RESUMEN

BACKGROUND AND PURPOSE: The thalamus seems to be profoundly involved in the cyclical recurrence of migraine clinical and neurophysiological features. Here possible structural changes in the thalamus of migraineurs were searched for by means of diffusion tensor (DT) magnetic resonance imaging (MRI). This MRI technique provides quantitative data on water molecule motion as a marker of tissue microstructure. METHODS: Twenty-four untreated migraine without aura (MO) patients underwent DT-MRI scans (3-T Siemens Gyroscan) during (n = 10) and between attacks (n = 14) and were compared with a group of 15 healthy volunteers (HVs). Fractional anisotropy (FA) and mean diffusivity (MD) were examined. RESULTS: During the interictal phase MO patients had a significantly higher FA and slightly lower MD values in bilateral thalami compared with HVs. During attacks, all MRI quantitative measurements in migraineurs were similar to those found in HVs. Right thalamic FA was positively correlated with the number of days since the last migraine attack in pooled patient data (r = 0.626, P = 0.003). CONCLUSIONS: These higher thalamic FA values noted during the interictal period which normalized during an attack are probably related to plastic peri-ictal modifications in regional branching and crossing of fibres. Whether these changes could be considered as the anatomical counterpart of the cyclical functional fluctuations previously observed in the neurophysiology of migraine remains to be determined.

Asunto(s)

Asunto(s)

RESUMEN

Recent advances in NMR-based screening methods have made it possible to screen larger libraries of molecules with higher throughput. However, experience shows that intelligent library design is important if NMR screening is to succeed in aiding our discovery of potent and useful lead compounds. This review presents the current state-of-the-art methodologies for designing primary and follow-up libraries for NMR screening. Diversity, drug-likeness and combinatorial libraries are discussed, and the inherent pitfalls of the NMR approach are addressed.

RESUMEN

BACKGROUND: Recently, it has been shown that nuclear magnetic resonance (NMR) may be used to identify ligands that bind to low molecular weight protein drug targets. Recognizing the utility of NMR as a very sensitive method for detecting binding, we have focused on developing alternative approaches that are applicable to larger molecular weight drug targets and do not require isotopic labeling. RESULTS: A new method for lead generation (SHAPES) is described that uses NMR to detect binding of a limited but diverse library of small molecules to a potential drug target. The compound scaffolds are derived from shapes most commonly found in known therapeutic agents. NMR detection of low (microM-mM) affinity binding is achieved using either differential line broadening or transferred NOE (nuclear Overhauser effect) NMR techniques. CONCLUSIONS: The SHAPES method for lead generation by NMR is useful for identifying potential lead classes of drugs early in a drug design program, and is easily integrated with other discovery tools such as virtual screening, high-throughput screening and combinatorial chemistry.

Asunto(s)

RESUMEN

Determining appropriate solvent conditions is a crucial first step for carrying out NMR spectroscopy of proteins, but rapid and efficient methods for doing so are currently lacking. Microdrop screening examines a large number of different solvent conditions using very small amounts of protein and minimal labor. Starting from one initial buffer condition, small aliquots of protein solution are combined with an array of solutions in which concentration, pH, buffer type, and added stabilizers are systematically varied. The protein concentration of each microliter-sized test drop ('microdrop') is gradually changed using vapor diffusion, and the solubility of the protein is determined by visual examination. A variety of analytical techniques may be applied to the contents of the microdrops to monitor enzymatic activity, aggregation, ligand binding, and protein folding.

Asunto(s)

RESUMEN

An estimated 1% of the global human population is infected by hepatitis C viruses (HCVs), and there are no broadly effective treatments for the debilitating progression of chronic hepatitis C. A serine protease located within the HCV NS3 protein processes the viral polyprotein at four specific sites and is considered essential for replication. Thus, it emerges as an attractive target for drug design. We report here the 2.5 angstrom resolution X-ray crystal structure of the NS3 protease domain complexed with a synthetic NS4A activator peptide. The protease has a chymotrypsin-like fold and features a tetrahedrally coordinated metal ion distal to the active site. The NS4A peptide intercalates within a beta sheet of the enzyme core.

