Presence Of TFE Research Articles

Conformational Properties of Five Peptides Corresponding to the Entire Sequence of Glutathione Transferase Domain II

Five peptides matching the helices α4, α5, α6, α7, and α8, spanning the entire sequence of domain II of pGSTP1-1, have been synthesized and their conformations analyzed by far-UV CD spectroscopy. The results show that a5, a7, and a8 peptides are unstructured in water/2,2,2-trifluoroethanol (TFE) solutions. The a4-peptide also adopts random conformations in aqueous solvent. Moreover, the relative low helical content (20%), estimated for this peptide in the presence of 30% (v/v) TFE, suggests that the sequence of this protein fragment does not possess sufficient information for a strong helical propensity. On the contrary, the synthesized a6 peptide, in the presence of TFE, showed a relevant structural autonomy with a helical content (41%) which was significantly higher than that estimated, under the same conditions, for all other peptides. More in general in the presence of solvents less polar than water, the isolated a6 peptide shows the same helical conformation adopted by the corresponding α6-helix in the hydrophobic core of the protein. A n-capping box motif, strictly conserved at the N-terminal of the α6-helix of all GST and related protein including eucaryotic translation elongation factor (EF1γ) and the yeast prion protein Ure2, plays an important role in the α-helix nucleation and stability of this protein fragment. The results suggest that the α6-helix might represent a nucleation site of GST folding and that the helical conformation of this region of the protein is an important requirement during earlier events of GST refolding.

Observation of the closing of individual hydrogen bonds during TFE-induced helix formation in a peptide.

Helix formation of an S-peptide analog, comprising the first 20 residues of Ribonuclease A and two additional N-terminal residues, was studied by measuring hydrogen bond (H-bond) (h3)J(NC') scalar couplings as a function of 2,2,2-trifluoroethanol (TFE) concentration. The (h3)J(NC') couplings give direct evidence for the closing of individual backbone N-H***O = C H-bonds during the TFE-induced formation of secondary structure. Whereas no (h3)J(NC') correlations could be detected without TFE, alpha-helical (i,i +4) H-bond correlations were observed for the amides of residues A5 to M15 in the presence of TFE. The analysis of individual coupling constants indicates that alpha-helix formation starts at the center of the S-peptide around residue E11 and proceeds gradually from there to both peptide ends as the TFE concentration is increased. At 60% to 90% TFE, well-formed alpha-helical H-bonds were observed for the amides hydrogens of residues K9 to Q13, whereas H-bonds of residues T5 to A8, H14, and M15 are affected by fraying. No intramolecular backbone H-bonds are present at and beyond the putative helix stop signal D16. As the (h3)J(NC') constants represent ensemble averages and the dependence of (h3)J(NC') on H-bond lengths is very steep, the size of the individual (h3)J(NC') coupling constants can be used as a measure for the population of a closed H-bond. These individual populations are in agreement with results derived from the Lifson-Roig theory for coil-to-helix transitions. The present work shows that the closing of individual H-bonds during TFE-induced helix formation can be monitored by changes in the size of H-bond scalar couplings.

Structure analysis of a fusogenic peptide sequence from the sea urchin fertilization protein bindin.

The structure of "B18", an 18-residue fusogenic peptide from the sea urchin fertilization protein bindin, was investigated in several membrane-mimicking environments with circular dichroism and nuclear magnetic resonance spectroscopy. The fully conserved peptide sequence represents the minimal functional part of the 24 kDa protein, which can bind to membranes and induce fusion of lipid vesicles. The B18 peptide undergoes a coil-helix transition in the presence of TFE, showing a transient tendency to self-associate. Its NMR structure in 30% TFE exhibits two helical regions at either side, connected by a flexible loop. In DPC and SDS detergent micelles, this loop becomes distinctly bent, presumably due to the high degree of curvature of the micelles. The loop contains a histidine-rich motif for binding zinc, which is required for the fusogenic function of the peptide. Therefore, we monitored the structural response of B18 and of recombinant bindin toward this ion. Like TFE, and in a mutually cooperative manner, zinc induces a partially helical structure in both the peptide and the protein. Complex formation via the histidine residues rigidifies the flexible loop and is accompanied by self-association of the molecules. The data suggest that the zinc-bound functional state is a continuous amphipathic alpha-helix, bearing some resemblance to a leucine zipper. Two hydrophobic patches on one face could favorably penetrate into a membrane, while two arginines on the other face could interact with lipid phosphate groups. The three-dimensional model of the B18 sequence thus contributes to a better understanding of peptide-induced vesicle fusion in general, and of the lipid-protein interactions of sperm bindin in particular.

