PubMed Health⌕ Search

SEARCH · PubMed Health

Results for “Solvent accessibility”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 901 records · Page 50Linked to original sources

Characterization of two fluorescent tryptophans in recombinant human granulocyte-colony stimulating factor: comparison of native sequence protein and tryptophan-deficient mutants.

In order to probe the role of the individual tryptophans of granulocyte-colony stimulating factor (G-CSF) in pH and guanidine HCl-induced fluorescence changes, site-directed mutagenesis was used to generate mutants replacing Trp118,Trp58, or both with phenylalanine. Neither Trp to Phe mutation affected the folding or activity of the recombinant G-CSF, and the material expressed in yeast behaved identically to that expressed in Escherichia coli. All of the G-CSF species responded to pH and guanidine HCl in qualitatively the same manner. Trp58 has a fluorescence maximum at 350 nm and is quenched to a greater extent by the addition of guanidine HCl, indicating that it is fully solvent-exposed. Trp118 has a fluorescence maximum at 344 nm, and is less solvent-accessible than Trp58. The analog in which both tryptophans have been replaced with phenylalanine shows only tyrosine fluorescence, with a peak at 304 nm which decreases with increasing pH. The intensity of the tyrosine fluorescence in this analog is much greater than that of the native sequence protein or single tryptophan mutants, indicating that energy transfer is taking place from tyrosine to tryptophan in these molecules. Below neutral pH the tyrosine fluorescence is much greater in the [Phe58]G-CSF than in the [Phe118]G-CSF, indicating that Trp58 might be a more efficient recipient of energy transfer from the tyrosine(s).

Cloning, Molecular↗

The chloroplast ATP synthase: structural changes during catalysis.

This article summarizes some of the evidence for the existence of light-driven structural changes in the epsilon and gamma subunits of the chloroplast ATP synthase. Formation of a transmembrane proton gradient results in: (1) a changed in the position of the epsilon subunit such that it becomes exposed to polyclonal antibodies and to reagents which selectively modify epsilon Lys109; (2) enhanced solvent accessibility of several sulfhydryl residues on the gamma subunit; and (3) release/exchange of tightly bound ADP from the enzyme. Theses and related experimental observations can, at least partially, be explained in terms of two different bound conformational states of the epsilon subunit. Evidence for structural changes in the enzyme which are driven by light or nucleotide binding is discussed with special reference to the popular rotational model for catalysis.

Catalysis↗

Sequencing and phylogenetic analysis of neurotoxin gene from an environmental isolate of Clostridium sp.: comparison with other clostridial neurotoxins.

A Clostridium sp. isolated from intestine of decaying fish exhibited 99% sequence identity with C. tetani at 16S rRNA level. It produced a neurotoxin that was neutralized by botulinum antitoxin (A+B+E) as well as tetanus antitoxin. The gene fragments for light chain, C-terminal and N-terminal regions of the heavy chain of the toxin were amplified using three reported primer sets for tetanus neurotoxin (TeNT). The neurotoxin gene fragments were cloned in Escherichia coli and sequenced. The sequences obtained exhibited approximately 98, 99 and 98% sequence identity with reported gene sequences of TeNT/LC, TeNT/HC and TeNT/HN, respectively. The phylogenetic interrelationship between the neurotoxin gene of Clostridium sp. with previously reported gene sequences of Clostridium botulinum A to G and C. tetani was examined by analysis of differences in the nucleotide sequences. Six amino acids were substituted at four different positions in the light chain of neurotoxin from the isolate when compared with the reported closest sequence of TeNT. Of these, four were located in the beta15 motif at a solvent inaccessible, buried region of the protein molecule. One of these substitutions were on the solvent accessible surface residue of alpha1 motif, previously shown to have strong sequence conservation. A substitution of two amino acids observed in N-terminal region of heavy chain were buried residues, located in the beta21 and beta37 motifs showing variability in other related sequences. The C-terminal region responsible for binding to receptor was conserved, showing no changes in the amino acid sequence.

Amino Acid Sequence↗

Setting up and optimization of membrane protein simulations.