Asunto(s)

RESUMEN

Dynamic NMR methods, such as differential line broadening and transferred NOE spectroscopy, are normally reserved for the study of small molecule ligand interactions with large protein receptors. Using a combination of isotope labeling and isotope edited NMR, we have extended these techniques to characterize interactions of a much larger protein/drug complex, FKBP-12/ FK506 with its receptor protein, calcineurin. In order to examine this multicomponent system by dynamic NMR methods, the 93 kDa, tightly bound FKBP-12/FK506/Cn complex was replaced with a lower affinity, rapidly exchanging system consisting of FKBP-12/FK506 (13 kDa), recombinant calcineurin subunit B (CnB) (20 kDa), and a synthetic peptide (4 kDa) corresponding to the B binding domain (BBD) of calcineurin catalytic subunit A (CnA). Analysis of 1H-13C HSQC data acquired for the FKBP-12/ 13C-FK506 and FKBP-12/13C-FK506/CnB/BBD complexes indicates that FKBP-12/FK506 and CnB/BBD are in fast exchange in the quaternary complex. Comparison of proton line widths shows significant broadening of resonances along the macrocycle backbone at 13-CH, 13-OMe, 15-OMe, 18-CH2, 20-CH, 21-CH, and 25-Me, as well as moderate broadening on the macrocycle backbone at 17-Me, 24-CH, and the pyranose 12-CH2 protons. The tri-substituted olefin and cyclohexyl groups also show moderate broadening at the 27-Me, 28-CH, and 30-CH2 positions, respectively. Unexpectedly, little line broadening was observed for the allyl resonances of FK506 in the quaternary complex, although 13C longitudinal relaxation measurements suggest this group also makes contacts with calcineurin. In addition, intermolecular transfer NOE peaks were observed for the allyl 37-CH2, 21-CH, 30-CH2, 13-OMe, 15-OMe, 17-Me, 25-Me, and 27-Me groups, indicating that these are potential sites on the FK506 molecule that interact with calcineurin.

Asunto(s)

RESUMEN

The ability of time-averaged restrained molecular dynamics (TARMD) to escape local low-energy conformations and explore conformational space is compared with conventional simulated-annealing methods. Practical suggestions are offered for performing TARMD calculations with ligand-receptor systems, and are illustrated for the complex of the immunosuppressant FK506 bound to Q50R,A95H,K98I triple mutant FKBP-13. The structure of (13)C-labeled FK506 bound to triple-mutant FKBP-13 was determined using a set of 87 NOE distance restraints derived from HSQC-NOESY experiments. TARMD was found to be superior to conventional simulated-annealing methods, and produced structures that were conformationally similar to FK506 bound to wild-type FKBP-12. The individual and combined effects of varying the NOE restraint force constant, using an explicit model for the protein binding pocket, and starting the calculations from different ligand conformations were explored in detail.

RESUMEN

The binding of the FK506/FKBP-12 complex to calcineurin (CN), its putative target for immunosuppression, involves recognition of solvent-exposed regions of the ligand as well as FKBP-12 residues near the active site. The R42K, H87V double mutation of FKBP-12 decreases the CN affinity of the complex by 550-fold [Aldape, R. A., Futer, O., DeCenzo, M. T., Jarrett, B. P., Murcko, M. A., & Livingston, D. J. (1992) J. Biol. Chem. 267, 16029-16032]. This work reports the solution structure of 13C-labeled FK506 bound to R42K, H87V FKBP-12. Assignments and NOE measurements at three mixing times were made from inverse-detected 1H-13C NMR experiments. Structures were calculated by several different methods, including distance geometry, restrained molecular dynamics, and molecular dynamics with time-averaged restraints. The NMR structures of the ligand are very well defined by the NOE restraints and differ slightly from the X-ray structure in regions that are involved in crystal packing. Comparison with the NMR structure of FK506 bound to wild-type FKBP-12 reveals that the R42K, H87V mutation causes the ligand backbone near C16 to move by 2.5 to 4.5 A, reorients 15-MeO by 90 degrees, and shifts 13-MeO by approximately 1.5 A. FK506 appears to undergo a concerted, mutationally induced shift in the binding pocket, with the greatest changes occurring in the effector region of the drug. The altered effector conformation of mutant-bound FK506 may perturb interactions between the drug and CN, thus accounting for the effect of the double mutation upon the CN inhibitory activity of the complex.

