Studies on the conformation of beta-endorphin and its constituent fragments in water and trifluorethanol by CD spectroscopy

Solution structure of human beta-endorphin in helicogenic solvents: an NMR study

Beta-endorphin is the largest natural opioid peptide. The knowledge of its bioactive conformation might be very important for the indirect mapping of the active site of opioid receptors. We have studied beta-endorphin in a variety of solution conditions with the goal of testing the intrinsic tendency of its sequence to assume a regular fold. We ran NMR experiments in water, dimethylsulfoxide and aqueous mixtures of methanol, ethylene glycol, trifluoroethanol, hexafluoracetone trihydrate and dimethylsulfoxide. The solvent in which the peptide is more ordered is the hexafluoracetone trihydrate/water mixture. The helical structure detected for beta-endorphin in this mixture at 300 K extends for the greater part of its address domain, hinting at a possible mechanism of interaction with opioid receptors: a two-point attachment involving an interaction of the helical part of the address domain (PLVTLFKNAIIKNAY) with one of the transmembrane helices and a classical interaction of the message domain (YGGF) with the receptor subsite common to all opioid receptors.

Reversed-phase chromatographic behaviour of beta-endorphin: evidence of conformational change

The effect of alteration in isocratic mobile phase constituents, composition of sample solution, flow-rate and column temperature on the reversed-phase chromatographic behaviour of beta-endorphin was investigated. Beta-Endorphin was shown to be particularly sensitive to the concentration of organic modifier within the mobile phase. The relative contact area of beta-endorphin was demonstrated to be less than that of the much smaller molecule, gamma-endorphin, indicating that beta-endorphin is in a folded form under the mobile phase conditions utilised. Buffer molarity and pH were implicated in the conformational transition of beta-endorphin. In addition, the micro-environment of beta-endorphin prior to its injection onto the high-performance liquid chromatographic (HPLC) column is crucial to its chromatographic behaviour. Manipulation of the sample solvent environment produced reversible conformational modifications ultimately resulting in asymmetric and even split peaks. This phenomenon was more clearly seen when altering HPLC flow-rate. Elevation of HPLC column temperature provided additional evidence of structural change in beta-endorphin, with further conformational forms of this molecule being observed at higher temperatures. This work suggests that the chromatography of beta-endorphin involves a complex mechanism of separation which cannot be adequately explained by the two-state model of kinetic processes.

Importance of the membrane in ligand-receptor interactions

NMR data that underscore the importance of the membrane in ligand-receptor interactions were obtained and analyzed. The following hypothesis for acetylcholine (ACh) binding to the acetylcholine receptor (AChR) is proposed: ACh first binds to the membrane, where it adopts its bioactive conformation, and it then rapidly diffuses along the membrane to bind to the AChR in its already-correct conformation. Data used to support this hypothesis include (a) the NMR-determined binding constant of KM = (2.8 +/- 0.6) x 10(3) M-1 for the binding of ACh to the asolectin membrane, (b) the lipid dependence of AChR activity, (c) the location of the ACh binding site close to the membrane surface, and (d) the conformation of ACh in its membrane-bound state. Additional experiments to test this hypothesis are proposed.

Circular dichroism and proteolysis of human beta-endorphin in surfactant and lipid solutions

The influence of micelles of sodium dodecyl sulfate, cetyltrimethylammonium bromide, lysophosphatidylcholine and dodecylphosphorylcholine on the content and stability of the ordered structure of human beta-endorphin and its 12-26 fragment has been investigated. The structure was determined by far-ultraviolet circular dichroism and the stability by the resistance of the polypeptide to proteolysis with trypsin and chymotrypsin, monitored by HPLC. The alpha-helix inducing effects of the amphipathic compounds were in the order anionic greater than zwitterionic greater than cationic. The protection against proteolysis was very marked, especially for trypsin, and it was proportional to the alpha-helix inducing potential of amphipathic compounds. However, the lower resistance to proteolysis of the highly structured 12-26 fragment suggests that factors other than secondary structure may be responsible for the resistance to proteolysis.

Secondary structure determination of human beta-endorphin by 1H NMR spectroscopy

The 1H NMR spectra of human beta-endorphin indicate that the peptide exists in random-coil form in aqueous solution but becomes helical in mixed solvent. Thermal denaturation NMR experiments show that in water there is no transition between 24 and 75 degrees C, while a slow noncooperative thermal unfolding is observed in a 60% methanol-40% water mixed solvent in the same temperature range. These findings are consistent with circular dichroism studies by other workers concluding that beta-endorphin is a random coil in water but that it forms 50% alpha-helix or more in mixed solvents. The peptide in the mixed water-methanol solvent was further studied by correlated spectroscopy (COSY) and nuclear Overhauser effect spectroscopy (NOESY) experiments. These allow a complete set of assignments to be made and establish two distinct stretches over which the solvent induces formation of alpha-helices: the first occurs between Tyr-1 and Thr-12 and the second between Leu-14 and extending to Lys-28. There is evidence that the latter is capped by a turn occurring between Lys-28 and Glu-31. These helices form at the enkephalin receptor binding site, which is at the amino terminus, and at the morphine receptor binding site, located at the carboxyl terminus [Li, C. H. (1982) Cell (Cambridge, Mass.) 31, 504-505]. Our findings suggest that these two receptors may specifically recognize alpha-helices.

