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Biomedical subjects

H R Guy

Publications and source records attributed to H R Guy.

46 records · Page 3Linked to original sources
PubMed1987

Molecular genetics of the VDAC ion channel: structural model and sequence analysis.

The voltage-dependent anion-selective channel of the outer mitochondrial membrane provides a unique system in which to study the molecular basis of voltage gating of ion flow. We have cloned and sequenced a cDNA coding for this protein in yeast. From the derived amino acid sequence, we have generated a preliminary model for the secondary structure of the protein which suggests that the protein forms a "beta-barrel" type structure. Comparison of the VDAC amino acid sequence with that of the bacterial porins has indicated that the two classes of molecules appear to be unrelated.

Amino Acid Sequence↗
PubMed1986

Molecular model of the action potential sodium channel.

Secondary and tertiary structural models of sodium channel transmembrane segments were developed from its recently determined primary sequence in Electrophorus electricus. The model has four homologous domains, and each domain has eight homologous transmembrane segments, S1 through S8. Each domain contains three relatively apolar segments (S1, S2 and S3) and two very apolar segments (S5 and S8), all postulated to be transmembrane alpha-helices. S4 segments have positively charged residues, mainly arginines, at every third residue. The model channel lining is formed by four S4 transmembrane alpha-helices and four negatively charged S7 segments. S7 segments are postulated to be short, partially transmembrane amphipathic alpha-helices in three domains and a beta-strand in the last domain. S7 segments are preceded by short apolar segments (S6) postulated to be alpha-helices in three domains and a beta-strand in the last domain. Positively charged side chains of S4 form salt bridges with negatively charged side chains on S7 and near the ends of S1 and S3. Putative extracellular segments that contain 5 of the 10 potential N-glycosylation sites link S5 to S6. Channel activation may involve a 'helical screw' mechanism in which S4 helices rotate around their axes as they move toward the extracellular surface.

Animals↗
PubMed1985

Amino acid side-chain partition energies and distribution of residues in soluble proteins.

Energies required to transfer amino acid side chains from water to less polar environments were calculated from results of several studies and compared with several statistical analyses of residue distributions in soluble proteins. An analysis that divides proteins into layers parallel with their surfaces is more informative than those that simply classify residues as exposed or buried. Most residues appear to be distributed as a function of the distance from the protein-water interface in a manner consistent with partition energies calculated from partitioning of amino acids between water and octanol phases and from solubilities of amino acids in water, ethanol, and methanol. Lys, Arg, Tyr, and Trp residues tend to concentrate near the water-protein interface where their apolar side-chain components are more buried than their polar side-chain components. Residue distributions calculated in this manner do not correlate well with side-chain solvation energies calculated from vapor pressures of side-chain analogs over a water phase. Results of statistical studies that classify residues as exposed to solvent or buried inside the protein interior appear to depend on the method used to classify residues. Data from some of these studies correlate better with solvation energies, but other data correlate better with partition energies. Most other statistical methods that have been used to evaluate effects of water on residue distributions yield results that correlate better with partition energies than with solvation energies.

Amino Acids↗
PubMed1984

A structural model of the acetylcholine receptor channel based on partition energy and helix packing calculations.

A structural model of the transmembrane portion of the acetylcholine receptor was developed from sequences of all its subunits by using transfer energy calculations to locate transmembrane alpha-helices and to calculate which helical side chains should be in contact with water inside the channel, with portions of other transmembrane helices, or with lipid hydrocarbon chains. "Knobs-into-holes" side chain packing calculations were used with other factors to stack the transmembrane alpha-helices together. In the model each subunit has the following structures in order along the sequence from the NH2 terminus: a large extracellular domain of undetermined structure, a short apolar alpha-helix that lies on the extracellular lipid surface of the membrane; three apolar transmembrane alpha-helices (I, II, and III), a cytoplasmic domain of undetermined structure, an amphipathic transmembrane alpha-helix (L) that forms the channel lining, a short extracellular alpha-helix, another apolar transmembrane alpha-helix (IV), and a small cytoplasmic domain formed by the COOH-terminal end of the chain. Three concentric layers form the pore. A bundle of five amphipathic L helices forms the channel lining. This bundle is surrounded by a bundle of 10 alternating II and III helices. Helices I and IV cover portions of the outer surface of the bundle formed by helices II and III. Positions of disulfide bridges are predicted and a mechanism for opening and closing conformational changes is proposed that requires tilting transmembrane helices and possibly a thiol-disulfide interchange reaction.