In this paper we describe a method for setting up an atomistic simulation of a membrane protein in a hydrated lipid bilayer and report the effect of differing electrostatic parameters on the drift in the protein structure during the subsequent simulation. The method aims to generate a suitable cavity in the interior of a lipid bilayer, using the solvent-accessible surface of the protein as a template, during the course of a short steered molecular dynamics simulation of a solvated lipid membrane. This is achieved by a two-stage process: firstly, lipid molecules whose headgroups are inside a cylindrical volume equivalent to that defined by the protein surface are removed; then the protein-lipid interface is optimized by applying repulsive forces perpendicular to the protein surface, and of gradually increased magnitude, to the remaining lipid atoms inside the volume occupied by the protein surface until it is emptied. The protein itself may then be inserted. Using the bacterial membrane proteins KcsA and FhuA as test cases, we show how the method achieves the formation of a suitable cavity in the interior of a dimyristoylphosphatidylcholine lipid bilayer without perturbing the configuration of the non-interfacial regions of the previously equilibrated lipid bilayer, even in cases of membrane proteins with irregular geometrical shapes. In addition, we compare subsequent simulations in which the long-range electrostatic interactions are treated via either a cut-off or a particle-mesh Ewald method. The results show that the drift from the initial structure is less in the latter case, especially for KcsA and for the non-core secondary structural elements (i.e. surface loops) of both proteins.

Bacterial Outer Membrane Proteins↗

C-Terminal 23 kDa polypeptide of soybean Gly m Bd 28 K is a potential allergen.

Gly m Bd 28 K is a major soybean (Glycine max Merr.) glycoprotein allergen. It was originally identified as a 28 kDa polypeptide in soybean seed flour. However, the full-length protein is encoded by an open reading frame (ORF) of 473 amino acids, and contains a 23 kDa C-terminal polypeptide of as yet unknown allergenic and structural characteristics. IgE-binding (allergenic potential) of the Gly m Bd 28 K protein including the 23 kDa C-terminal portion as well as shorter fragments derived from the full-length ORF were evaluated using sera from soy-sensitive adults. All of these sera contained IgE that efficiently recognized the C-terminal region. Epitope mapping demonstrated that a dominant linear C-terminal IgE binding epitope resides between residues S256 and A270. Alanine scanning of this dominant epitope indicated that five amino acids, Y260, D261, D262, K264 and D266, contribute most towards IgE-binding. A model based on the structure of the beta subunit of soybean beta-conglycinin revealed that Gly m Bd 28 K contains two cupin domains. The dominant epitope is on the edge of the first beta-sheet of the C-terminal cupin domain and is present on a potentially solvent-accessible loop connecting the two cupin domains. Thus, the C-terminal 23 kDa polypeptide of Gly m Bd 28 K present in soy products is allergenic and apparently contains at least one immunodominant epitope near the edge of a cupin domain. This knowledge could be helpful in the future breeding of hypoallergenic soybeans.

Alanine↗

Structural, biochemical and electrostatic basis of serotype specificity in bovine enteroviruses.

We have performed immunostructural analyses of three closely related picornaviruses in order to gain understanding of the biochemical and structural basis of serotype specificity. We carried out sequence alignments of the capsid regions of three bovine enterovirus strains: VG-5-27 and M-4 from serotype 1 and PS-87 from serotype 2. Using our knowledge of the three dimensional and antigenic structure of strain VG-5-27 and the high levels of sequence identity between the strains, we have calculated the structures and solvent-accessible electrostatic potentials of the epitopes of all three viruses. We have demonstrated the viability of the molecular models of the epitopes of the M-4 and PS-87 strains. In each of the strains, we have explained the serotype specificities in terms of specific physical and chemical properties, and identified individual residues which are pivotal in determination of antibody recognition. These changes are in agreement with the known cross-reactivity of peptide and antiviral sera, showing that it is possible to derive structures for short variable sections of proteins of high sequence identity using molecular modelling which are significant in terms of biological function. We believe this study to be a novel approach in the analysis of virus serotype specificity.

Amino Acid Sequence↗

Thermal stability of the [Fe(SCys)(4)] site in Clostridium pasteurianum rubredoxin: contributions of the local environment and Cys ligand protonation.