Asunto(s)

RESUMEN

Backbone dynamics of the ligand- (FK506-) bound protein FKBP-12 (107 amino acids) have been examined using 15N relaxation data derived from inverse-detected two-dimensional 1H-15N NMR spectra. A model free formalism [Lipari & Szabo (1982) J. Am. Chem. Soc. 104, 4546-4559] was used to derive the generalized order parameter (S2), the effective correlation time for internal motions (tau e), and the chemical-exchange line width (R(ex)) based on the measured 15N relaxation rate constants (R1, R2) and 1H-15N heteronuclear NOEs. The final optimized overall correlation time (tau m) was 9.0 ns. The average order parameter (S2) describing the amplitude of motions on the picosecond time scale was found to be 0.88 +/- 0.04, indicating that internal flexibility is restricted along the entire polypeptide chain. In contrast to results obtained for uncomplexed FKBP, the 80's loop (residues 82-87) surrounding the ligand binding site was found to be rigidly fixed, indicating that internal motions at this site are damped significantly due to stabilizing noncovalent interactions with the FK506 molecule. Structural implications of these differences in picosecond mobility as well as possible implications for calcineurin recognition are discussed.

Asunto(s)

RESUMEN

Homonuclear two-dimensional and three-dimensional 1H-NMR spectroscopy has been utilized to study the 15.9-kDa protein soybean leghemoglobin. NMR experiments were performed on the diamagnetic carbon monoxide complex at two temperatures and two pH values. Sequence-specific assignments have been made for 94% of the backbone and approximately 70% of the expected side-chain resonances. The secondary structure of leghemoglobin in solution has been determined on the basis of NOE connectivity patterns, hydrogen exchange and chemical-shift analyses. Leghemoglobin consists of seven helices and, unlike mammalian myoglobins, is missing the D helix. Instead an extended loop, the CE loop, is observed which might have importance for ligand entry into and exit from the protein interior. The hydrogen exchange behavior for the F helix and at the beginning of the A helix suggests different dynamic stability compared to other helical regions in leghemoglobin. Population of a second protein conformation, in which there is perturbation at the A-G-H helix interface, is observed at low pH.

Asunto(s)

RESUMEN

Backbone dynamics of the major tacrolimus (FK506) binding protein (FKBP-12, 107 amino acids) have been studied using 15N relaxation data derived from proton-detected two-dimensional 1H-15N NMR spectroscopy. 15N spin-lattice relaxation rate constants (R1), spin-spin relaxation rate constants (R2), and heteronuclear NOEs were determined for over 85% of the backbone amide 15N nuclei. A model free formalism [Lipari, G., & Szabo, A. (1982) J. Am. Chem. Soc. 104, 4546-4559; Lipari, G., & Szabo, A. (1982) J. Am. Chem. Soc. 104, 4559-4570] was used to derive values for the generalized order parameter (S2), the effective correlation time for internal motions (tau e), and the chemical exchange line width (Rex) for each N-H bond vector. The final optimized overall correlation time (tau m) was 9.2 ns. The average order parameter (S2) describing the amplitude of motions on the picosecond time scale was found to be 0.88 +/- 0.06. Motions on the picosecond time scale are restricted at the N and C termini, consistent with previous NMR structural studies indicating well-defined beta-strands in these regions. With the exception of the flap region from resides 82 to 87, no regions appear to be significantly disordered on the picosecond time scale. Residues in several regions of the protein exhibit high Rex terms, indicating possible motions on the millisecond to microsecond time scale due to chemical exchange and/or conformational averaging effects. Possible effects of tacrolimus (FK506) binding on FKBP-12 dynamics are discussed in the context of previously determined solution structures for FKBP-12 in the uncomplexed [Michnick et al. (1991) Science 252, 836-839; Moore et al. (1991) Nature 351, 248-250] and complexed [Meadows et al. (1993) Biochemistry 32, 754-765] states.

