beta-Endorphin: formation of alpha-helix in lipid solutions.

Abstract

Human 8-endorphin adopts a partial helical conformation in aqueous solutions of cerebroside sulfate, ganglioside GMI, phosphatidylserine, and phosphatidic acid, but not of cerebroside and phosphatidylcholine, as evidenced by circular dichroic spectra. Addition of Ca2+ to the peptide in cerebroside sulfate solution can break up the helix; at. 10 mM Ca2+ the peptide (12 ,M) essentially exists in an unordered form.For comparison, sheep ,-lipotropin in acidic cerebroside sulfate solution (pH < 4) also has a partial helical -conformation, whereas human adrenocorticotropin does not. The conformation of the complex between human ,Bendorphin and lipids may be related to the opiatelike function of this peptide hormone. The opiate receptor has not yet been isolated and purified, but many studies have indicated that anionic groups at the receptor site are probably involved in the opiate receptor interactions and the prime mode is an electrostatic interaction between the protonated nitrogen of the opiates such as morphine and the anionic groups at the binding site. The discovery of enkephalins (1) has stimulated much interest in this class of opioid peptides. Several related peptides termed endorphins were later found in mammalian tissues (2-8). Although their physiological function is not yet known, these peptides are reported to possess opiatelike activity in many in vitro experiments (2-7). However,only O3-endorphins are analgesic in in vivo experiments (9, 10). The structure-function relationship dictates that these peptides bound to the, cell membrane must assume some ordered conformation to elicit their biological activities.The amino acid sequence of human f-endorphin is (5)Tyr-Gly-Gly-Phe-Met-Thr-Ser -Glu-Lys-Ser-Gln-Thr-Pro-Leu-Val -Thr-Leu-Phe-Lys-Asn-Ala-Ile -Ile -Lys-Asn-Ala-Tyr-Lys-Lys-Gly-Glu (Camel (3-endorphin has His-27 and Gln-31 instead of Tyr and Glu). The sequence of camel 3-endorphin is identical with the COOH-terminal 31 amino acid residues of sheep f3-lipotropin (11,12). Both 3-endorphin and (3-lipotropin in methanol or sodium dodecyl sulfate solution were previously shown to have a helical structure that can be as much as one-half of either peptide molecule (13). We report here on the conformation of f-endorphin in several lipid solutions. These lipids not only fulfill the requirement of the electrostatic interaction but also simulate a hydrophobic environment in the membrane that is conducive to the formation of ordered structure in the peptide molecule. MATERIAL AND METHODSHuman 3-endorphin and corticotropin (ACTH) were synthesized as described (10,14 was purified from an n-butanol solution (15). The cerebroside sulfate liposomes were prepared by suspending the lipid (1.5-2 mM) in water. The mixed micelles of lipid and C16E13.5 in water were prepared by drying the lipid from a chloroform solution under reduced pressure and dissolving it in the surfactant with a flash-mixer (15). For molar ratios less than 5, the phospholipids were solubilized at 500C, and the solution remained clear upon ...

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

“…This type of behavior is well documented in several helical peptides [69] and it has been specifically observed that a unique feature of TM helices is an apparently higher content of Pro [70]. It is interesting to observe that formation of a helical segment in the sequence T 6 SEKSQT 12 , although compatible with some predictions [25,27,66,67], would lead to unfavorable interactions inside the receptor. That is, a helix with even a partially polar surface could not interact well with the very hydrophobic surfaces of the TM helices.…”

“…Introduction of restraints corresponding to only two intramolecular hydrogen bonds (those involving residues A 21 -L 17 and I 22 -F 18 ) in the DYANA torsion angle dynamics (TAD) procedure yielded a very regular h-helix from Pro 13 to Tyr 27 . In order to improve the quality of the structure determination we resorted first to restrained MM and finally to restrained MD simulated annealings on the ten best structures generated by DYANA.…”

“…Most early studies, both experimental [23][24][25][26][27][28][29][30][31][32][33] and predictive [25,27] pointed to the presence of helical segments but failed to identify a consensus helical stretch. CD spectroscopy studies in water found no significant sign of secondary structure [23] but studies in alcohol-water mixtures [23][24][25][26][27] or micelles [24,27,31] indicated a helical content varying from 0 to 50%. Previous conformational analyses of i-endorphin based on NMR include a study in water consistent with an essentially random coil conformation [28], a photo-CIDNP study in an aqueous solution of phospholipid micelles [29,30] that revealed limited accessibility of Tyr-27 and a study in a mixture of methanol and water (60:40, v/v) that identified two long helical stretches encompassing nearly the entire length of the molecule: between residues 1 and 12 and between residues 14 and 28 [33].…”

Conformational changes in β‐endorphin as studied by electrospray ionization mass spectrometry

“…Solutions that contain as little as 10% methanol and trifluoroethanol have been shown to induce helicity in BE 24. The preference for helicity has also been reported in the presence of sodium dodecyl sulfate23 and various lipids 26. However, some conflicting proposals have emerged from these studies with respect to the conformational states and the locations of the helix.…”

“…Previous work using other instrumental techniques has also provided evidence of secondary structure in BE 23–29. CD spectroscopy studies have shown that BE exists as a random coil in water, but it assumes as much as up to 50% α‐helical form or more in mixed solvent systems 23,.…”