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A preference-based free-energy parameterization of enzyme-inhibitor binding. Applications to HIV-1-protease inhibitor design.

The interface between protein receptor-ligand complexes has been studied with respect to their binary interatomic interactions. Crystal structure data have been used to catalogue surfaces buried by atoms from each member of a bound complex and determine a statistical preference for pairs of amino-acid atoms. A simple free energy model of the receptor-ligand system is constructed from these atom-atom preferences and used to assess the energetic importance of interfacial interactions. The free energy approximation of binding strength in this model has a reliability of about +/- 1.5 kcal/mol, despite limited knowledge of the unbound states. The main utility of such a scheme lies in the identification of important stabilizing atomic interactions across the receptor-ligand interface. Thus, apart from an overall hydrophobic attraction (Young L, Jernigan RL, Covell DG, 1994, Protein Sci 3:717-729), a rich variety of specific interactions is observed. An analysis of 10 HIV-1 protease inhibitor complexes is presented that reveals a common binding motif comprised of energetically important contacts with a rather limited set of atoms. Design improvements to existing HIV-1 protease inhibitors are explored based on a detailed analysis of this binding motif.

Alcohols↗

Multiple scattering x-ray absorption studies of Zn2+ binding sites in bacterial photosynthetic reaction centers.

Binding of transition metal ions to the reaction center (RC) protein of the photosynthetic bacterium Rhodobacter sphaeroides has been previously shown to slow light-induced electron and proton transfer to the secondary quinone acceptor molecule, Q(B). On the basis of x-ray diffraction at 2.5 angstroms resolution a site, formed by AspH124, HisH126, and HisH128, has been identified at the protein surface which binds Cd(2+) or Zn(2+). Using Zn K-edge x-ray absorption fine structure spectroscopy we report here on the local structure of Zn(2+) ions bound to purified RC complexes embedded into polyvinyl alcohol films. X-ray absorption fine structure data were analyzed by combining ab initio simulations and multiparameter fitting; structural contributions up to the fourth coordination shell and multiple scattering paths (involving three atoms) have been included. Results for complexes characterized by a Zn to RC stoichiometry close to one indicate that Zn(2+) binds two O and two N atoms in the first coordination shell. Higher shell contributions are consistent with a binding cluster formed by two His, one Asp residue, and a water molecule. Analysis of complexes characterized by approximately 2 Zn ions per RC reveals a second structurally distinct binding site, involving one O and three N atoms, not belonging to a His residue. The local structure obtained for the higher affinity site nicely fits the coordination geometry proposed on the basis of x-ray diffraction data, but detects a significant contraction of the first shell. Two possible locations of the second new binding site at the cytoplasmic surface of the RC are proposed.

Absorptiometry, Photon↗

Conserved positions for ribose recognition: importance of water bridging interactions among ATP, ADP and FAD-protein complexes.

Analysis of the spatial arrangement of protein and water atoms that form polar interactions with ribose has been performed for a structurally non-redundant dataset of ATP, ADP and FAD-protein complexes. The 26 ligand-protein structures were separated into two groups corresponding to the most populated furanose ring conformations (N and S-domains). Four conserved positions were found for S-domain protein-ligand complexes and five for N-domain complexes. Multiple protein folds and secondary structural elements were represented at a single conserved position. The following novel points were revealed: (i) Two complementary positions sometimes combine to describe a putative atomic spatial location for a specific conserved binding spot. (ii) More than one third of the interactions scored were water-mediated. Thus, conserved spatial positions rich in water atoms are a significant feature of ribose-protein complexes.

Adenosine Diphosphate↗

Synthesis, cytotoxic effect, and structure-activity relationship of Pd(II) complexes with coumarin derivatives.

