X-Ray crystal structures of Candida albicans dihydrofolate reductase: high resolution ternary complexes in which the dihydronicotinamide moiety of NADPH is displaced by an inhibitor.

X-ray crystallographic analysis of 5-(4'-substituted phenyl)sulfanyl-2,4-diaminoquinazoline inhibitors in ternary complex with Candida albicans dihydrofolate reductase (DHFR) and NADPH revealed two distinct modes of binding. The two compounds with small 4'-substituents (H and CH3) were found to bind with the phenyl group oriented in the plane of the quinazoline ring system and positioned adjacent to the C-helix. In contrast, the more selective inhibitors with larger 4'-substituents (tert-butyl and N-morpholino) were bound to the enzyme with the phenyl group perpendicular to the quinazoline ring and positioned in the region of the active site that typically binds the dihydronicotinamide moiety of NADPH. The cofactor appeared bound to DHFR but with the disordered dihydronicotinamide swung away from the protein surface and into solution. This unusual inhibitor binding mode may play an important role in the high DHFR selectivity of these compounds and also may provide new ideas for inhibitor design.

Preparation, characterization, and the crystal structure of the inhibitor ZK-807834 (CI-1031) complexed with factor Xa.

Factor Xa plays a critical role in the formation of blood clots. This serine protease catalyzes the conversion of prothrombin to thrombin, the first joint step that links the intrinsic and extrinsic coagulation pathways. There is considerable interest in the development of factor Xa inhibitors for the intervention in thrombic diseases. This paper presents the structure of the inhibitor ZK-807834, also known as CI-1031, bound to factor Xa and provides the details of the protein purification and crystallization. Results from mass spectrometry indicate that the factor Xa underwent autolysis during crystallization and the first EGF-like domain was cleaved from the protein. The crystal structure of the complex shows that the amidine of ZK-807834 forms a salt bridge with Asp189 in the S1 pocket and the basic imidazoline fits snugly into the S4 site. The central pyridine ring provides a fairly rigid linker between these groups. This rigidity helps minimize entropic losses during binding. In addition, the structure reveals new interactions that were not found in the previous factor Xa/inhibitor complexes. ZK-807834 forms a strong hydrogen bond between an ionized 2-hydroxy group and Ser195 of factor Xa. There is also an aromatic ring-stacking interaction between the inhibitor and Trp215 in the S4 pocket. These interactions contribute to both the potency of this compound (K(I) = 0.11 nM) and the >2500-fold selectivity against homologous serine proteases such as trypsin.

Design, synthesis, and in vitro biological activity of benzimidazole based factor Xa inhibitors.

Inhibitors based on the benzimidazole scaffold showed subnanomolar potency against Factor Xa with 500-1000-fold selectivity against thrombin and 50-100-fold selectivity against trypsin. The 2-substituent on the benzimidazole ring had a strong impact on the FXa inhibitory activity. Crystallography studies suggest that the 2-substituent may have a conformational effect favoring the extended binding conformation.

Allosteric inhibitors of inducible nitric oxide synthase dimerization discovered via combinatorial chemistry.

Potent and selective inhibitors of inducible nitric oxide synthase (iNOS) (EC ) were identified in an encoded combinatorial chemical library that blocked human iNOS dimerization, and thereby NO production. In a cell-based iNOS assay (A-172 astrocytoma cells) the inhibitors had low-nanomolar IC(50) values and thus were >1,000-fold more potent than the substrate-based direct iNOS inhibitors 1400W and N-methyl-l-arginine. Biochemical studies confirmed that inhibitors caused accumulation of iNOS monomers in mouse macrophage RAW 264.7 cells. High affinity (K(d) approximately 3 nM) of inhibitors for isolated iNOS monomers was confirmed by using a radioligand binding assay. Inhibitors were >1,000-fold selective for iNOS versus endothelial NOS dimerization in a cell-based assay. The crystal structure of inhibitor bound to the monomeric iNOS oxygenase domain revealed inhibitor-heme coordination and substantial perturbation of the substrate binding site and the dimerization interface, indicating that this small molecule acts by allosterically disrupting protein-protein interactions at the dimer interface. These results provide a mechanism-based approach to highly selective iNOS inhibition. Inhibitors were active in vivo, with ED(50) values of <2 mg/kg in a rat model of endotoxin-induced systemic iNOS induction. Thus, this class of dimerization inhibitors has broad therapeutic potential in iNOS-mediated pathologies.