Interaction of beta-endorphin with sodium dodecyl sulfate in aqueous solution. 1H-NMR investigation

1H-NMR spectra at 600 MHz and 270 MHz and photo-chemically induced dynamic nuclear polarization (photo-CIDNP) spectra at 360 MHz of beta-endorphin in the presence of sodium dodecyl sulfate (SDS) micelles are reported and discussed in terms of structural changes and immobilization upon binding. On addition of micelles several NH protons show slow H-D exchange rate in 2H2O at p2H 4.6, 25 degrees C, which indicates that some regions of the polypeptide are buried and shielded from the direct interaction with the solvent. Moreover, in the methyl region of the spectrum strong changes are detected both in chemical shifts and line-widths, suggesting an appreciable interaction between the hydrophobic residues of beta-endorphin and the detergent micelles. All aromatic residues are strongly affected by the presence of SDS, supporting the notion that beta-endorphin can interact with both the hydrophobic and hydrophilic portion of the micelles. At physiological pH photo-CIDNP experiments indicate that SDS has about the same immobilizing effect on Tyr-1 and Tyr-27 as n-dodecylphosphorylcholine.

A classification of peptide ligands based on their predicted receptor conformation

Peptides containing fewer than 50 amino acids show little ordered structure under physiological conditions. In this paper it is shown that in the receptor environment, secondary structure could be induced in small peptides that involves 87% of all the amino acid residues. The statistical methods of Chou and Fasman are used to predict the conformation of 41 peptide hormones or neuromodulators in the proteinaceous environment of the receptor, and four distinct conformational groupings are elucidated. beta-bend, beta-structure and alpha-helical conformation are possible for distinct groups of linear peptides, and disulfide bridge containing peptides show a common beta-bend beta-structure conformation at the receptor. In the predicted receptor conformation, the peptides show hydrophobic and hydrophilic domains that must reflect the distribution of corresponding regions in the ligand-binding site of the receptor. The predicted ligand conformation should allow a more rational approach to interpreting existing structure activity studies and the design of new analogs of pharmacological interest.

Investigation by photochemically-induced dynamic nuclear polarization and nuclear Overhauser enhancement 1H-NMR of the interaction between beta-endorphin and phospholipid micelles

The photochemically-induced dynamic nuclear polarization technique has been used to investigate the access of a photoexcited flavin dye to tyrosyl and histidyl residues in [Met]enkephalin and human and camel beta-endorphins, both alone and in the presence of n-dodecylphosphorylcholine micelles. The results indicate that the mode of binding of Tyr-1, but not of residue 27, is similar in the two endorphins and differs from that of Tyr-1 in [Met]enkephalin. In human beta-endorphin, accessibility and mobility of Tyr-27 are strongly reduced in the presence of lipid at physiological pH, whereas in camel beta-endorphin His-27 becomes immobilized only at high pH. Moreover, nuclear Overhauser enhancement experiments suggest a rigidifying influence of the peptide on the polar head groups of the micelles.

beta-Endorphin: demonstration of a tertiary structure in aqueous solution

Difference spectra generated during thermolysin digestion of camel beta-endorphin at pH 8.2 or at pH 6.5 indicate rapid blue shifting of the near-UV absorption bands of the NH2-terminal tyrosine. A similar spectral change is not observed for the NH2-terminal tyrosine in [Met]enkephalin when it is digested under similar conditions. These results suggest that enzymatic digestion destroys or alters some structural interaction between the NH2-terminal tyrosyl residue of the endorphin and a residue(s) within the COOH-terminal segment of the molecule. Peptide mapping of the digest as a function of time suggests that cleavage of the bond linking the alanine-21 and isoleucine-22 residues produces most of the observed effect. These data provide evidence for the existence of a tertiary structure for beta-endorphin in aqueous solutions.

Isolation of the C-fragment and C'-fragment of lipotropin from pig pituitary and C-fragment from brain

Three novel peptides derived from lipotropin, the C-Fragment (residues 61-91), C'-Fragment (61-87) and N-Fragment (1-38), were isolated from pig pituitary, and the C-Fragment was shown to be present in brain. The experimental procedures developed for their isolation are described. The formation of each of the fragments involves enzymic cleavage of lipotropin at consecutive basic residues, with specificity identical with that involved in the activation of known prohormones. In brain assays C-Fragment exhibits a range of biological activities related to its ability to act as an inhibitory neurotransmitter.