Thermal denaturation of the mesophilic rubredoxin from Clostridium pasteurianum occurs through a number of temperature-dependent steps, the last and irreversible one being release of iron from the [Fe(2+)(SCys)(4)] site. We show here that thermally induced [Fe(2+)(SCys)(4)] site destruction is largely determined by the local environment, and not directly connected to thermostability of the native polypeptide fold of rubredoxin. Hydrophobic residues on the protein surface, V8 and L41, that shield the [Fe(SCys)(4)] site from solvent and form N-H(.)S hydrogen bonds to the metal-coordinating sulfurs, were mutated to residues with both uncharged and charged side chains. On these mutated rubredoxins the temperature dependence was measured for: (1) global unfolding of the protein by NMR, (2) loss of Fe(2+)at various ionic strengths and pH values, (3) the rates of non-denaturing displacement of Fe(2+) by Cd(2+) or Zn(2+). For reversible temperature-dependent changes in the global protein folding that occur prior to loss of iron, no thermostability differences were found among the wild-type, V8A, V8D, L41R, and L41D rubredoxins. However, for irreversible loss of iron from the [Fe(2+)(SCys)(4)] site, relative to the wild-type protein, L41R was more thermostable, V8A was somewhat less thermostable, and the acidic mutants L41D, V8D and [V8D, L41D] showed dramatically lowered thermostability. Lower pH facilitated - both kinetically and thermodynamically - thermally induced iron release, likely through protonation of ligand cysteines' thiols. For all of the rubredoxins a direct correlation was found between the midpoint temperature for thermally induced Fe(2+) loss and the rate of non-denaturing Fe(2+) displacement by Cd(2+) or Zn(2+) at room temperature. A mechanism is proposed involving transient movement of residue-8 and -41 side chains, allowing, and, in the case of negatively charged side chains, also facilitating, attack of a ligand cysteine by the incoming positively charged species (H(+), Cd(2+), or Zn(2+)). Thus, localized charge density and solvent accessibility modulate the stability of Fe(2+) ligation in rubredoxin. However, the reduced [Fe(SCys)(4)] site does not control the thermostability of the native polypeptide fold of rubredoxin.

Cadmium↗

Redox properties of the photosystem II cytochromes b559 and c550 in the cyanobacterium Thermosynechococcus elongatus.

Redox properties of cytochrome b559 (Cyt b559) and cytochrome c550 (Cyt c550) have been studied by using highly stable photosystem II (PSII) core complex preparations from a mutant strain of the thermophilic cyanobacterium Thermosynechococcus elongatus with a histidine tag on the CP43 protein of PSII. Two different redox potential forms for Cyt b559 are found in these preparations, with a midpoint redox potential ( E'(m)) of +390 mV in about half of the centers and +275 mV in the other half. The high-potential form, whose E'(m)is pH independent, can be converted into the lower potential form by Tris washing, mild heating or alkaline pH incubation. The E'(m) of the low-potential form is significantly higher than that found in other photosynthetic organisms and is not affected by pH. The possibility that the heme of Cyt b559 in T. elongatus is in a more hydrophobic environment is discussed. Cyt c550 has a higher E'(m)when bound to the PSII core (-80 mV at pH 6.0) than after its extraction from the complex (-240 mV at pH 6.0). The E'(m) of Cyt c550 bound to PSII is pH independent, while in the purified state an increase of about 58 mV/pH unit is observed when the pH decreases below pH 9.0. Thus, Cyt c550 seems to have a single protonateable group which influences the redox properties of the heme. From these electrochemical measurements and from EPR controls it is proposed that important changes in the solvent accessibility to the heme and in the acid-base properties of that protonateable group could occur upon the release of Cyt c550 from PSII.

Cyanobacteria↗

Contribution of the [FeII(SCys)4] site to the thermostability of rubredoxins.