Prolonged circulating lives of single-chain Fv proteins conjugated with polyethylene glycol: a comparison of conjugation chemistries and compounds.

The utility of single-chain Fv proteins as therapeutic agents would be substantially broadened if the circulating lives of these minimal antigen-binding polypeptides were both prolonged and adjustable. Poly(ethylene glycol) (PEG) bioconjugate derivatives of the model single-chain Fv, CC49/218 sFv, were constructed using six different linker chemistries that selectively conjugate either primary amines or carboxylic acid groups. Activated PEG polymers with molecular weights of 2000, 5000, 10 000, 12 000, and 20 000 were included in the sFv bioconjugate evaluation. Additionally, the influence of PEG conjugate geometry in branched PEG strands (U-PEG) and the effect of multimeric PEG-sFv bioconjugates on circulating life and affinity were examined. Although random and extensive PEG polymer conjugations have been achievable in highly active derivatives of the prototypical PEG-enzymes, PEGylation of CC49/218 sFv required stringent adjustment of reaction conditions in order to preserve antigen-binding affinity as measured in either mucin-specific or whole cell immunoassays. Purified bioconjugates with PEG:sFv ratios of 1:1 through 2:1 were identified as promising candidates which exhibit sFv affinity (K(d)) values within 2-fold of the unmodified sFv protein. Interestingly, PEG conjugation to carboxylic acid moieties, using a PEG-hydrazide chemistry, achieved significant activity retention in bioconjugates at a higher PEG:sFv ratio (5:1) than with any of the amine-reactive activated PEG polymers. Prolonged circulating life in mice was demonstrated for each of the PEG conjugates. An increase in PEG polymer length was found to be more effective for serum half-life extension than a corresponding increase in total PEG mass. For example, CC49/218 sFv conjugated to either one strand of PEG-20000, or four strands of PEG-5000, displayed about 20- or 14-fold increased serum half-life, respectively, relative to the unmodified sFv. The demonstrated suitability of established random conjugation chemistries for PEGylation of sFv proteins, in conjunction with innovative site-specific conjugation methods, indicates that production of a panoply of sFv proteins with both engineered affinity and tailored circulating life may now be achievable.

Conformations of trypsin-bound amidine inhibitors of blood coagulant factor Xa by double REDOR NMR and MD simulations.

Double rotational-echo double resonance (double REDOR) has been used to investigate the bound conformations of (13)C,(15)N,(19)F-labeled factor Xa inhibitors to bovine trypsin. Carbon-fluorine dipolar couplings were measured by (13)C{(19)F} REDOR with natural-abundance background interferences removed by (13)C{(15)N} REDOR. The conformations of the bound inhibitors were characterized by molecular dynamics (MD) simulations of binding restrained by double REDOR-determined intramolecular C-F distances. A symmetrical bisamidine inhibitor and an asymmetrical monoamidine-monoamine inhibitor of the same general shape had distinctly different conformations in the bound state. According to the MD models, these differences arise from specific interactions of the amidine and amine groups with the active-site residues of trypsin and nearby water molecules.

Crystallographic analysis of potent and selective factor Xa inhibitors complexed to bovine trypsin.

Factor Xa is a serine protease which activates thrombin (factor IIa) and plays a key regulatory role in the blood-coagulation cascade. Factor Xa is, therefore, an important target for the design of anti-thrombotics. Both factor Xa and thrombin share sequence and structural homology with trypsin. As part of a factor Xa inhibitor-design program, a number of factor Xa inhibitors were crystallographically studied complexed to bovine trypsin. The structures of one diaryl benzimidazole, one diaryl carbazole and three diaryloxypyridines are described. All five compounds bind to trypsin in an extended conformation, with an amidinoaryl group in the S1 pocket and a second basic/hydrophobic moiety bound in the S4 pocket. These binding modes all bear a resemblance to the reported binding mode of DX-9065a in bovine trypsin and human factor Xa.

