Biomedical subjects

I D Kuntz

Publications and source records attributed to I D Kuntz.

At least 19 recordsLinked to original sources

PubMed1995

Solution structure of the GTPase activating domain of alpha s.

We have used heteronuclear three-dimensional NMR spectroscopy to determine the solution structure of a 141 residue protein containing the GTPase activating domain from the alpha chain of the heterotrimeric G protein Gs. The domain contains six alpha-helices and is stable and structured in solution despite having been excised from the intact Gs protein. The N-terminal ten and C-terminal 11 residues of the protein are unstructured in solution while the core is well determined by the 2483 distance and torsion restraints derived from the NMR spectra. The final ensemble of 14 structures, generated with a hybrid distance geometry/simulated annealing protocol, have an average to-the-mean backbone root-mean-square deviation of 0.39 A for the core residues 89 to 201. The majority of the structure is remarkably similar to that observed for the cognate domains in crystal structures of the homologous proteins alpha t and alpha i1. However, the orientations of the second helix and the subsequent interhelical loops differ markedly among the three proteins. This structural divergence, along with functional studies of chimeric proteins, suggests that this region of the domain interacts with either the downstream effector adenylyl cyclase or with some other intermediary protein.

Amino Acid Sequence↗

PubMed1995

Complete 1H, 13C, and 15N assignments and secondary structure of the GTPase activating domain of Gs.

Complete 1H, 13C, and 15N assignments for backbone and side-chain atoms of the 145 residue GTPase activating domain of Gs are presented. The combination of gradient-enhanced versions of the HNCACB and CBCA(CO)NNH pulse sequences provided enough information to obtain sequential backbone assignments for residues 2-145 of the polypeptide, as well as assignments of asparagine and glutamine side-chain amides. HBHA(CO)NNH, HCCH-TOCSY, and 13C/15N NOESY-HSQC experiments yielded side-chain 1H and 13C assignments. Chemical shift data and 15N NOESY-HSQC experiments provided information on the secondary structure of the domain, which is similar to that observed in the cognate domain in transducin, a related G protein. The functionally essential C-terminal 15 residues are disordered in solution. These assignments provide a basis for determining the solution structure of the domain.

Amino Acid Sequence↗

PubMed1995

New molecular shape descriptors: application in database screening.

Geometric descriptors are becoming popular tools for encoding molecular shape, for use in database screening and clustering calculations. They provide condensed representations of complex objects and, as a consequence, can usually be compared quite rapidly. Here we present a number of new descriptors and methods for the quantification of molecular shape similarity. The techniques are tested using two different biological systems, with particular emphasis on their potential utility as methods for prescreening shape-based database searches. Results are compared with data sets produced using the DOCK program. We find that such similarity evaluations are useful for finding molecules with complementary shape, and that they contain an enriched number of potential DOCK hits when compared to the original databases. Significant limitations in the utility of such DOCK prescreens are discussed, and potential solutions are considered.

Binding Sites↗

PubMed1995

Flexible ligand docking using a genetic algorithm.

Two computational techniques have been developed to explore the orientational and conformational space of a flexible ligand within an enzyme. Both methods use the Genetic Algorithm (GA) to generate conformationally flexible ligands in conjunction with algorithms from the DOCK suite of programs to characterize the receptor site. The methods are applied to three enzyme-ligand complexes: dihydrofolate reductase-methotrexate, thymidylate synthase-phenolpthalein and HIV protease-thioketal haloperidol. Conformations and orientations close to the crystallographically determined structures are obtained, as well as alternative structures with low energy. The potential for the GA method to screen a database of compounds is also examined. A collection of ligands is evaluated simultaneously, rather than docking the ligands individually into the enzyme.

Algorithms↗

PubMed1995

BUILDER v.2: improving the chemistry of a de novo design strategy.

Significant improvements have been made to the de novo drug design program BUILDER. The BUILDER strategy is to find molecule templates that bind tightly to 'hot spots' in the target receptor, and then generate bridges to join these templates. In this paper, the bridging algorithm has been further developed to improve the chemical sense and diversity of the bridges, as well as the robustness of the technique. The improved algorithm is then applied to rebuild known bridges in methotrexate and HIV protease. Finally, the entire BUILDER approach is tested by rebuilding methotrexate de novo.

Algorithms↗

PubMed1994

Docking analysis of a series of benzylamino acetylcholinesterase inhibitors with a phthalimide, benzoyl, or indanone moiety.