We report the influence of the substituent at the N atom of the ligands on the synthesis, biological activity, and stability of Pd(II) complexes of the general formula PdL(2). The compounds adopt a cis or trans configuration with respect to the substituent at the nitrogen atom. Sterically hindered substituents promote the formation of trans isomers, whereas when the nitrogen atom is unsubstituted, cis isomers are formed. The compounds were characterized by elemental analysis, infrared and 1H NMR spectroscopies, and electrospray mass spectrometry. The complexes were also studied using X-ray diffraction and computational DFT methods. Both complexes cis-3a and trans-3c exhibit square-planar geometries around the Pd(II) atom. The cytotoxic effects of these complexes were examined on two human leukemia cell lines, HL-60 and NALM-6. Pd complex cis-3a showed significant cytotoxic activity. The effects exhibited by this complex were comparable to those reported for carboplatin. Loigand 2a was not cytotoxic. Computational analysis carried out at the PB/B3LYP/LACVP**//mPW1PW91/LanL2DZ level showed excellent correlation between the energy difference of the cis and trans isomers and the cytotoxic activity, rendering computations a useful predictive tool for the design of new drugs.

Antineoplastic Agents↗

Gold cluster carbonyls: vibrational spectroscopy of the anions and the effects of cluster size, charge, and coverage on the CO stretching frequency.

We report the vibrational spectra of the carbonyl complexes of anionic gold clusters in the range of the CO stretching frequency as measured in the gas phase using IR multiple photon dissociation spectroscopy. The investigated complexes contain between 3 and 14 Au atoms and up to 7 CO ligands. Special attention is given to the complexes that exhibit saturation CO coverage as well as to the monocarbonyl species. In conjunction with data from the corresponding cationic complexes we quantify how the CO stretching frequency varies with the charge state of the gold cluster. Our results provide a size- and charge-dependent basis to interpret values of the CO stretching frequency measured for CO on deposited gold clusters in terms of the charge states of the clusters.

Journal Article↗

Macromolecular assemblies: greater than their parts.

Increasingly powerful methods of analysis have opened up complex macromolecular assemblies to scrutiny at atomic detail. They reveal not only examples of assembly from preformed and prefolded components, but also examples in which the act of assembly drives changes to the components. In the most extreme of these examples, some of the components only achieve a folded state when the complex is formed. Striking results have appeared for systems ranging from the already mature field of virus structure and assembly, where notable progress has been made for rather complex capsids, to descriptions of ribosome structures in atomic detail, where recent results have emerged at breathtaking speed.

Crystallography↗

PreBI: prediction of biological interfaces of proteins in crystals.

PreBI is a server that predicts biological interfaces in protein crystal structures, according to the complementarity and the area of the interface. The server accepts a coordinate file in the PDB format, and all of the possible interfaces are generated automatically, according to the symmetry operations given in the coordinate file. For all of the interfaces generated, the complementarities of the electrostatic potential, hydrophobicity and shape of the interfaces are analyzed, and the most probable biological interface is identified according to the combination of the degree of complementarity derived from the database analyses and the area of the interface. The results can be checked through an interactive viewer, and the most probable complex can be downloaded as atomic coordinates in the PDB format. PreBI is available at http://pre-s.protein.osaka-u.ac.jp/~prebi/.

Crystallography, X-Ray↗

Bis(dimethyl sulfoxide-S)tetrakis(&mgr;-p-hydroxybenzoato-O:O')dirhodium(II)-tetra kis(&mgr;-butyrato-O:O')bis(dimethyl sulfoxide-S)dirhodium(II) cocrystal ethanol disolvate

The title structure, [Rh(2)(C(7)H(5)O(3))(4)(C(2)H(6)OS)(2)]. [Rh(2)(C(4)H(7)O(2))(4)(C(2)H(6)OS)(2)].2C(2)H(6)O, contains two discrete neutral Rh-Rh dimers cocrystallized as the ethanol disolvate. Each dimer is situated on an inversion center. The butyrate chain displays disorder in one C-atom position. In each dimer, the dimethyl sulfoxide ligand (dmso) is bound via S, as expected. The ethanol is a hydrogen-bond acceptor for one p-hydroxybenzoate hydroxyl group and acts as a hydrogen-bond donor to the dmso O atom of a neighboring p-hydroxybenzoate dirhodium complex. A third hydrogen bond is formed from the other p-hydroxybenzoate hydroxyl group to the dmso O atom of a butyrate-dirhodium complex.

Journal Article↗

Structural basis for cooperative DNA binding by CAP and lac repressor.

