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D W Urry

Publications and source records attributed to D W Urry.

At least 127 records · Page 7Linked to original sources
PubMed1980

The malonyl gramicidin channel: NMR-derived rate constants and comparison of calculated and experimental single-channel currents.

Malonyl gramicidin is incorporated into lysolecithin micelles in a manner which satisfies a number of previously demonstrated criteria for the formation of the transmembrane channel structure. By means of sodium-23 nuclear magnetic resonance, two binding sites are observed: a tight site and a weak site with binding constants of approximately 100 M-1 and 1 M-1 respectively. In addition, off-rate constants from the two sites were estimated from NMR analyses to be kofft congruent to 3 X 10(5)/sec and koffw congruent to 2 X 10(7)/sec giving, with the binding constants, the on-rate constants, kont congruent to 3 X 10(7)/Msec and konw congruent to 2 X 10(7)/Msec. Five different multiple occupancy models with NMR-restricted energy profiles were considered for the purpose of calculating single-channel currents as a function of voltage and concentration utilizing the four NMR-derived rate constants (and an NMR-limit placed on a fifth rate constant for intrachannel ion translocation) in combination with Eyring rate theory for the introduction of voltage dependence. Using the X-ray diffraction results of Koeppe et al. (1979) for limiting the positions of the tight sites, the two-site model and a three-site model in which the weak sites occur after the tight site is filled were found to satisfactorily calculate the experimental currents (also reported here) and to fit the experimental currents extraordinarily well when the experimentally derived values were allowed to vary to a least squares best fit. Surprisingly the "best fit" values differed by only about a factor of two from the NMR-derived values, a variation that is well within the estimated experimental error of the rate constants. These results demonstrate the utility of ion nuclear magnetic resonance to determine rate constants relevant to transport through the gramicidin channel and of the Eyring rate theory to introduce voltage dependence.

Colloids↗
PubMed1980

15N-NMR of repeat peptides of tropoelastin. The tetrapeptide.

The 15N-NMR data obtained in chloroform and methanol are reported for Boc-L-Val1-L-Pro2-Gly3-Gly4-OMe, a repeat tetrapeptide sequence of tropoelastin. Deuterium substitution for labile peptide protons was carried out to delineate solvent-exposed and solvent-shielded peptide nitrogens. Solvent perturbation of the peptide nitrogen resonances together with the hydrogen-deuterium substitution reaffirms the previously derived conformation of the molecule in chloroform which contains a beta-turn (type II) and a 14-membered hydrogen bond to form a cross-beta structure.

Chloroform↗
PubMed1980

Repeat polypeptide models of elastin as substrates for lysyl oxidase.

Synthetic repeat polypeptides analogous in sequence to the valine-rich regions of elastin have been tested as substrates for purified bovine aorta lysyl oxidase. These polypeptides, HCO(phi-Pro-Gly-Gly)n-Val-OMe, HCO(Val-Pro-Gly-phi-Gly)n-Val-OMe, and HCO-Val-(Ala-Pro-Gly-phi-Gly-Val)n-OMe, where phi = Val or Lys at approximately a 4:1 ratio and where n greater than or equal to 40, are models of the tetra-, penta-, or hexapeptide repeat sequences found in elastin. alpha-Aminoadipic delta-semialdehyde is generated in each of these upon incubation with lysyl oxidase at 37 degrees C, whereas the aldol and anhydrolysinonorleucine bifunctional cross-linkages were formed only in the incubation of enzyme with polypentapeptide. Incubation of the polypentapeptide at 55 degrees C, which enhances coacervation of the peptide, increases aldehyde formation and generates a much higher ratio of cross-linkages to aldehyde than occurred at 37 degrees C. These results demonstrate that lysyl oxidase can oxidize lysine in synthetic polypeptides and suggest important conformational aspects of lysyl oxidase substrates which may control substrate potential as well as the ability of peptidyl aldehyde, once formed by the enzyme, to condense to cross-linkage products.

Amino Acid Oxidoreductases↗
PubMed1980

Conformational studies on the gramicidin A transmembrane channel in lipid micelles and liposomes.

The interaction of gramicidin A with lysolecithin micelles and with lecithin liposomes is demonstrated by circular dichroism to result in several metastable conformational states. A stable state can be obtained after extensive heating when the gramicidin A was added dry or in ethanol solution to the phospholipid dispersion but the stable state is readily obtained when gramicidin A is added in a trifluoroethanol solution. The circular dichroism of the stable conformational states is characterized by negative ellipticity below 205 nm and principally by a positive 220 nm band on which is superposed a weak 230 nm band (the latter likely arising from tryptophan side chains). The stable conformational state is considered to be that of the functional transmembrane channel primarily on the basis of extensive studies on its interaction with sodium ions.

Circular Dichroism↗
PubMed1980

Rate theory calculation of gramicidin single-channel currents using NMR-derived rate constants.

By means of 23Na NMR, two ion binding sites were observed in phospholipid-packaged gramicidin channels and the four associated rate constants were approximated. Limits also were placed on a fifth rate constant for an intrachannel ion translocation. By using Eyring rate theory to introduce voltage dependence, these rate constants were used in steady-state-current equations for calculation of gramicidin single-channel currents for two- and three-site models. Calculated single-channel currents are compared with previously published experimental single-channel currents obtained by electrical measurements on Na+ transport across gramicidin-doped planar lipid bilayers. The calculated results for the two- and three-site models compare favorably with the experimental results. Accordingly, it is demonstrated that NMR-derived rate constants can be coupled with Eyring rate theory to calculate currents through a transmembrane channel and to do so within levels of variation that compare with the differences obtained on planar lipid bilayers formed with different lipids.