The enzyme-binding mode of a series of acetylcholinesterase inhibitors has been analyzed on the basis of the crystal structure of the Torpedo enzyme using docking programs DOCK and directed-DOCK. The inhibitors have a benzyl group connected to tertiary ammonium nitrogen at one end and a phthalimide, benzoyl, or indanone moiety at the other. Our modeling results have indicated that the benzyl group interacts with Trp 84, which is located near the bottom of the binding pocket and is postulated to be the quaternary ammonium binding site for acetylcholine. The other aromatic ring has been found to interact with Trp 279 at the peripheral hydrophobic site. In addition, the hydrogen-bonding interaction between a carbonyl group of the inhibitor and Tyr 121 OH seems to play an important role. Our active-orientation model is, at least qualitatively, consistent with structure-activity data for more than 50 compounds and should be useful for the design of more potent inhibitors.

Acetylcholinesterase↗

PubMed1994

Finding potential DNA-binding compounds by using molecular shape.

For the first time a general shape-search docking algorithm (DOCK) has been applied to the minor and major grooves of A-, B- and Z-type DNA dodecamers and to an intercalation site in a B-DNA-type hexamer. Both experimentally and theoretically derived geometries for the various DNA fragments were used. The DOCK searches were carried out on a subset of the Cambridge Crystallographic Database, consisting of almost 10,000 molecules. One of the molecules that scored best in terms of the DOCK algorithm was CC-1065, a potent antitumor agent known to (covalently) bind the AT-rich parts of the minor groove of B-DNA. Several known DNA-binding agents also scored highly. Molecules with shapes complementary to A-, B- and Z-type DNA were indicated by DOCK. In addition, compounds were extracted from the database that might be selective for the GC-rich regions of the minor groove of B-DNA. Many of the compounds in the present study may serve as a starting point for further molecular design of novel DNA-binding ligands.

Algorithms↗

PubMed1994

Evaluating docked complexes with the HINT exponential function and empirical atomic hydrophobicities.

Methods that predict geometries of ligands binding to receptor molecules can facilitate ligand discovery and yield information on the factors governing complementarity. Here, the use of atomic hydrophobicities in evaluating binding modes has been examined with four ligand-receptor complexes of known structure. In each system, hundreds of hypothetical binding orientations were generated with DOCK and evaluated using the HINT (Hydropathic INTeractions) exponential function and atomic hydrophobic constants. In three of the four systems, the experimental binding mode received the best HINT score; in the fourth system, the experimental binding mode scored only slightly lower than a similar, apparently reasonable orientation. The HINT function may be generally useful as a scoring method in molecular docking.

Binding Sites↗

PubMed1993

Novel heteronuclear methods of assignment transfer from a diamagnetic to a paramagnetic protein: application to rat cytochrome b5.

15N and 1H resonance assignments for backbone and side-chain resonances of both equilibrium forms of rat ferricytochrome b5 have been obtained, using a combination of novel heteronuclear assignment transfer methods from the known assignments of the diamagnetic protein [Guiles, R. D., Basus, V. J., Kuntz, I. D., & Waskell, L. A. (1992) Biochemistry 31, 11365-11375] and computational methods which depend on an accurate determination of the orientation of the components of the susceptibility tensor. The transfer of amide proton resonance assignments takes advantage of the apparent insensitivity of amide 15N resonances to pseudocontact effects, evident in overlays of 15N-1H heteronuclear correlation spectra. Amide-proton resonance assignments tentatively transferred from the known diamagnetic assignments to the paramagnetic form of the protein were confirmed using conventional assignment strategies employing 600-MHz COSY, HOHAHA, and NOESY spectra of the oxidized protein. As was observed in rat ferrocytochrome b5, more than 40% of all residues exhibited NMR detectable heterogeneity due to the two different orientations of the heme. Complete assignment of both forms enabled accurate determination of the orientation of the susceptibility tensor for both conformations of the heme. The orientation of the z-component of the susceptibility tensors for the two forms are indistinguishable, while the in-plane components appear to differ by about 6 degrees. Differences in the orientation of the in-plane susceptibility components are undoubtedly due dominantly to the relative axial rotation of the heme of between 5 degrees and 10 degrees indicated by the NOESY contacts to the protein observed in the spectra of the ferrocytochrome [Guiles, R. D., Basus, V. J., Kuntz, I. D., & Waskell, L. A. (1992) Biochemistry 31, 11365-11375; Pochapsky, T. C., Sligar, S. G., McLachlan, S. J., & LaMar, G. N. (1990) J. Am. Chem. Soc. 112, 5258-5263].

Amino Acid Sequence↗

PubMed1993

Structure of a non-peptide inhibitor complexed with HIV-1 protease. Developing a cycle of structure-based drug design.

A stable, non-peptide inhibitor of the protease from type 1 human immunodeficiency virus has been developed, and the stereochemistry of binding defined through crystallographic three-dimensional structure determination. The initial compound, haloperidol, was discovered through computational screening of the Cambridge Structural Database using a shape complementarity algorithm. The subsequent modification is a non-peptidic lateral lead, which belongs to a family of compounds with well characterized pharmacological properties. This thioketal derivative of haloperidol and a halide counterion are bound within the enzyme active site in a mode distinct from the observed for peptide-based inhibitors. A variant of the protease cocrystallized with this inhibitor shows binding in the manner predicted during the initial computer-based search. The structures provide the context for subsequent synthetic modifications of the inhibitor.