Crystal structures of zinc-free and -bound heme domain of human inducible nitric-oxide synthase. Implications for dimer stability and comparison with endothelial nitric-oxide synthase.

The crystal structures of the heme domain of human inducible nitric-oxide synthase (NOS-2) in zinc-free and -bound states have been solved. In the zinc-free structure, two symmetry-related cysteine residues form a disulfide bond. In the zinc-bound state, these same two cysteine residues form part of a zinc-tetrathiolate (ZnS(4)) center indistinguishable from that observed in the endothelial isoform (NOS-3). As in NOS-3, ZnS(4) plays a key role in stabilizing intersubunit contacts and in maintaining the integrity of the cofactor (tetrahydrobiopterin) binding site of NOS-2. A comparison of NOS-2 and NOS-3 structures illustrates the conservation of quaternary structure, tertiary topology, and substrate and cofactor binding sites, in addition to providing insights on isoform-specific inhibitor design. The structural comparison also reveals that pterin binding does not preferentially stabilize the dimer interface of NOS-2 over NOS-3.

Protein fold analysis of the B30.2-like domain.

The B30.2-like domain occurs in some members of a diverse and growing family of proteins containing zinc-binding B-box motifs, whose functions include regulation of cell growth and differentiation. The B30.2-like domain is also found in proteins without the zinc-binding motifs, such as butyrophilin (a transmembrane glycoprotein) and stonustoxin (a secreted cytolytic toxin). Currently, the function for the B30.2-like domain is not clear and the structure of a protein containing this domain has not been solved. The secondary structure prediction methods indicate that the B30.2-like domain consists of fifteen or fewer beta-strands. Fold recognition methods identified different structural topologies for the B30.2-like domains. Secondary structure prediction, deletion and lack of local sequence identity at the C-terminal region for certain members of the family, and packing of known core structures suggest that a structure containing two beta domains is the most probable of these folds. The most C-terminal sequence motif predicted to be a beta-strand in all B30.2-like domains is a potential subdomain boundary based on the sequence-structure alignments. Models of the B30.2-like domains were built based on immunoglobulin-like folds identified by the fold recognition methods to evaluate the possibility of the B30.2 domain adopting known folds and infer putative functional sites. The SPRY domain has been identified as a subdomain within the B30.2-like domain. If the B30.2-like domain is a subclass of the SPRY domain family, then this analysis would suggest that the SPRY domains are members of the immunoglobulin superfamily.

Design, synthesis, and activity of 2,6-diphenoxypyridine-derived factor Xa inhibitors.

A novel series of 2,6-diphenoxypyridines has been designed to inhibit factor Xa, a serine protease strategically located in the coagulation cascade. The evolution from the photochemically unstable bisamidine (Z,Z)-BABCH to potent bisamidine compounds with a pyridine heterocycle as the core scaffold has been achieved. The most potent compound in the series, 6h, has a Ki for human factor Xa of 12 nM. The selectivity of 6h against bovine trypsin and human thrombin was greater than 90- and 1000-fold, respectively. Two proposed modes of binding of 6h to factor Xa are made based on the crystal structures of 6h by itself and of 6h bound to bovine trypsin.

Interaction between terminal complement proteins C5b-7 and anionic phospholipids.

We have recently shown that C5b-6 binds to the erythrocyte membrane via an ionic interaction with sialic acid before the addition of C7 and subsequent membrane insertion. In this study we assessed the role of anionic lipids in the binding of the terminal complement proteins to the membrane and the efficiency of subsequent hemolysis. Human erythrocytes were modified by insertion of dipalmitoyl phosphatidylcholine (DPPC), dipalmitoyl phosphatidylserine (DPPS), dipalmitoyl phosphatidylethanolamine (DPPE), or dipalmitoyl phosphatidic acid (DPPA). Lipid incorporation and the hemolytic assays were done in the presence of 100 micromol/L sodium orthovanadate to prevent enzymatic redistribution of lipid. We found that the neutral lipids, DPPC and DPPE, did not affect C5b-7 uptake or hemolysis by C5b-9. In contrast, the two acidic phospholipids, DPPS and DPPA, caused a dose-dependent increase in both lysis and C5b-7 uptake. We conclude that the presence of anionic lipids on the exterior face of the membrane increases C5b-7 uptake and subsequent hemolysis. It is known that sickle cell erythrocytes have increased exposure of phosphatidylserine on their external face and are abnormally sensitive to lysis by C5b-9. The data presented here provide a plausible mechanism for this increased sensitivity.