Binding Sites↗
PubMed1979

Formation of ionic channels in black lipid membranes by succinic derivatives of gramicidin A.

Different succinyl derivatives of Gramicidin A were synthesized and their activity was investigated with different methods on lipid bilayer membranes. The succinyl derivatives of Gramicidin A can be classified as three different types, the O-succinyl derivative, the N-succinyl derivative and the N-O-succinyl derivative of Gramicidin A. An O-pyromellityl-N-succinyl gramicidin was synthesized which can be attributed to the latter class. It was found that O-succinyl gramicidin behaves like the unmodified Gramicidin A despite a charge effect on single-channel conductance, arising from the negative charge of the succinic residue at the mouth of the channel. The activity of N-succinyl and N-O-succinyl gramacidin and of O-pyromellityl-N-succinyl-gramicidin depends strongly on the pH of the electrolyte solution. It is demonstrated that at low pH (less than or equal to 5) the N-succinyl derivatives show high activity, whereas at high pH (greater than or equal to 7) the activity is sharply reduced or disappears totally. From these experiments it can be concluded that, for the formation of a dimeric gramicidin channel, the hydrogen of the formyl group can be replaced by a protonated carboxylic group of a succinic residue. Further results, obtained by measurement of the single-channel conductance and of the reaction rate constants for the channel formation, are discussed in terms of the structural basis of the single stranded model for the gramicidin channel. On this basis the double stranded helix can be excluded and an interesting head-to-head single stranded beta(pi L.D.) helical channel is described which contains carboxyl groups at the head-to-head junction.

Electric Conductivity↗
PubMed1979

Nuclear magnetic resonance studies on a cyclic dodecapeptide analogue of a repeat hexapeptide of tropoelastin: evaluation of secondary structure.

The cyclododecapeptide, (Ala1-Pro2-Gly3-Val4-Gly5-Val6)2, was synthesized and its secondary structure was evaluated from extensive studies in dimethyl sulphoxide, trifluoroethanol and water using NMR methods. A selective decoupling technique in 13C-NMR has been utilized in order to assign the C=O carbon resonances. Temperature dependence of the peptide NH protons and the solvent perturbation of the peptide NH and C=O resonances show the occurrence in all solvents of a beta-turn (a 10-membered H-bond between the Val4 NH and Ala1 C=O) and a gamma-turn, an 11-membered H-bond between the Gly3 NH and the Gly5 C=O; and a possible 14-membered H-bond between the Ala1 NH and the Val4 C=O in dimethyl sulphoxide and trifluoroethanol. These secondary structural features are compared with the linear polyhexapeptide and found the the beta-turn and the gamma-turn are the common conformational features of these peptide systems.

Amino Acid Sequence↗
PubMed1979

N-acetyl gramicidin: single-channel properties and implications for channel structure.

Substitution of a methyl group for the N-terminal hydrogen of gramicidin greatly increased the rate of dissociation of conductive channels in lipid bilayer membranes. The finding of short lifetimes for conductive channels, comparable to those seen for the neuromuscular junction, lends support to the head-to-head dimer structure for the conductive channel.

Cations, Monovalent↗
PubMed1978

Proton magnetic resonance and conformational energy calculations of repeat peptides of tropoelastin. A permutation of the hexapeptide.

The conformation of a hexapeptide sequence occurring in tropoelastin is discussed from the results obtained using a combined approach of theoretical conformational energy calculations on HCO-Val-Ala-Prb-Gly-OMe and 1h nmr studies on t-Boc-Val-Ala-Pro-Gly-Val-Gly-OMe in a dilute solution of methanol. Both studies have reasonable concurrence with respect to the preferred conformation of the hexapeptide and an analysis of the combined results suggests that the hexapeptide is stabilized by a beta-turn involving the Ala1,iC=O and Val4,iNH groups and a gamma-turn involving Gly5,iC=O and Gly3,iNH groups. A weaker interaction between Gly3,iC=O and Gly5,iNH groups is also found to be possible. Conformational features of the first valyl residue in the sequence Val-Ala-Pro-Gly-Val-Gly and the last valyl residue in Ala-Pro-Gly-Val-Gly-Val are compared and found to have similar torsion angles. The implications of such a similarity are discussed with respect to the conformation of the polyhexapeptide.

Amino Acid Sequence↗
PubMed1978

Cyclic analog of elastin polyhexapeptide exhibits an inverse temperature transition leading to crystallization.

The cyclic dodecapeptide analog of the linear polyhexapeptide of tropoelastin crystallizes from water on raising of the temperature and thereby demonstrates an inverse temperature transition implying dominant intermolecular hydrophobic interactions. The temperature profiles of turbidity (TP tau) of the cyclododecapeptide are analogous to those of the polyhexapeptide where increases in concentration lead to translations of the profiles to lower temperature without sharpening of the transition. The demonstration of increase in order with increase in temperature for the cyclododecapeptide in water and the similarity of TPtau's lends credence to the view that increases in temperature lead to increases in order, specifically, for the linear polyhexapeptide and, generally, for the precursor protein of the elastic fiber wherein the repeating hexapeptide occurs.

Crystallization↗