The thermostabilities of Fe(2+) ligation in rubredoxins (Rds) from the hyperthermophile Pyrococcus furiosus (Pf) and the mesophiles Clostridium pasteurianum (Cp) and Desulfovibrio vulgaris (Dv) were compared. Residue 44 forms an NH.S(Cys) hydrogen bond to one of the cysteine ligands to the [Fe(SCys)(4)] site, and substitutions at this location affect the redox properties of the [Fe(SCys)(4)] site. Both Pf Rd and Dv Rd have an alanine residue at position 44, whereas Cp Fd has a valine residue. Wild-type proteins were examined along with V44A and A44V "exchange" mutants of Cp and Pf Rds, respectively, in order to assess the effects of the residue at position 44 on the stability of the [Fe(SCys)(4)] site. Stability of iron ligation was measured by temperature-ramp and fixed-temperature time course experiments, monitoring iron release in both the absence and presence of more thiophilic metals (Zn(2+), Cd(2+)) and over a range of pH values. The thermostability of the polypeptide fold was concomitantly measured by fluorescence, circular dichroism, and (1)H NMR spectroscopies. The A44V mutation strongly lowered the stability of the [Fe(II)(SCys)(4)] site in Pf Rd, whereas the converse V44A mutation of Cp Rd significantly raised the stability of the [Fe(II)(SCys)(4)] site, but not to the levels measured for wild-type Dv Rd. The region around residue 44 is thus a significant contributor to stability of iron coordination in reduced Rds. This region, however, made only a minor contribution to the thermostability of the protein folding, which was found to be higher for hyperthermophilic versus mesophilic Rds, and largely independent of the residue at position 44. These results, together with our previous studies, show that localized charge density, solvent accessibility, and iron site/backbone interactions control the thermostability of the [Fe(SCys)(4)] site. The iron site thermostability does make a minor contribution to the overall Rd thermostability. From a mechanistic standpoint, we also found that attack of displacing ions (H(+), Cd(2+)) on the Cys42 sulfur ligand at the [Fe(SCys)(4)] site occurs through the V8 side and not the V44 side of the iron site.

Amino Acid Sequence↗

Artificial di-iron proteins: solution characterization of four helix bundles containing two distinct types of inter-helical loops.

Peptide-based models have an enormous impact for the development of metalloprotein models, as they seem appropriate candidates to mimic both the structural characteristics and reactivity of the natural systems. Through the de novo design of four-helix bundles, we developed the DF (Due Ferri) family of artificial proteins, as models of di-iron and di-manganese metalloproteins. The goal of our research is to elucidate how the electrostatic environment, polarity and solvent accessibility of the metal-binding site, influence the functional properties of di-iron proteins. The first two subsets of the DF protein family, DF1 and DF2, consist of two non-covalently associated helix-loop-helix motifs, which bind the di-metal cofactor near the center of the structure. The DF2 subset was designed to improve the properties of DF1: DF2 and DF2t have several changes in their sequences to improve solubility and metal ion access, as well as a change in the loop connecting the two helices. In order to evaluate how these changes affect the overall structure of the model proteins, we solved the NMR structures of the di-Zn(II) complexes of DF2 and DF2t, and compared these structures with those recently obtained from X-ray crystallography. Further, we examined the thermodynamic consequences associated with the mutations, by measuring the stability of DF2t in the presence of different metal ions, and comparing the results with the data already obtained for DF2. Taken together, analysis of all the data showed the importance of the turn conformation in the design and stability of four-helix bundle.

Amino Acid Sequence↗

Axial ligation and polypeptide matrix effects on the reduction potential of heme proteins probed on their cyanide adducts.

The enthalpic and entropic changes accompanying the reduction reaction of the six-coordinate cyanide adducts of cytochrome c, microperoxidase-11 and a few plant peroxidases were measured electrochemically. Once the compensating changes in reduction enthalpy and entropy due to solvent reorganization effects are factorized out, it is found that cyanide binding stabilizes enthalpically the ferriheme following the order: cyochrome c > peroxidase > microperoxidase-11. The effect is inversely correlated to the solvent accessibility of the heme. Comparison of the reduction thermodynamics for the cyanide adducts of cytochrome c and plant peroxidases with those for microperoxidase-11 and myoglobin, respectively, yielded an estimate of the consequences of protein encapsulation and of the anionic character of the proximal histidine on the reduction potential of the heme-cyanide group. Insertion of the heme-CN group into the folded peptide chain of cyt c induces an enthalpy-based decrease in E degrees ' of approximately 100 mV, consistent with the lower net charge of the oxidized as compared to the reduced iron center, whereas a full imidazolate character of the proximal histidine stabilizes enthalpically the ferriheme by approximately 400 mV. The latter value should be best considered as an upper limit since it also includes some solvation effects arising from the nature of the protein systems being compared.

Cyanides↗

Oxidation-state-dependent reactions of cytochrome c with the trioxidocarbonate(*1-) radical: a pulse radiolysis study.