Synthesis, characterization, and structure-activity relationships of amidine-substituted (bis)benzylidene-cycloketone olefin isomers as potent and selective factor Xa inhibitors.

Factor Xa (FXa) is a trypsin-like serine protease that plays a key role in blood coagulation linking the intrinsic and extrinsic pathways to the final common pathway of the coagulation cascade. During our initial studies, we observed facile photochemical conversion of the known FXa/tPA inhibitor, BABCH ¿(E,E)-2, 7-bis(4-amidinobenzylidene)cycloheptan-1-one, 1a, to the corresponding (Z,Z) olefin isomer, 1c (FXa K(i) = 0.66 nM), which was over 25,000 times more potent than the corresponding (E,E) isomer (1a, FXa K(i) = 17 000 nM). In order to determine the scope of this observation, we expanded on our initial investigation through the preparation of the olefin isomers in a homologous series of cycloalkanone rings, 4-substituted cyclohexanone analogues, and modified amidine derivatives. In most cases the order of potency of the olefin isomers was (Z,Z) > (E,Z) > (E,E) with the cycloheptanone analogue (1c) showing the most potent factor Xa inhibitory activity. In addition, we found that selectivity versus thrombin (FIIa) can be dramatically improved by the addition of a carboxylic acid group to the cycloalkanone ring as seen with 8c (FXa K(i) = 6.9 nM, FIIa K(i) > 50,000 nM). Compounds with one or both of the amidine groups substituted with N-alkyl substituents or replaced with amide groups led to a significant loss of activity. In this report we have demonstrated the importance of the two amidine groups, the cycloheptanone ring, and the (Z,Z) olefin configuration for maximum inhibition of FXa within the BABCH template. The results from this study provided the foundation for the discovery of potent, selective, and orally active FXa inhibitors.

Single-chain Fv with manifold N-glycans as bifunctional scaffolds for immunomolecules.

Unlike natural antibodies, single-chain Fv (sFv) proteins normally lack asparagine-linked glycosylation. Many designed immunoconjugates and other therapeutics currently employ the advantageous conjugation chemistry or targeting properties provided by the glycoprotein oligosaccharide domain. sFv proteins with engineered N-glycan designs were evaluated in Pichia pastoris for glycosylation efficiency, expression level, oligosaccharide chain length and composition, and affinity. In contrast to nearly all natural glycoproteins, the engineered attachment of N-glycans conveniently near the polypeptide C-terminus was found to produce the optimal results. Furthermore, the percentage modification and chain length of the attached mannose chains were controllable by the use of tandem and overlapping Asn-X-Thr tripeptide sites. The glycosylated sFv mannose chains could be effectively conjugated to polyethylene glycol and the resulting conjugate displayed a 10-fold increased circulating life in mice. The potential to control polymer:sFv or drug:sFv molar ratios by site-specific conjugation may substantially improve the therapeutic efficacy of these minimal antigen-binding molecules.

X-ray crystallographic studies of Candida albicans dihydrofolate reductase. High resolution structures of the holoenzyme and an inhibited ternary complex.

The recent rise in systemic fungal infections has created a need for the development of new antifungal agents. As part of an effort to provide therapeutically effective inhibitors of fungal dihydrofolate reductase (DHFR), we have cloned, expressed, purified, crystallized, and determined the three-dimensional structure of Candida albicans DHFR. The 192-residue enzyme, which was expressed in Escherichia coli and purified by methotrexate affinity and cation exchange chromatography, was 27% identical to human DHFR. Crystals of C. albicans DHFR were grown as the holoenzyme complex and as a ternary complex containing a pyrroloquinazoline inhibitor. Both complexes crystallized with two molecules in the asymmetric unit in space group P21. The final structures had R-factors of 0.199 at 1.85-A resolution and 0.155 at 1.60-A resolution, respectively. The enzyme fold was similar to that of bacterial and vertebrate DHFR, and the binding of a nonselective diaminopyrroloquinazoline inhibitor and the interactions of NADPH with protein were typical of ligand binding to other DHFRs. However, the width of the active site cleft of C. albicans DHFR was significantly larger than that of the human enzyme, providing a basis for the design of potentially selective inhibitors.