Asunto(s)

RESUMEN

The complex of the immunosuppressant FK506 bound to FKBP-12 has been studied in solution using 1H and inverse-detected 13C NMR methods. The resonances of bound, 13C-labelled FK506 were assigned and a set of 66 intraligand NOE distance restraints were used to calculate the structure of the bound ligand by distance geometry and restrained molecular dynamics methods. The structure of bound FK506 in solution closely resembles that seen in the X-ray structure [17], except for the allyl region. The differences reflect the influence of intermolecular crystal contacts and have implications for interpretation of the interaction of the FK506/FKBP complex with its putative biological receptor.

Asunto(s)

RESUMEN

The three-dimensional solution structure of reduced (CuI) plastocyanin from French bean leaves has been determined by distance geometry and restrained molecular dynamics methods using constraints obtained from 1H n.m.r. (nuclear magnetic resonance) spectroscopy. A total of 1244 experimental constraints were used, including 1120 distance constraints, 103 dihedral angle constraints and 21 hydrogen bond constraints. Stereospecific assignments were made for 26 methylene groups and the methyls of 11 valines. Additional constraints on copper co-ordination were included in the restrained dynamics calculations. The structures are well defined with average atomic root-mean-square deviations from the mean of 0.45 A for all backbone heavy atoms and 1.08 A for side-chain heavy atoms. French bean plastocyanin adopts a beta-sandwich structure in solution that is similar to the X-ray structure of reduced poplar plastocyanin; the average atomic root-mean-square difference between 16 n.m.r. structures and the X-ray structure is 0.76 A for all backbone heavy atoms. The conformations of the side-chains that constitute the hydrophobic core of French bean plastocyanin are very well defined. Of 47 conserved residues that populate a single chi 1 angle in solution, 43 have the same rotamer in the X-ray structure. Many surface side-chains adopt highly preferred conformations in solution, although the 3J alpha beta coupling constants often indicate some degree of conformational averaging. Some surface side-chains are disordered in both the solution and crystal structures of plastocyanin. There is a striking correlation between measures of side-chain disorder in solution and side-chain temperature factors in the X-ray structure. Side-chains that form a distinctive acidic surface region, believed to be important in binding other electron transfer proteins, appear to be disordered. Fifty backbone amide protons form hydrogen bonds to carbonyls in more than 60% of the n.m.r. structures; 45 of these amide protons exchange slowly with solvent deuterons. Ten hydrogen bonds are formed between side-chain and backbone atoms, eight of which are correlated with decreased proton exchange. Of the 60 hydrogen bonds formed in French bean plastocyanin, 56 occur in the X-ray structure of the poplar protein; two of the missing hydrogen bonds are absent as a result of mutations. It appears that molecular dynamics refinement of highly constrained n.m.r. structures allows accurate prediction of the pattern of hydrogen bonding.

Asunto(s)

RESUMEN

The reaction of trans-diamminedichloroplatinum(II) (trans-DDP), the inactive isomer of the anticancer drug cisplatin, with the single-stranded deoxydodecanucleotide d(CCTCGAGTCTCC) in aqueous solution at 37 degrees C was monitored by reversed-phase HPLC. Consumption of the dodecamer follows pseudo-first-order reaction kinetics with a rate constant of 1.25 (4) x 10(-4) s-1. Two intermediates, shown to be monofunctional adducts in which Pt is coordinated to the guanine N7 positions, were trapped with NH4(HCO3) and identified by enzymatic degradation analysis. These monofunctional adducts and a third, less abundant, one are rapidly removed from the DNA by thiourea under mild conditions. When allowed to react further, the monofunctional intermediates formed a single main product that was characterized by 1H NMR spectroscopy and enzymatic digestion as the bifunctional 1,3-intrastrand cross-link trans-[Pt(NH3)2[d(CCTCGAGTCTCC)-N7-G(5),N7-G(7]]). Binding of the trans-[Pt(NH3)2]2+ moiety to the guanosine N7 positions decreases the pKa at N1 and leads to destacking of the intervening A(6) base. The double-stranded trans-DDP-modified and unmodified DNAs were obtained by annealing the complementary strand to the corresponding single strands and then studied by 31P and 1H NMR and UV spectroscopy. trans-DDP binding does not induce large changes in the O-P-O bond or torsional angles of the phosphodiester linkages in the duplex, nor does it significantly alter the UV melting temperature. trans-DDP binding does, however, cause the imino protons of the platinated duplex to exchange rapidly with solvent by 50 degrees C, a phenomenon that occurs at 65 degrees C for the unmodified duplex. A structural model for the platinated double-stranded oligonucleotide was generated through molecular dynamics calculations. This model reveals that the trans-DDP bifunctional adduct can be accommodated within the double helix with minimal distortion of the O-P-O angles and only local disruption of base pairing and destacking of the platinated bases. The model also predicts hydrogen bond formation involving coordinated ammine ligands that bridge the two strands.