The reaction of the trioxidocarbonate(*1-) radical (CO (3) (*-) , "carbonate radical anion") with cytochrome c was studied by pulse radiolysis at alkaline pH and room temperature. With iron(III) cytochrome c, CO (3) (*-) reacts with the protein moiety with rate constants of (5.1 +/- 0.6) x 10(7) M(-1) s(-1) (pH 8.4, I approximately 0.27 M) and (1.0 +/- 0.2) x 10(8) M(-1) s(-1) (pH 10, I = 0.5 M). The absorption spectrum of the haem moiety was not changed, thus, amino acid radicals produced on the protein do not reduce the haem. The pH-dependent difference in rate constants may be attributed to differences in ionization states of amino acids and to the change in the conformation of the protein. With iron(II) cytochrome c, CO (3) (*-) oxidizes the haem quantitatively, presumably via electrostatic guidance of the radical to the solvent-accessible haem edge, with a different pH dependence: at pH 8.4, the rate constant is (1.1 +/- 0.1) x 10(9) M(-1) s(-1) and, at pH 10, (7.6 +/- 0.6) x 10(8) M(-1) s(-1). We propose that CO (3) (*-) oxidizes the iron center directly, and that the lower rate observed at pH 10 is due to the different charge distribution of iron(II) cytochrome c.

Carbonates↗

Molecular statistics of cytochrome c: structural plasticity and molecular environment.

Nuclear magnetic resonance experiments performed on yeast mitochondrial cytochrome c (Cytc), a paradigmatic electron transfer protein, reveal that the two oxidation states have similar structures, but different mobility: despite the few structural differences compared with the reduced form, the oxidized form displays a larger unfolding propensity. Molecular dynamics simulations performed on both NMR reduced and NMR oxidized forms show that the reduced form has a larger solvent-accessible surface area (SASA). Starting from this observation, a molecular statistical approach was then applied in order to correlate the molecular surface to molecular mobility. Simulations started from biased initial conditions corresponding to different molecular sizes were combined with the maximal constrained entropy method. The NMR structure of oxidized Cytc is more suited to expose a smaller SASA than the NMR structure of the reduced form, but the accessible conformational landscape at 300 K around the NMR oxidized structure is flatter than for the NMR reduced structure. Protein configurations of smaller SASA and size display larger plasticity when they resemble the NMR oxidized structure, whereas they are more rigid when they resemble the NMR reduced structure. Implications of the results for the protein properties during its functional process are discussed.

Computer Simulation↗

Ab initio structure solution of a dimeric cytochrome c3 from Desulfovibrio gigas containing disulfide bridges.

The 1.2 A resolution crystal structure of the 29 kDa di-tetrahaem cytochrome c3 from the sulfate reducing bacterium Desulfovibrio gigas was solved by ab initio methods, making this the largest molecule to be solved by this procedure. The actual refined model of the cysteine-linked dimeric molecule reveals that this molecule is very similar to the non-covalently linked symmetrical dimer of the di-tetrahaem cytochrome c3 from Desulfomicrobium norvegicum. Each monomer has the typical polypeptide fold, haem arrangement and iron coordination found for the tetrahaem cytochrome c3 molecules. The interface between the covalently linked monomers in the asymmetric unit of the crystal shows a pseudo two-fold arrangement, disturbed from symmetry by crystal packing forces. The fact that D. gigas contains a dimeric tetrahaem cytochrome with solvent accessible disulfide bridges and that this cytochrome specifically couples hydrogen oxidation to thiosulfate reduction in bacterial extracts provides an interesting aspect related to disulfide exchange reactions in this microorganism.

Crystallography, X-Ray↗

The influence of axial ligands on the reduction potential of blue copper proteins.

