aqueous or DMSO solutions of free trisaccharide by either 1H or 13C NMR experiments. In DMSO, a different intramolecular hydrogen bond between Abe O-2 and Man O-4 was observed due to a solvent-induced shift in the conformational equilibria (relative to aqueous solution). Molecular modeling of the trisaccharide in the binding site and as the free ligand suggested that the protein imposes an induced fit on the antigen, primarily resulting in a shift of the Gal-Man phi torsional angle. This reduces the interproton separation between Abe H-3 and Gal H-1 with a marked increase in the intensity of the previously weak NOEs between the protons of the noncovalently linked galactose and abequose residues. The impact of the conformational shift on gross trisaccharide topology is sufficiently small that binding modes inferred from functional group replacements are not impaired.

Crystal structure of the complex of human alpha-thrombin and nonhydrolyzable bifunctional inhibitors, hirutonin-2 and hirutonin-6.

The crystal structure of the complexes of hirutonin-2 and hirutonin-6 with human alpha-thrombin have been solved and refined to R-factors of 0.169 (2.0 A resolution) and 0.162 (2.1 A), respectively. Hirutonins belong to a family of bifunctional inhibitors bearing a noncleavable moiety mimicking the scissile bond. Hirutonin-2 is an analog of (D)Phe-Pro-Arg-Gly-hirudin49-65; hirutonin-6 has the same N-terminal tripeptide connected to a shortened fibrinogen exosite-binding part by a short, nonpeptidyl linker. The hirutonin-6 molecule is well defined in the electron density with the exception of the C-terminal Leu-h61. The linker follows near the bottom of the canyon connecting the active site with the exosite, forms a short antiparallel beta-sheet-like arrangement with Leu40-Leu41 and makes van der Waals contacts with Glu39-Leu40-Leu41 of thrombin. In the thrombin-hirutonin-2 complex, the N- and C-terminal parts of the inhibitor are well ordered (except the C-terminal Gln-h65) while the central portion of the linker is partially disordered. The glycine analog in the P1' position of hirutonin-2 assumes a conformation similar to that of the canonical form (Bode and Huber (1992) Eur. J. Biochem. 204:433-451) and supports the identification of the S1' site as restricted by His57, Trp60D, Lys60F, and the Cys42-Cys58 disulfide bridge. The carbonyl oxygen of the P1 arginine residue is located in the oxyanion hole formed by the NH groups of Gly193 and Ser195, while the carbonyl carbon is positioned within a short distance, 2.8 A, from the O gamma of Ser195. This resembles the conformation of the substrate-like inhibitors bound to other serine proteases. The N-terminal (D)Phe-Pro-Arg fragment common to both inhibitors binds to thrombin in a fashion very similar to that of other inhibitors having this motif. The binding of the C-terminus of hirutonins to the fibrinogen-binding exosite is similar to that observed in hirudin and hirulog complexes.