Asunto(s)

RESUMEN

The reaction of trans-[Pt(NH3)2Cl2] with the sodium salt of [d(ApGpGpCpCpT)]2 in aqueous solution at 37 degrees C was monitored by reversed-phase high-performance liquid chromatography and UV spectroscopy. Two intermediates, most likely monofunctional adducts, were observed, which subsequently formed one predominant single-stranded product, as well as several polymeric species proposed to be interstrand cross-linked products. The single-stranded adduct was structurally characterized by 1H NMR spectroscopy. From the pH dependence of the chemical shifts, two-dimensional homonuclear chemical shift correlation (COSY) spectroscopy, and one- and two-dimensional nuclear Overhauser effect (NOESY) experiments, the platinum(II) moiety was found to be coordinated to the N7 positions of adenine(1) and guanine(3), with the intervening guanine(2) base destacked from its neighboring residues. This intrastrand 1,3 adduct induces changes in the backbone torsion angles and causes the deoxyribose ring of adenine(1) to switch from a C2'-endo to a predominantly C3'-endo conformation. The other deoxyribose rings retain B DNA type conformations. The structure of trans-[Pt(NH3)2[d(ApGpGpCpCpT)-N7-A(1),N7-G(3)]] differs from those previously reported for cis-DDP 1,2- and 1,3-intrastrand oligonucleotide adducts but is consistent with the structures of trans-DDP 1,3-intrastrand adducts of two previously reported trinucleotides.

Asunto(s)

RESUMEN

Cytochrome c can be modified by [(NH3)5RuII/III-] specifically at the imidazole moiety of histidine 33, and we have recently discussed the thermodynamics and kinetics of electron transfer within this modified protein. X-ray crystal structures of the oxidized and reduced forms of tuna cytochrome c indicate that the separation between the haem group of cytochrome c and the ruthenium label is 12-16 A. Internal electron transfer from the [(NH3)5RuII-] centre to the Fe(III) haem centre occurs with a rate constant k congruent to 53 s-1 (25 degrees C) (delta H = 3.5 kcal mol-1, delta S = -39 EU), as measured by pulse radiolysis. The measured unimolecular rate constant, k congruent to 53 s-1, is on the same timescale as a number of conformational changes that occur within the cytochrome c molecule. These results raise the question of whether electron transfer or protein conformational change is the rate limiting step in this process. We describe here an experiment that probes this intramolecular electron transfer step further. It involves reversing the direction of electron transfer by changing the redox potential of the ruthenium label. Electron transfer in the new ruthenium-cytochrome c derivative described here is from haem(II) to the Ru(III) label, whereas in (NH3)5Ru-cytochrome c the electron transfer is from Ru(II) to haem(III). Intramolecular electron transfer from haem(II) to Ru(III) in the new ruthenium-cytochrome c described here proceeds much slower (greater than 10(5) times) than the electron transfer from Ru(II) to haem(III) in the (NH3)5Ru-cytochrome c. We therefore conclude that electron transfer in cytochrome c is directional, with the protein envelope presumably involved in this directionality.

Asunto(s)