Single chain antigen binding protein (sFv CC49): first human studies in colorectal carcinoma metastatic to liver.

BACKGROUND: An sFv fragment of the anti-TAG-72 monoclonal antibody CC49 has been developed and has shown promise in improved targeting to colorectal carcinoma in animal studies. In this study the authors report their initial experience in human patients after intravenous injection. METHODS: Five patients with colorectal carcinoma metastatic to the liver were studied prior to surgery. High performance liquid chromatography showed a low level of aggregation (< 10% complex formation), before and after radiolabeling with iodogen. Prior to radiolabeling, 123I was brought to the dry form, phosphate buffer added and titrated to a pH of 7, with diluted hydrochloric acid. 123I was injected in doses of 26, 12, 27, 25 and 1 millicurie, respectively, and labeled to a 5-mg fragment. Single photon emission computed tomography and whole body imaging were performed at 4-6 hours, and 24 hours, respectively, after injection. RESULTS: The agent was rapidly cleared from the blood with biphasic clearance T-1/2 of 30 minutes and 10.5 hours, respectively. Distribution from whole body imaging confirmed rapid equilibration with extracellular fluid, and clearance T-1/2 from the body was comparable to the slower component of blood clearance. The spleen was visualized in all patients, and the testes were imaged in 67% of male patients. Renal excretion was noted with early uptake and clearance from the renal parenchyma except in one patient in whom renal parenchyma retention was intense. Although image quality was suboptimal, tumor was visualized in all five patients in both primary and metastatic lesions. At surgery, (16-24 hours postinjection), the tumor retained significant concentrations of the radiotracer, with metastatic tumor/normal liver ratios of approximately 1:5-3:1. No patient had any associated symptom or change in biochemical and hematopoietic status. CONCLUSIONS: This study showed that sFv is safe, tissue equilibration and clearance is rapid, and early, same-day imaging of the primary and metastatic tumors is feasible in patients colorectal carcinoma. Further studies are warranted to define a more optimal mass of sFv CC49 dose for tumor targeting.

Response of SCC-12F, a human squamous cell carcinoma cell line, to complement attack.

We studied the response of a human squamous cell carcinoma cell line, SCC-12F, to human complement attack and found that the cells were completely resistant to complement lysis. In the absence of lysis, there was significant C3 deposition and C5b-9 deposition on the cells. Removal of the lipid-linked complement regulatory proteins CD59 and decay-accelerating factor (DAF) by treatment of the cells with phosphatidylinositol-specific phospholipase C (PIPLC) resulted in increased C3b and C5b-9 deposition on the cells and a slight increase in cell death. Treatment of the cells with complement caused them to release membrane vesicles containing the terminal complement proteins. In addition, complement induced SCC-12F to produce significant amounts of prostaglandin F2alpha (PGF2alpha). We conclude that CD59 and DAF are important in the resistance of SCC-12F to complement and that these cells produce membrane vesicles and PGF2alpha in response to complement attack. These responses, in the absence of cell death, may be important in the pathogenesis of inflammatory skin disease in which complement is deposited.

Structure of human plasminogen kringle 4 at 1.68 a and 277 K. A possible structural role of disordered residues.

