Amphipathic helix and its relationship to the interaction of calcitonin with phospholipids

Abstract

Salmon, porcine, and human calcitonins interact with phosphatidylglycerol to form water-soluble complexes, but these peptides do not interact with the zwitterionic lipids phosphatidylcholine or sphingomyelin. The calcitonins are more helical in the presence of dimyristoylphosphatidylglycerol than in its absence, but human calcitonin is considerably less helical than the other two, particularly in the presence of the lipid. This may explain the previously reported faster rate of degradation of human compared with salmon calcitonin in vivo. The ability of human calcitonin to solubilize dimyristoylphosphatidylglycerol and to alter the phase transition properties of this phospholipid while maintaining a low content of helix indicates that the presence of an amphipathic helix is not a requirement for these effects. The binding of salmon calcitonin to dimyristoylphosphatidylglycerol has been studied by determining the dependence of the circular dichroism properties of the peptide on the concentration of lipid. At 25 degrees C, salmon calcitonin binds to five molecules of dimyristoylphosphatidylglycerol with an affinity constant of 1 X 10(5) M-1. Little change in these parameters is observed at 38 degrees C, and the complex is stable over a wide range of temperatures both above and below the phase transition temperature. The rate of reaction of salmon calcitonin with dimyristoylphosphatidylglycerol is rapid at or above the phase transition temperature of the lipid but not at low temperatures. Salmon calcitonin also interacts with egg phosphatidylglycerol. These results demonstrate that salmon calcitonin can react with phosphatidylglycerol at or above its phase transition temperature to form complexes which are at least kinetically stable both above and below the phase transition temperature. Salmon calcitonin can solubilize mixtures of dimyristoylphosphatidylglycerol and dimyristoylphosphatidylcholine containing 25% or more of the former phospholipid. The helical content of the peptide in the presence of these lipid mixtures is dependent on the fraction of the lipid which is phosphatidylglycerol, with larger fractions of this lipid leading to the formation of a higher helical content. At 25% phosphatidylglycerol, salmon calcitonin can solubilize the lipid mixture without much increase in the helix content of the peptide, again demonstrating that an amphipathic helical structure is not required for the solubilization of phospholipids. Ionic bonding appears to be an important component in the binding of the cationic calcitonins to phospholipids. Salmon calcitonin binds to the acidic phospholipids phosphatidylinositol and phosphatidic acid, but not to zwitterionic phospholipids. In addition, high concentrations of NaCl cause the dissociation of the complex between salmon calcitonin and dimyristoylphosphatidylglycerol.(ABSTRACT TRUNCATED AT 400 WORDS)

Secondary structure of the pore‐forming colicin A and its C‐terminal fragment

“…As to the conformations involved in pore formation, although some membrane proteins such as matrix porin of E. coli outer membrane have been shown to adopt a P-sheet structure, there is much evidence that protein segments interacting with lipids are a-helical. Amphipathic a-helices were proposed for serum apolipoproteins [IS], inelittin [19], calcitonin [20] and hormones or synthetic analogs of hormones [21]. Also the N,N'-dicyclohexyl carbodiimide proteolipid of the H '…”

Structure/activity relationship of eel calcitonin

“…The first region, including the positions from 8 to 16, may contribute to keep the amphiphilic a-helix structure, which is essential for preserving the CT activity [8,. This speculation was supported in part by evidence that retention times of analogs K12-Kl6 on an analytical RP-HPLC, a parameter of content of the amphiphilicity [25,26], were much shorter than others except K25 (Fig.…”

Folding analysis of hormonal polypeptide calcitonins and the oxidized calcitonins using electrospray ionization mass spectrometry combined with H/D exchange

“…The ability to form ␣-helix in the presence of TFE has been reported to be ECT Ͼ salmonCT (sCT) Ͼ porcine CT (pCT) Ͼ human CT (hCT) [4 -7]. The presence of the ␣-helix in the central region of CTs has been reported to be important for the interaction with membrane lipids and for hypocalcemic activity [8]. Using SDS solution, the difference of ␣-helix content among species has correlated with the activity [4], and another report has suggested that conformational flexibility is also an important parameter for the activity [9].…”