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E Braswell

Publications and source records attributed to E Braswell.

7 recordsLinked to original sources
PubMed1994

Electrostatic interactions in collagen-like triple-helical peptides.

Collagen-like peptides with potential for ion pair formation were studied to investigate the role of electrostatic interactions in the triple-helix conformation. Three peptides--(POG)10, the EK-containing peptide (POG)4EKG(POG)5, and T3-487, a peptide with 18 residues of type III collagen and a C-terminal (GPO)4 tail--all form stable triple helices in aqueous solution, with melting temperatures of 58, 46, and 26 degrees C, respectively, at neutral pH. The thermal stabilities of these peptides correlate with their imino acid content, which is 66%, 60%, and 41%, respectively. Variation of pH over the range of 1-13 led to 8-9 degrees C changes in the Tm of the EK-containing peptide and peptide T3-487, with the greatest stability seen at pH values where both acidic and basic residues are ionized. Equilibrium ultracentrifugation shows these peptides are largely trimeric at low temperature, with no hexamers or larger aggregates, indicating that the pH-dependent stability arises from intramolecular interaction. Computer modeling indicates both intrachain ion pairs and interchain ion pairs can form and stabilize the triple helix. Studies of the pH dependence of the thermal stability of (POG)10 and the N-terminal acetylated form of T3-487 indicate that repulsion of the three charged N-terminal or C-terminal ends has a destabilizing effect. Taking into account these end effects, the energy contribution of two oppositely charged residues in a triple helix which are sterically capable of participating in ion pairs and backbone hydrogen bonding is 0.5-1 kcal/mol ion pair. It is possible that the stabilizing influence of ion pairs arises indirectly, through elimination of like charge repulsion, formation of ion pairs in the single chain form, or solvent effects.

Amino Acid Sequence↗
PubMed1994

Human immunodeficiency virus-1 reverse transcriptase heterodimer stability.

Structural and biochemical evidence strongly supports a heterodimeric (p66p51) active form for human immunodeficiency virus-1 reverse transcriptase (RT). Heterodimer stability was examined by sedimentation analysis as a function of temperature and ionic strength. Using NONLIN regression software, monomer-dimer-trimer and monomer-dimer-tetramer association models gave the best fit to the analytical ultracentrifuge sedimentation equilibrium data. The heterodimer is the predominant form of RT at 5 degrees C, with a dimerization Ka value of 5.2 x 10(5) M-1 for both models. Ka values of 2.1 x 10(5) and 3.8 x 10(5) M-1 were obtained for the respective association models at 20 degrees C. RT in 50 and 100 mM Tris, pH 7.0, completely dissociates at 37 degrees C and behaves as an ideal monomeric species. The dissociation of RT as a function of increasing temperature was also observed by measuring the decrease in sedimentation velocity (sw,20). If the stabilization of the heterodimer was due primarily to hydrophobic interactions we would anticipate an increase in the association from 21 degrees C to 37 degrees C. The opposite temperature dependence for the association of RT suggests that electrostatic and hydrogen bond interactions play an important role in stabilizing heterodimers. To examine the effect of ionic strength on p66p51 association we determined the changes in sw,20 as a function of NaCl concentration. There is a sharp decrease in sw,20 between 0.10 and 0.5 M NaCl, leading to apparent complete dissociation. The above results support a major role for electrostatic interactions in the stabilization of the RT heterodimer.

Cold Temperature↗
PubMed1993

Characterization of collagen-like peptides containing interruptions in the repeating Gly-X-Y sequence.

Glycine is found as every third residue along the entire length of triple helices in fibrillar collagens, but the triple-helix regions of nonfibrillar collagens and other proteins usually contain one or more interruptions in this repeating pattern. A set of four peptides was designed to model the effect of interruptions in the (Gly-X-Y)n repeating pattern on triple-helix formation, stability, and folding. Into the middle of the stable triple-helical peptide (Pro-Hyp-Gly)10, an interruption was introduced representing one of the four possible categories: a glycine deletion, a deletion of a hydroxyproline (Y position), an alanine insertion, or a glycine to alanine substitution. As shown by sedimentation equilibrium, NMR, and CD studies, the introduction of an interruption still allowed formation of trimers in solution, but with marked decrease in stability. The degree of destabilization and the thermodynamic basis for the loss of stability depended on the type of interruption. The glycine substitution and alanine insertion were the least disruptive, followed by the hydroxyproline deletion, with the glycine deletion being the most destabilizing. Our results suggest that the breaks in these peptides affect both the triple-helical conformation and the monomer conformation. These studies provide a basis for considering the structural and functional consequences of different kinds of interruptions in collagen.

Amino Acid Sequence↗
PubMed1991

Solution structure of neuronal bungarotoxin determined by two-dimensional NMR spectroscopy: sequence-specific assignments, secondary structure, and dimer formation.

The solution structure of neuronal bungarotoxin (nBgt) has been studied by using two-dimensional 1H NMR spectroscopy. Sequence-specific assignments for over 95% of the backbone resonances and 85% of the side-chain resonances have been made by using a series of two-dimensional spectra at four temperatures. From these assignments over 75% of the NOESY spectrum has been assigned, which has in turn provided 582 distance constraints. Twenty-seven coupling constants (NH-alpha CH) were determined from the COSY spectra, which have provided dihedral angle constraints. In addition, hydrogen exchange experiments have suggested the probable position of hydrogen bonds. The NOE constraints, dihedral angle constraints, and the rates of amide proton exchange suggest that a triple-stranded antiparallel beta sheet is the major component of secondary structure, which includes 25% of the amino acid residues. A number of NOE peaks were observed that were inconsistent with the antiparallel beta-sheet structure. Because we have confirmed by sedimentation equilibrium that nBgt exists as a dimer, we have reinterpreted these NOE constraints as intermolecular interactions. These constraints suggest that the dimer consists of a six-stranded antiparallel beta sheet (three from each monomer), with residues 55-59 forming the dimer interface.

Amino Acid Sequence↗