Catabolite gene activator protein (CAP) and lac repressor (LR) are celebrated transcription-regulating proteins that bind to DNA cooperatively forming a ternary complex with the promoter loop. Here we present a multiscale model of the ternary complex derived from crystal structures of the proteins and a continuous structure of the DNA loop built using the theory of elasticity. We predict that the loop is underwound in the binary complex with the LR, whereas in the ternary complex with the LR and CAP, the loop is overwound and extended due to an upstream relocation of a DNA binding hand of LR. The computed relocation distance matches the experimental observations and the energy balance of the system explains the cooperativity effect. Using the multiscale approach, we build an all-atom model of the ternary complex that suggests a series of further experimental investigations.

Binding, Competitive↗

Docking enzyme-inhibitor complexes using a preference-based free-energy surface.

We present a docking scheme that utilizes both a surface complementarity screen as well as an energetic criterion based on surface area burial. Twenty rigid enzyme/inhibitor complexes with known coordinate sets are arbitrarily separated and reassembled to an average all-atom rms (root mean square) deviation of 1.0 A from the native complexes. Docking is accomplished by a hierarchical search of geometrically compatible triplets of surface normals on each molecule. A pruned tree of possible bound configurations is built up using successive consideration of larger and larger triplets. The best scoring configurations are then passed through a free-energy screen where the lowest energy member is selected as the predicted native state. The free energy approximation is derived from observations of surface burial by atom pairs across the interface of known enzyme/inhibitor complexes. The occurrence of specific atom-atom surface burial, for a set of complexes with well-defined secondary structure both in the bound and unbound states, is parameterized to mimic the free energy of binding. The docking procedure guides the inhibitor into its native state using orientation and distance-dependent functions that reproduce the ideal model of free energies with an average rms deviation of 0.9 kcal/mol. For all systems studied, this docking procedure identifies a single, unique minimum energy configuration that is highly compatible with the native state.

Enzyme Inhibitors↗

Refined structure of the aminobenzolamide complex of human carbonic anhydrase II at 1.9 A and sulphonamide modelling of bovine carbonic anhydrase III.

The binding of aminobenzolamide to human carbonic anhydrase (HCA II) has been investigated by X-ray crystallography. The three dimensional atomic structure of the enzyme inhibitor complex has been refined at 1.9 A resolution. The crystallographic R-factor is 17.8%. All inhibitor atoms are clearly possible to identify from the difference electron density map in the active site of the enzyme. The nitrogen of the sulphonamide group of the inhibitor is bound as a fourth ligand to the zinc ion, the other three are all histidyl residues. The binding conformation of the sulphonamide groups is similar to the previously described sulphonamide inhibitors. One of the oxygens of the outer sulphonamido group of the inhibitor forms a hydrogen bond to the amino group of Gln 92. The higher affinity of the benzolamide inhibitor compared with acetazolamide can be accounted for by the strong aromatic and hydrophobic interactions between the amino benzene ring of the inhibitor and the residues Phe 131 and Leu 198. In modelling studies of bovine carbonic anhydrase III (BCA III) it was evident that Phe 198 prevents an optimal interaction with sulphonamides.

Animals↗

Preparation and characterization of new hybrid organic/inorganic systems derived from calcium (alpha-aminoalkyl)-phosphonates and -phosphonocarboxylates.

We have studied the phenomenon of calcium complexation by lab synthesized amphiphilic (alpha-aminoalkyl)-phosphonocarboxylic or -phosphonic acids. The electrical conductivity of aqueous solutions of sodium salts of all these acids was measured versus the volume of a calcium salt solution added. It appeared that calcium complexes are formed in a Ca/P atomic ratio close to 1. Calcium phosphonocarboxylates and calcium phosphonates were also precipitated by mixing aqueous solutions of disodium salts of phosphorus amphiphiles and calcium nitrate solutions. Before chemical analysis, these complexes were calcined to remove the organic part. In the mineralized products, calcium and phosphate were assayed: the Ca/P atomic ratio was equal to 1. X-ray diffraction and IR spectroscopy showed that they are made entirely of beta pyrophosphate (Ca2P2O7), a result in agreement with previous chemical analysis. The chemical formula of the starting calcium complexes could be written as CaL2H2O (L=ligand). The SEM micrographs of these complexes show plate-like structures. XRD patterns are characteristic of layered structures. These facts suggest that calcium complexes are composed of alternating bimolecular layers of calcium alkylphosphonocarboxylates or calcium alkylphosphonates, the chains being tilted and partially interdigitated.

