SynopsisThe CD of glucagon, secretin, and vasoactive intestinal peptide has been studied as a function of temperature in water and in aqueous solutions of dodecyl sulfate, phosphatidyl glycerol, and L-a-phosphatidic acid (dipalmitoyl). The anionic detergent and lipids induce helix formation in all three peptides, with the amount of induced helical content increasing in the order glucagon < secretin < vasoactive intestinal peptide. These observations are subject to quantitative rationalization using a matrix formulation for the configuration partition function. In this formulation the major conformational consequences of the interaction with anionic lipids or detergents is an increase in the probability for helix formation by arginyl, histidyl, and lysyl residues. The region in which helix formation is maximal is found to he a t amino acid residues 13-20 in all three peptides. Other studies have implicated this portion of the polypeptide chain in receptor binding. Thus, the helical segment induced by interaction with anionic lipids may play an important physiological role.The gastrointestinal hormones glucagon,' s e~r e t i n ,~.~ and vasoactive intestinal p e~t i d e * ?~ have amino acid sequences ( Fig. 1) that suggest they are derived from a common ancestor.6-s Similarities are not confined to their covalent structures. While each peptide displays certain unique a c t i~n s ,~-l~ they also have many biological activities in common.12 One area of great current interest is their potential involvement as neurotransmitters.None of these peptides is highly ordered in a dilute aqueous solution. The present study is also of interest from a purely theoretical point of view. While the amino acid sequences of glucagon, secretin, and vasoactive intestinal peptide exhibit extensive homology, those differences which do occur are responsible for the prediction that these peptides should show significantly different helical content in the presence of anionic lipids. If the configuration partition function is formulated in the manner used earlier for other peptides and proteins,l3-I6 the helical content in the presence of an anionic lipid is predicted to increase in the order glucagon < secretin < vasoactive intestinal peptide. Thus, the behavior of these peptides provides a sensitive test for the validity of assumptions used in the treatment of processes by which anionic lipids induce order in polypeptides and proteins. Glucagon H S O G T -F T S D EXPERIMENTALMaterials were purchased from the following sources: vasoactive intestinal peptide from Vega Biochemicals; secretin from United States Biochemical Corporation; glucagon, L-cr-phosphatidic acid (dipalmitoyl), and L-a-lysophosphatidylcholine (palmitoyl) from Sigma Chemical Company; phosphatidyl glycerol from Calbiochem; and sodium dodecyl sulfate from Matheson, Coleman, and Bell Manufacturing Chemists.CD measurements were conducted on protein solutions prepared by dilution of a stock solution with distilled deionized water. The pH was measured with a Beckman 3500 pH meter...

SynopsisConformational properties have been examined for three proteins which are disordered when dissolved in water but become partially ordered in the presence of anionic lipids. The three proteins, which play important roles in the central nervous system, are myelin basic protein, P-endorphin, and P-lipotropin. When evaluated using matrix methods, the helical content of each protein is predicted to be vanishingly small in water, in agreement with experiment. Unperturbed root-mean-square radii of gyration are also evaluated for these proteins in water using generator matrices, which have seen wide application to synthetic polymers. Agreement between computed and measured dimensions is found to be excellent. Having successfully described the conformations of myelin basic protein, P-endorphin, and 0-lipotropin in water, attention is then directed to the changes induced upon interaction with anionic lipids or detergents. Computations predict an increase in helical content, with numerical results being in quite good agreement with experimental observations using several anionic lipids. Examination of the helix-propagation-probability profiles reveals an interesting feature of regions where this probability is high. When folded into a a-helix, these regions show one surface where the only side chains are hydrophobic. Charged side chains (with positive charges predominating) are found on the other surface of the helical segment. The arrangement of side chains on these helices is thus well suited to promote favorable interactions with a membrane containing anionic lipids. Examples of the occurrence of these helices are provided by amino acid residues 13-25 and 130-157 in myelin basic protein and residues 17-29 in &endorphin.Several proteins of intense current interest are essentially disordered in water but become partially ordered in the presence of anionic lipids. The helical content of these proteins in water is poorly described1,2 by popular empirical predictive method^^-^ based on the measured properties of globular proteins in the crystalline state. That such is the case is hardly surprising. In the disordered state each amino acid residue is exposed to solvent, while in the globular state many amino acid residues are shielded from the solvent. Since the helix-forming tendency of the various amino acid residues is well known to be solvent dependent, it is to be expected that any empirical method designed to treat globular proteins will fail if applied t o proteins which are substantially disordered.

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SynopsisA configuration partition function, which incorporates concepts embodied in the amphipathic helix hypothesis, has been formulated for a polypeptide in the presence of zwitterionic phospholipid. An enhanced probability is assigned to helix formation in any region of the polypeptide chain where side chains bearing charges of opposite sign will be situated on the same side of the a-helix but displaced from one another by one turn. This situation will arise when residues i -4 (or i -3) and i bear charges of opposite sign and residues i -4 (or i -3) through i are in a helical state. Illustrative calculations are performed for polypeptide chains in which the generalized nonionic amino acid residue serving as host has Zimm-Bragg parameters of o = lo-*, s = l. These calculations define conditions under which two interacting charged pairs can cooperate in a synergistic helix augmentation even when the two pairs are separated by significantly more than four generalized nonionic amino acid residues. Furthermore, the two interacting charged pairs, as well as the intervening amino acid residues, may become helical as one unit. Significant augmentation in helicity is observed with plausible values for the enhanced probability assigned to helix formation for an interacting pair. This model predicts correctly that glucagon and secretin, but not vasoactive intestinal peptide, undergo a coil-to-helix transition in the presence of zwitterionic phospholipid. This prediction is made with plausible values for the parameter used to express the helicity enhancement. The experimental observation with zwitterionic phospholipids is the direct opposite of that seen for these three peptides in the presence of anionic lipids and detergents. In anionic lipids the amount of induced helicity is in the following order: glucagon < secretin < vasoactive intestinal peptide. Results obtained with these three peptides demonstrate that the nature of the head group of the lipid is important for lipid-protein interaction and that the resulting conformational changes can be rationalized by matrix methods.