Amino Acid Sequence↗
PubMed1981

Structural models of the nicotinic acetylcholine receptor and its toxin-binding sites.

Models of the protein structure of agonist-, competitive antagonist-, and snake neurotoxin-binding sites were designed using the sequence of the first 54 residues of the acetylcholine receptor (AChR) alpha subunit from Torpedo californica. These models are based on the premise that the N-terminal portions of the subunits form the outermost extracellular surface of the AChR and that agonists bind to this portion. The models were developed by predicting the secondary structure of the alpha-subunit N-terminal segment from its sequence, then using these predictions to fold the segment into tertiary structures that should bind snake neurotoxins, agonists, and antagonists. Possible gating mechanisms and quaternary structures are suggested by the proposed tertiary structures of the subunits. Experiments are suggested to test aspects of the models.

Animals↗
PubMed1978

Effects of organophosphate insecticides on the cholinergic receptors of frog skeletal muscle.

The organophosphates octamethyl pyrophosphoramide, Bidrin, and phosphoric acid 2,2-dichlorovinyl dimethyl ester inhibit the membrane voltage response of frog sartorius muscles to carbamylcholine in a manner expected of either a slowly reversing competitive inhibitor or a noncompetitive inhibitor. The inhibition reverses with a time course of minutes, depending upon the temperature. The inhibition by these compounds is reduced by d-tubocurarine but is unaffected by hexamethonium. This may indicate that the organophosphate binding sites are near the cholinergic binding site since both d-tubocurarine and hexamethonium are competitive inhibitors of cholinergic agonists and d-tubocurarine is the larger compound.

Acetylcholine↗
PubMed1977

Effect of denervation on the permeability of acetylcholine-activated channels to organic cations.

Experiments were perfomed on chronically denervated frog sartorius muscles to determine the permeability of the acetylcholine-activated channels to organic cations. The membrane voltage response to bath-applied acetylcholine was measured with the moving electrode when the muscles were bathed in normal Ringer and in Ringer in which all of the Na+ had been replaced with an organic cation. The magnitude of the maximum voltage response was used to estimate the permeability of the channel to the organic cation. These results were compared with those which have been reported for innervated frog sartorius muscles (Maeno, Edwards, and Anraku, 1977). It is concluded that the permeability to a wide range of organic cations is virtually identical for the extrajunctional channels which develop following denervation and the channels with are localized at the junctional region of innervated muscles.

Acetylcholine↗
PubMed1976

A study of the effects of p-chloromercuribenzene sulfonic acid on acetylcholine-induced responses of molluscan neurons.

The effects of the organic mercurial p-chloromercuribenzene sulfonic acid (PCMBS) on the acetylcholine receptors of two types of neurons from the marine gastropod Anisodoris nobilis were studied. Cell types were distinguished by the ion-specific conductance change resulting from applied cholinergic agents: in certain cells a conductance increase to Cl resulted (Cl-neurons); in other cells a cation conductance increase resulted (D cells). Exposure of the Cl-neurons to 1 mM PCMBS at 4 degrees C had two distinct effects: 1) the sensitivity of the membrane conductance to microelectrophoretically applied carbamylcholine was blocked in a few minutes; and 2) the membrane conductance slowly increased with an inversion potential which was the same as that of the cholinergically induced response. d-Turbocurarine did not appreciably reduce this conductance increase. However, applying carbamylcholine during the exposure period to PCMBS reduced the PCMBS response. The cholinergic response of D cells was affected to a much lesser degree by PCMBS.

4-Chloromercuribenzenesulfonate↗