Journal Article↗

Quantum computational analysis for drug resistance of HIV-1 reverse transcriptase to nevirapine through point mutations.

Quantum chemical calculation has been carried out to analyze binding interactions of nevirapine to HIV-1 reverse transcriptase (RT) and single point mutants Lys103 --> Asn (K103N) and Tyr181--> Cys (Y181C). In this study, the entire system of HIV-1 RT/nevirapine complex with over 15,000 atoms is explicitly treated by using a recently developed MFCC (molecular fractionation with conjugate caps) approach. Quantum calculation of protein-drug interaction energy is performed at Hartree-Fock and DFT levels. The RT-nevirapine interaction energies are computed at fixed geometries given by the crystal structures of the HIV-1 RT/nevirapine complexes from protein data bank (PDB). The present calculation provides a quantum mechanical interaction spectrum that explicitly shows interaction energies between nevirapine and individual amino-acid fragments of RT. Detailed interactions that are responsible for drug resistance of two major RT mutations are elucidated based on computational analysis in relation to the crystal structures of binding complexes. The present result provides a qualitative molecular understanding of HIV-1 RT drug resistance to nevirapine and gives useful guidance in designing improved inhibitors with better resistance to RT mutation.

Anti-HIV Agents↗

"Venetian blinds" mechanism of solvation/desolvation in palladium(II) wheel-and-axle organic-inorganic diols.

A family of organic-inorganic wheel-and-axle diols (Pd(LOH)(2)Cl(2), Pd(LOH)(2)(CH(3))Cl, Pd(LOH)(2)(CH(3)COO)(2), LOH = alpha-(4-pyridyl)benzhydrol) and several corresponding solvates are synthesized and characterized by single-crystal X-ray diffraction analysis. Their structures are compared to investigate the factors governing the modes of solid state association, the propensity to clathration, and the structural basis of guest inclusion. In all the complexes, the palladium coordination is a slightly distorted square. The LOH ligands coordinate Pd(2+) by means of the 4-pyridyl ring. In the chloride complexes solvation occurs with a 1:2 host/guest ratio by hydrogen bonding between the terminal -OH groups of the complex diol and one acceptor atom on the guest, and it is further assisted by guest stacking between host aryl rings. All solvates are organized in layers with practically invariant metrics, while the layers may be assembled in different arrangements. The structures of the nonsolvate compounds are related to the metrics of the solvate forms by rotation of the complex molecules within the layer plane. In all cases the nonsolvates are completely converted into the corresponding crystalline solvate forms by exposure to the vapor of the guest, and conversely they are quantitatively recovered from the solvate upon removal of the guest by mild conditions. On the basis of the structural data, it is proposed that the solvation/desolvation process proceeds by a concerted rotation of the complex molecules in the layer plane. The structural analysis of Pd(LOH)(2)(CH(3)COO)(2) and of its tetrahydrofuran monosolvate form suggests that the first step of the solid/gas solvation process may imply the clathration of 1 mol of guest between the aryl rings, which successively triggers the collective reorientation of the host molecules.

Journal Article↗

NMR detection of arginine-ligand interactions in complexes of Lactobacillus casei dihydrofolate reductase.

1H-NMR and 15N-NMR signal assignments have been made for the eight arginine residues in Lactobacillus casei dihydrofolate reductase in its binary complex with methotrexate and in its ternary complex with methotrexate and NADPH. 1H-NMR chemical shifts for the guanidino groups of two of the arginines (Arg57 and Arg43) were sensitive to different modes of binding of the guanidino groups with charged oxygen atoms of the ligands. In the complexes formed with methotrexate, Arg57 showed four non-equivalent NH eta proton signals indicating hindered rotation about the N epsilon-C zeta and C zeta-N eta bonds. The NH eta 12 and NH eta 22 protons showed large downfield shifts, which would be expected for a symmetric end-on interaction of these protons with the charged oxygen atoms of a carboxylate group in methotrexate. These effects were not observed for the complex formed with trimethoprim, which does not contain any carboxylate groups. In the complex formed with NADPH present, Arg43 showed a large downfield chemical shift for its NH epsilon proton and a retardation of its rate of exchange with water. This pattern of deshielding contrasts with that detected for Arg57 and is that expected for a side-on interaction of the guanidino group protons with charged oxygen atoms of the ribose 2'-phosphate group of NADPH.