Binding Sites↗

PubMed1993

Orientational sampling and rigid-body minimization in molecular docking.

The biological activities of proteins depend on specific molecular recognition and binding. Computational methods for predicting binding modes can facilitate the discovery and design of ligands and yield information on the factors governing complementarity. The DOCK suite of programs has been applied to several systems; here, the degree of orientational sampling required to reproduce and identify known binding modes, with and without rigid-body energy minimization, is investigated for four complexes. There is a tradeoff between sampling and minimization. The known binding modes can be identified with intensive sampling alone (10,000 to 20,000 orientations generated per system) or with moderate sampling combined with minimization. Optimization improves energies significantly, particularly when steric clashes are present, and brings many orientations closer to the experimentally observed position. Whether or not minimization is performed, however, sampling must be sufficient to find at least one structure in the vicinity of the presumed true binding mode. Hybrid approaches combining docking and minimization are promising and will become more viable with the use of faster algorithms and the judicious selection of fewer orientations for minimization.

Carboxypeptidases↗

PubMed1993

Mutational and secondary structural analysis of the basolateral sorting signal of the polymeric immunoglobulin receptor.

The 17-juxtamembrane cytoplasmic residues of the polymeric immunoglobulin receptor contain an autonomous basolateral targeting signal that does not mediate rapid endocytosis (Casanova, J. E., G. Apodaca, and K. E. Mostov. Cell. 66:65-75). Alanine-scanning mutagenesis identifies three residues in this region, His656, Arg657, and Val660, that are most essential for basolateral sorting and two residues, Arg655 and Tyr668, that play a lesser role in this process. Progressive truncations suggested that Ser664 and Ile665 might also play a role in basolateral sorting. However, mutation of these residues to Ala or internal deletions of these residues did not affect basolateral sorting, indicating that these residues are probably not required for basolateral sorting. Two-dimensional NMR spectroscopy of a peptide corresponding to the 17-mer signal indicates that the sequence Arg658-Asn-Val-Asp661 has a propensity to adopt a beta-turn in solution. Residues COOH-terminal to the beta-turn (Arg662 to Arg669) seem to take up a nascent helix structure in solution. Substitution of Val660 with Ala destabilizes the turn, while mutation of Arg657 to Ala does not appear to affect the turn structure. Neither mutation detectably altered the stability of the nascent helix in the COOH-terminal portion of the peptide.

Amino Acid Sequence↗

PubMed1993

Matching chemistry and shape in molecular docking.

We have added a chemical filter to the ligand placement algorithm of the molecular docking program DOCK. DOCK places ligands in receptors using local shape features. Here we label these shape features by chemical type and insist on complementary matches. We find fewer physically unrealistic complexes without reducing the number of complexes resembling the known ligand-receptor configurations. Approximately 10-fold fewer complexes are calculated and the new algorithm is correspondingly 10-fold faster than the previous shape-only matching. We tested the new algorithm's ability to reproduce three known ligand-receptor complexes: methotrexate in dihydrofolate reductase, deoxyuridine monophosphate in thymidylate synthase and pancreatic trypsin inhibitor in trypsin. The program found configurations within 1 A of the crystallographic mode, with fewer non-native solutions compared with shape-only matching. We also tested the program's ability to retrieve known inhibitors of thymidylate synthase and dihydrofolate reductase by screening molecular databases against the enzyme structures. Both algorithms retrieved many known inhibitors preferentially to other compounds in the database. The chemical matching algorithm generally ranks known inhibitors better than does matching based on shape alone.

Algorithms↗

PubMed1992

Sequence-specific 1H and 15N resonance assignments for both equilibrium forms of the soluble heme binding domain of rat ferrocytochrome b5.

15N and 1H resonance assignments for backbone and side-chain resonances of both equilibrium forms of rat ferrocytochrome b5 have been obtained, using 15N-1H heteronuclear correlation methods employing globally 15N-labeled protein. Unlike other cytochrome b5 species assigned to date (Guiles et al., 1990) the rat cytochrome exists as an equilibrium distribution of conformers in nearly equal abundance (Lee et al., 1990). The ratio of conformers present in all other species variants is approximately 1:9. More than 40% of all residues of the rat protein exhibit NMR-detectable heterogeneity due to the 180 degrees rotation of the heme about the alpha, gamma-meso axis. NOESY and HOHAHA relayed 15N-1H double-DEPT heteronuclear correlation methods were an indispensible tool for the deconvolution of a system with this level of heterogeneity. Differences in the resonance assignments between the two equilibrium conformers were found to be as great as differences between species variants we have previously reported. On the basis of the magnitude and extent of the observed chemical shift differences and specific NOESY connectivities observed in the two isomers, we believe the two equilibrium conformers differ not only by a simple back-to-front flip of the heme but also by an additional rotation about an axis normal to the heme plane as has been previously suggested by Pochapsky et al. (1990). A short segment of the protein at the N-terminus could not be assigned, presumably due to rapid exchange of solvent-accessible amide protons in this disordered segment of the protein. Assignments for 93 of the 98 residues of this 12-kDa protein have been obtained.