Despite considerable effort to elucidate the functional role of the kringle domains, relatively little is known about interactions with other protein domains. Most of the crystal structures describe the interactions at the kringle active site. This study suggests a novel way to interpret structural results such as disorder located away from an active site. The crystal structure of human plasminogen kringle 4 (PGK4) has been refined against 10-1.68 A resolution X-ray data (R(merge) = 3.7%) to the standard crystallographic R = 14.7% using the program X-PLOR. The crystals of PGK4 showed significant instability in cell dimensions (changes more than 1.5 A) even at 277 K. The refinement revealed structural details not observed before [Mulichak, Tulinsky & Ravichandran (1991). Biochemistry, 30, 10576-10588], such as clear density for additional side chains and more extensive disorder. Discrete disorder was detected for residues S73, S78, T80, S89, S91, S92, Ml12, S132, C138 and K142. Most of the disordered residues form two patches on the surface of the protein. This localized disorder suggests that these residues may play a role in quaternary interactions and possibly form an interface with the other domains of proteins that contain kringles, such as plasminogen. Although, an additional residue D65 was refined at the beginning of the sequence, still more residues near the peptide cleavage site must be disordered in the crystal.

Interaction between complement proteins C5b-7 and erythrocyte membrane sialic acid.

The initial phase of membrane attack by complement is the interaction between C5b6, C7, and the cell membrane that leads to the insertion of C5b-7. Here we investigate the role of sialic acid residues in the assembly of C5b-7 intermediates on erythrocyte cell membranes. We find that C5b6 binds to glycophorin, whereas C5 or C6 does not bind, and desialylation of the glycophorin abolishes C5b6 binding. Complement lysis is inhibited by either masking glycophorin sialic acid with F(ab) fragments of an mAb, or by removal of the sialylated region of glycophorin by mild trypsinization. Gangliosides inhibit C5b-7 deposition when added to the aqueous phase. Asialogangliosides and synthetic gangliosides lacking the carboxylic acid residue have no inhibitory activity. We conclude that C5b6 binds to sialylated molecules on the erythrocyte surface. We propose a new model of membrane attack in which C5b6 initially binds to membranes via ionic forces. C7 then binds to C5b6, disrupting the ionic interaction and leading to the exposure of hydrophobic domains. Sialic acid is known to inhibit complement activation. Thus, these findings reveal a paradoxical role for sialic acid in complement attack; the presence of sialic acid inhibits the generation of C5b6, but once the membrane attack pathway is initiated, sialic acid enhances complement lysis.

Identification of C1q as the heat-labile serum cofactor required for immune complexes to stimulate endothelial expression of the adhesion molecules E-selectin and intercellular and vascular cell adhesion molecules 1.

To examine the role of complement components as regulators of the expression of endothelial adhesive molecules in response to immune complexes (ICs), we determined whether ICs stimulate both endothelial adhesiveness for leukocytes and expression of E-selectin and intercellular and vascular cell adhesion molecules 1 (ICAM-1 and VCAM-1). We found that ICs [bovine serum albumin (BSA)-anti-BSA] stimulated endothelial cell adhesiveness for added leukocytes in the presence of complement-sufficient normal human serum (NHS) but not in the presence of heat-inactivated serum (HIS) or in tissue culture medium alone. Depletion of complement component C3 or C8 from serum did not prevent enhanced endothelial adhesiveness stimulated by ICs. In contrast, depletion of complement component C1q markedly inhibited IC-stimulated endothelial adhesiveness for leukocytes. When the heat-labile complement component C1q was added to HIS, the capacity of ICs to stimulate endothelial adhesiveness for leukocytes was completely restored. Further evidence for the possible role of C1q in mediating the effect of ICs on endothelial cells was the discovery of the presence of the 100- to 126-kDa C1q-binding protein on the surface of endothelial cells (by cytofluorography) and of message for the 33-kDa C1q receptor in resting endothelial cells (by reverse transcription-PCR). Inhibition of protein synthesis by cycloheximide blocked endothelial adhesiveness for leukocytes stimulated by either interleukin 1 or ICs in the presence of NHS. After stimulation with ICs in the presence of NHS, endothelial cells expressed increased numbers of adhesion molecules (E-selectin, ICAM-1, and VCAM-1). Endothelial expression of adhesion molecules mediated, at least in part, endothelial adhesiveness for leukocytes, since leukocyte adhesion was blocked by monoclonal antibodies directed against E-selectin. These studies show that ICs stimulate endothelial cells to express adhesive proteins for leukocytes in the presence of a heat-labile serum factor. That factor appears to be C1q.