Arginine↗

New approaches to drug discovery and development: a mechanism-based approach to pharmaceutical research and its application to BNP7787, a novel chemoprotective agent.

Any approach applied to drug discovery and development by the medical community and pharmaceutical industry has a direct impact on the future availability of improved, novel, and curative therapies for patients with cancer. By definition, drug discovery is a complex learning process whereby research efforts are directed toward uncovering and assimilating new knowledge to create and develop a drug for the purpose of providing benefit to a defined patient population. Accordingly, a highly desirable technology or approach to drug discovery should facilitate both effective learning and the application of newly discovered observations that can be exploited for therapeutic benefit. However, some believe that drug discovery is largely accomplished by serendipity and therefore appropriately addressed by screening a large number of compounds. Clearly, this approach has not generated an abundance of new drugs for cancer patients and suggests that a tangibly different approach in drug discovery is warranted. We employ an alternative approach to drug discovery, which is based on the elucidation and exploitation of biological, pharmacological, and biochemical mechanisms that have not been previously recognized or fully understood. Mechanism-based drug discovery involves the combined application of physics-based computer simulations and laboratory experimentation. There is increasing evidence that agreement between simulations based on the laws of physics and experimental observations results in a higher probability that such observations are more accurate and better understood as compared with either approach used alone. Physics-based computer simulation applied to drug discovery is now considered by experts in the field to be one of the ultimate methodologies for drug discovery. However, the ability to perform truly comprehensive physics-based molecular simulations remains limited by several factors, including the enormous computer-processing power that is required to perform the formidable mathematical operations and data processing (e.g. memory bandwidth, data storage and retrieval). Another major consideration is the development of software that can generate an appropriate and increasingly complex physical representation of the atomic arrangements of biological systems. During the past 17 years, we have made tremendous progress in addressing some of these obstacles by developing and optimizing physics-based computer programs for the purpose of obtaining increasingly accurate and precise information and by improving the speed of computation. To perform physics-based simulations that involve complex systems of biological and pharmaceutical interest, we have developed methods that enable us to exceed Moore's law. This has been accomplished by parallel processing as well as other methods that have enabled us to study more complex and relevant molecular systems of interest. This paper provides an overview of our approach to drug discovery and describes a novel drug, currently in clinical development, which has directly resulted from the application of this approach.

Animals↗

Sugar interaction with biologically active cis-PtCl2(NH3)2 (anticancer) and its inactive trans isomer.

The interaction of D-glucuronic and D-gluconic acids with cis- and trans-PtCl2(NH3)2 (cisplatin and transplatin) has been investigated in aqueous solution and solid complexes of the type cis-[PtL(NH3)2]L.H2O and trans-[PtL2(NH3)2]L.H2O, where L = D-glucuronate or D-gluconate anions, are isolated and characterized by means of Fourier transform-infrared and 1H-NMR spectroscopy, and molar conductivity and X-ray powder diffraction measurements. Spectroscopic and other evidence indicated that the sugar anions bind monodentately in trans-[PtL2(NH3)2].H2O and bidentately in cis-[PtL(NH3)2]L.H2O complexes through the carboxylate oxygen atoms and other sugar donor groups. The strong sugar intermolecular hydrogen-bonding network is altered to that of the sugar-OH...NH3(H2O)...OH-sugar, upon platinum-ammine interaction. The D-glucuronate anion has the beta-anomer configuration both in the free salt and in these platinum-sugar complexes.

Cisplatin↗

Raman spectroscopic evidence for cooperative C-H...O interactions in the acetaldehyde-CO2 complex.

In carbonyl compounds having a hydrogen atom attached to the alpha-carbon or directly to the carbonyl carbon, our recent ab initio and solvation studies have predicted the presence of a weak C-H...O interaction with CO2. This intermolecular interaction acts cooperatively with the Lewis acid-Lewis base interaction that exists between CO2 and the carbonyl group. This communication provides the first experimental evidence for this cooperative interaction in the acetaldehyde-CO2 complex using Raman spectroscopy at 25.0 degrees C in the gas phase. The implications of these interactions in the solvation of carbonyl-based CO2-philes are also discussed.

Journal Article↗