Effect of lipids on the conformation of an antigenic determinant for experimental autoimmune neuritis.

The conformation of SP-26, the synthetic peptide (residues 53-78) of myelin P2 protein that causes experimental autoimmune neuritis (EAN) in the peripheral nervous system, has been studied in lipid environments and in water/trifluoroethanol (TFE) mixture using circular dichroism (CD) spectroscopy and Fourier transform infra-red (FTIR) spectroscopy. SP-26 becomes more structured on binding with micelles formed from anionic lipids and in the presence of TFE. On the other hand, zwitterionic lipids have a denaturing effect or no effect on the conformation of SP-26. The present results suggest that the disease-modulating effects of various lipids are closely related to their ability to induce conformational change in the antigenic region of P2 protein.

Nonnative Capping Structure Initiates Helix Folding in an Annexin I Fragment. A 1H NMR Conformational Study

A 21-residue peptide, P1AQFD5ADELR10AAMKG15LGTDE20D, corresponding to the (helix A)-loop motif of the second repeat of human annexin I, was synthesized and studied by 2D proton NMR. The conformational properties of the peptide were characterized at different temperatures in pure aqueous solution and in a TFE/H2O (1:4 v/v) mixture. In pure aqueous solution, the peptide adopts a preferred conformation, comprising both elements of native and nonnative structures. A high alpha helix content is present in the DADELRA segment, which corresponds to an initiation site in the middle of the native alpha helix sequence. At the N-terminus flanking region, a particular nonnative folding is revealed by the J(NH-CH alpha) coupling constants and a set of unusual NOE connectivities which correspond to a helix interrupt at the first D residue. Addition of relatively small amount of TFE restores the native helix fold at the C-terminus but not at the N-terminus. On the contrary, the nonnative N-terminus structure is clearly stabilized by TFE. Our data indicate that this structure comprises (i) an Asp5-x-x-Glu8 N-terminal capping box, as recently named by Harper and Rose [Harper, E. T., & Rose, G. D. (1993) Biochemistry 32, 7605-7609], (ii) a (i,i + 3) Asp7-x-x-Arg10 salt bridge, and (iii) a hydrophobic cluster centered on Phe4 which mainly interacts with Leu9 but also with Ala2, Ala6, and Ala12 in a dynamic way. This structure is rather stable since it is still observed at 293 K in aqueous solution and 313 K in the presence of TFE. It constitutes a very potent initiation site of the alpha helix structure. This is, however, a nonnative structure involving highly conserved residues in the whole annexin family and thus may play an important role in the folding pathway as a transient "compacting helper".

A single-stranded amphipathic .alpha.-helix in aqueous solution: Design, structural characterization, and its application for determining .alpha.-helical propensities of amino acids

In order to investigate the positional effect of alpha-helical propensities of amino acids in an amphipathic alpha-helix, an amphipathic alpha-helical model peptide (Ac-Glu-Ala-Glu-Lys-Ala-Ala-Lys-Glu-Ala-Glu-Lys-Ala-Ala-Lys-Glu-Ala-Glu-Lys- amide) was designed and characterized by circular dichroism and 2D-NMR spectroscopies. This peptide contains 65% alpha-helical structure in solution, and its monomeric molecular weight in aqueous solution was determined by size-exclusion chromatography. The independence of alpha-helical structure and stability on peptide concentration demonstrates that helix formation of this peptide is a monomolecular process. To compare the effect of substitutions in the hydrophobic and hydrophilic face of the helix on structure and stability, a leucine, alanine, or glycine was individually substituted in the hydrophobic face (position 9) or hydrophilic face (position 7) of the model peptide. The change in helix content and stability upon substitution was measured by circular dichroism studies in the absence and presence of TFE or urea. The results indicate that each amino acid has a different helix propensity when it is located in the hydrophobic face versus hydrophilic face and the effect of substitution is more significant in the hydrophobic face. This single-stranded amphipathic alpha-helical peptide provides an appropriate model system to determine helix propensities of amino acids on both hydrophobic and hydrophilic faces.