Animals↗

PubMed1992

Structure-based strategies for drug design and discovery.

Most drugs have been discovered in random screens or by exploiting information about macromolecular receptors. One source of this information is in the structures of critical proteins and nucleic acids. The structure-based approach to design couples this information with specialized computer programs to propose novel enzyme inhibitors and other therapeutic agents. Iterated design cycles have produced compounds now in clinical trials. The combination of molecular structure determination and computation is emerging as an important tool for drug development. These ideas will be applied to acquired immunodeficiency syndrome (AIDS) and bacterial drug resistance.

Acquired Immunodeficiency Syndrome↗

PubMed1992

Disulfide bond-coupled folding of bovine pancreatic trypsin inhibitor derivatives missing one or two disulfide bonds.

The disulfide bond-coupled folding and unfolding mechanism (at pH 8.7, 25 degrees C in the presence of oxidized and reduced dithiothreitol) was determined for a bovine pancreatic trypsin inhibitor mutant in which cysteines 30 and 51 were replaced with alanines so that only two disulfides, between cysteines 14 and 38 and cysteines 5 and 55, remain. Similar studies were made on a chemically-modified derivative of the mutant retaining only the 5-55 disulfide. The preferred unfolding mechanism for the Ala30/Ala51 mutant begins with reduction of the 14-38 disulfide. An intramolecular rearrangement via thiol-disulfide exchange, involving the 5-55 disulfide and cysteines 14 and/or 38, then occurs. At least five of six possible one-disulfide bond species accumulate during unfolding. Finally, the disulfide of one or more of the one-disulfide bond intermediates (excluding that with the 5-55 disulfide) is reduced giving unfolded protein. The folding mechanism seems to be the reverse of the unfolding mechanism; the observed folding and unfolding reactions are consistent with a single kinetic scheme. The rate constant for the rate-limiting intramolecular folding step--rearrangements of other one-disulfide bond species to the 5-55 disulfide intermediate--seems to depend primarily on the number of amino acids separating cysteines 5 and 55 in the unfolded chain. The energetics and kinetics of the mutant's folding mechanism are compared to those of wild-type protein [Creighton, T. E., & Goldenberg, D. P. (1984) J. Mol. Biol. 179, 497] and a mutant missing the 14-38 disulfide [Goldenberg, D. P. (1988) Biochemistry 27, 2481]. The most striking effects are destabilization of the native structure and a large increase in the rate of unfolding.

Alanine↗

PubMed1992

Comparison of solution structures of mutant bovine pancreatic trypsin inhibitor proteins using two-dimensional nuclear magnetic resonance.

Structural perturbations due to a series of mutations at the 30-51 disulfide bond of bovine pancreatic trypsin inhibitor have been explored using NMR. The mutants replaced cysteines at positions 30 and 51 by alanine at position 51 and alanine, threonine, or valine at position 30. Chemical shift changes occur in residues proximate to the site of mutation. NOE assignments were made using an automated procedure, NASIGN, which used information from the wild-type crystal structure. Intensity information was utilized by a distance geometry algorithm, VEMBED, to generate a series of structures for each protein. Statistical analyses of these structures indicated larger averaged structural perturbations than would be expected from crystallographic and other information. Constrained molecular dynamics refinement using AMBER at 900 K was useful in eliminating structural movements that were not a necessary consequence of the NMR data. In most cases, statistically significant movements are shown to be those greater than approximately 1 A. Such movements do not appear to occur between wild type and A30A51, a result confirmed by crystallography (Eigenbrot, C., Randal, M., & Kossiakoff, A.A., 1990, Protein Eng. 3, 591-598). Structural alterations in the T30A51 or V30A51 mutant proteins near the limits of detection occur in the beta-loop (residues 25-28) or C-terminal alpha-helix, respectively.

Amino Acid Sequence↗

PubMed1992

A fast and efficient method for 2D and 3D molecular shape description.

A new formalism for molecular shape description is described. The formalism, based on considering each molecule as a collection of its 3-atom submolecules, is applied to both the graph theory and geometrical coordinate representations of molecules. The timing results for shape description of several databases indicate that this new method is applicable to large databases. Furthermore, results from clustering a small database show good agreement with clustering results obtained by a distance-matching algorithm.

Algorithms↗