The reduction potentials of blue copper sites vary between 180 and about 1000 mV. It has been suggested that the reason for this variation is that the proteins constrain the distance between the copper ion and its axial ligands to different values. We have tested this suggestion by performing density functional B3LYP calculations on realistic models of the blue copper proteins, including solvent effects by the polarizable continuum method. Constraining the Cu-S(Met) bond length to values between 245 and 310 pm (the range encountered in crystal structures) change the reduction potential by less than 70 mV. Similarly, we have studied five typical blue copper proteins spanning the whole range of reduction potentials: stellacyanin, plastocyanin, azurin, rusticyanin, and ceruloplasmin. These studies included the methionine (or glutamine) ligand as well as the back-bone carbonyl oxygen group that is a ligand in azurin and is found at larger distances in the other proteins. The active-site models of these proteins show a variation in the reduction potential of about 140 mV, i.e., only a minor part of the range observed experimentally (800 mV). Consequently, we can conclude that the axial ligands have a small influence on the reduction potentials of the blue copper proteins. Instead, the large variation in the reduction potentials seems to arise mainly from variations in the solvent accessibility of the copper site and in the orientation of protein dipoles around the copper site.

Bacterial Proteins↗

Binding free energy of selected anticancer compounds to DNA--theoretical calculations.

Many studies have elucidated structures and thermodynamics of complexes formed by different ligands with DNA. However, in most cases structural and free energy binding studies were not correlated with each other because of the problem of identifying which experimental free energy of binding corresponds to which experimental DNA-ligand structure. In the present work, Poisson-Boltzmann and solvent-accessible surface area methods were used to predict unknown modes of interaction between DNA and three different ligands: mitoxantrone and two pyrimidoacridine derivatives. In parallel, experimental measurements of binding free energy for the studied complexes were performed to compare experimental and calculated values. Our studies showed that the calculated values of free energy are only close to experimental data for some models of interaction between ligands and DNA. Based on this correlation, the most likely models of DNA-ligand complexes were postulated: (i) mitoxantrone and one derivative of pyrimidoacridine, both with two charged side chains, intercalate from the minor groove of DNA and bind with both chains in this groove; (ii) pyrimidoacridine, with only one side chain, very likely does not intercalate into DNA at all. Additionally, the non-electrostatic and electrostatic parts of the calculated binding free energy for the DNA-ligands studied are discussed.

Antineoplastic Agents↗

ADME evaluation in drug discovery. 1. Applications of genetic algorithms to the prediction of blood-brain partitioning of a large set of drugs.

In this study, the relationships between the brain-blood concentration ratio of 96 structurally diverse compounds with a large number of structurally derived descriptors were investigated. The linear models were based on molecular descriptors that can be calculated for any compound simply from a knowledge of its molecular structure. The linear correlation coefficients of the models were optimized by genetic algorithms (GAs), and the descriptors used in the linear models were automatically selected from 27 structurally derived descriptors. The GA optimizations resulted in a group of linear models with three or four molecular descriptors with good statistical significance. The change of descriptor use as the evolution proceeds demonstrates that the octane/water partition coefficient and the partial negative solvent-accessible surface area multiplied by the negative charge are crucial to brain-blood barrier permeability. Moreover, we found that the predictions using multiple QSPR models from GA optimization gave quite good results in spite of the diversity of structures, which was better than the predictions using the best single model. The predictions for the two external sets with 37 diverse compounds using multiple QSPR models indicate that the best linear models with four descriptors are sufficiently effective for predictive use. Considering the ease of computation of the descriptors, the linear models may be used as general utilities to screen the blood-brain barrier partitioning of drugs in a high-throughput fashion.

Algorithms↗

Inactive and active states and supramolecular organization of GPCRs: insights from computational modeling.

Herein we make an overview of the results of our computational experiments aimed at gaining insight into the molecular mechanisms of GPCR functioning either in their normal conditions or when hit by gain-of-function or loss-of-function mutations. Molecular simulations of a number of GPCRs in their wild type and mutated as well as free and ligand-bound forms were instrumental in inferring the structural features, which differentiate the mutation- and ligand-induced active from the inactive states. These features essentially reside in the interaction pattern of the E/DRY arginine and in the degree of solvent exposure of selected cytosolic domains. Indeed, the active states differ from the inactive ones in the weakening of the interactions made by the highly conserved arginine and in the increase in solvent accessibility of the cytosolic interface between helices 3 and 6. Where possible, the structural hallmarks of the active and inactive receptor states are translated into molecular descriptors useful for in silico functional screening of novel receptor mutants or ligands. Computational modeling of the supramolecular organization of GPCRs and their intracellular partners is the current challenge toward a deep understanding of their functioning mechanisms.

Animals↗