Throughout the biological world, a 30 A hydrophobic film typically delimits the environments that serve as the margin between life and death for individual cells. Biochemical and biophysical findings have provided a detailed model of the composition and structure of membranes, which includes levels of dynamic organization both across the lipid bilayer (lipid asymmetry) and in the lateral dimension (lipid domains) of membranes. How do cells apply anabolic and catabolic enzymes, translocases and transporters, plus the intrinsic physical phase behaviour of lipids and their interactions with membrane proteins, to create the unique compositions and multiple functionalities of their individual membranes?

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Membrane lipids: where they are and how they behave.

Antimicrobial peptides are an abundant and diverse group of molecules that are produced by many tissues and cell types in a variety of invertebrate, plant and animal species. Their amino acid composition, amphipathicity, cationic charge and size allow them to attach to and insert into membrane bilayers to form pores by 'barrel-stave', 'carpet' or 'toroidal-pore' mechanisms. Although these models are helpful for defining mechanisms of antimicrobial peptide activity, their relevance to how peptides damage and kill microorganisms still need to be clarified. Recently, there has been speculation that transmembrane pore formation is not the only mechanism of microbial killing. In fact several observations suggest that translocated peptides can alter cytoplasmic membrane septum formation, inhibit cell-wall synthesis, inhibit nucleic-acid synthesis, inhibit protein synthesis or inhibit enzymatic activity. In this review the different models of antimicrobial-peptide-induced pore formation and cell killing are presented.

Antimicrobial peptides: pore formers or metabolic inhibitors in bacteria?

T e purpose of this review is to assess the nature and magnitudes of the dominant forces in protein folding. Since proteins are only marginally stable at room temperature,’ no type of molecular interaction is unimportant, and even small interactions can contribute significantly (positively or negatively) to stability (Alber, 1989a,b; Matthews, 1987a,b). However, the present review aims to identify only the largest forces that lead to the structural features of globular proteins: their extraordinary compactness, their core of nonpolar residues, and their considerable amounts of internal architecture. This review explores contributions to the free energy of folding arising from electrostatics (classical charge repulsions and ion pairing), hydrogen-bonding and van der Waals interactions, intrinsic propensities, and hydrophobic interactions. An earlier review by Kauzmann (1959) introduced the importance of hydrophobic interactions. His insights were particularly remarkable considering that he did not have the benefit of known protein structures, model studies, high-resolution calorimetry, mutational methods, or force-field or statistical mechanical results. The present review aims to provide a reassessment of the factors important for folding in light of current knowledge. Also considered here are the opposing forces, conformational entropy and electrostatics. The process of protein folding has been known for about 60 years. In 1902, Emil Fischer and Franz Hofmeister independently concluded that proteins were chains of covalently linked amino acids (Haschemeyer & Haschemeyer, 1973) but deeper understanding of protein structure and conformational change was hindered because of the difficulty in finding conditions for solubilization. Chick and Martin (191 1) were the first to discover the process of denaturation and to distinguish it from the process of aggregation. By 1925, the denaturation process was considered to be either hydrolysis of the peptide bond (Wu & Wu, 1925; Anson & Mirsky, 1925) or dehydration of the protein (Robertson, 1918). The view that protein denaturation was an unfolding process was

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Dominant forces in protein folding

Prediction of protein conformation.

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Principles Of Fluorescence Spectroscopy

Determinants of lipids acyl chain specificity: A tale of two enzymes.

The formation of an amphipathic helix in the central portion of calcitonin contributes to the potency of this hormone. We have synthesized a number of analogs of salmon calcitonin, containing deletions in the region of the peptide which is thought to form an amphipathic helix. There is no direct relationship between the hydrophobic moment of the helix and the biological activity of the peptide. For example, salmon des-Leu19-calcitonin and des-Ser13-calcitonin both have lower helical hydrophobic moments but have greater or equal biological potency compared with the native hormone. We suggest that other conformational features, such as flexibility and helix-forming potential, are also important in determining biological potency.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1432-1033.1986.tb09841.x

The hydrophobic moment of the amphipathic helix of salmon calcitonin and biological potency.

The low aqueous solubility of phospholipids makes necessary the use of lipid carriers in studies ranging from lipid traffic and metabolism to the engineering of model membranes bearing lipid transverse asymmetry. One particular lipid carrier that has proven to be particularly useful is methyl-β-cyclodextrin (MβCD). In order to assess the interaction of MβCD with structurally different phospholipids, the present work reports the results of isothermal titration calorimetry in conjunction with dynamic light scattering measurements. The results showed that the interaction of MβCD with large unilamellar vesicles composed of a single type of lipid led to the solubilization of the lipid vesicle and, consequently, the complexation of MβCD with the lipids. This interaction is dependent on the nature of the lipid headgroup with a preferable interaction with phosphatidylglycerol in comparison to phosphatidylcholine. It was also possible to show a role played by the phospholipid backbone on this interaction. In many ...

Thermodynamics of Methyl-β-cyclodextrin-Induced Lipid Vesicle Solubilization: Effect of Lipid Headgroup and Backbone.

Studies of membrane physical properties and their role in biological function.

Many molecular features contribute to the antimicrobial activity of peptides. One aspect that contributes to the antimicrobial activity of a peptide, in many cases, results from the fact that many antimicrobial peptides are polycationic and the lipids on the surface of bacteria are often anionic. In certain cases this can result in the clustering of anionic lipids as a result of the binding of the cationic peptide to the surface of the bacterial membrane. This lipid clustering can be detrimental to the viability of the bacteria to which the peptide binds. Several factors, including the charge, size, and conformational flexibility of the peptide, will determine the efficiency of lipid clustering. In addition, the lipid composition of the bacterial membrane is very variable, and it plays a critical role in this mechanism. As a result, one can test the importance of this factor by determining the species specificity of the antimicrobial activity of the peptide. The molecular mechanism by which lipid clustering affects bacterial viability is uncertain in many cases. This phenomenon can be used to increase the antimicrobial potency of peptides in some case and can also predict the bacterial species specificity of some agents.

Richard M. Epand

Papers

Determinants of lipids acyl chain specificity: A tale of two enzymes.

The hydrophobic moment of the amphipathic helix of salmon calcitonin and biological potency.

Thermodynamics of Methyl-β-cyclodextrin-Induced Lipid Vesicle Solubilization: Effect of Lipid Headgroup and Backbone.

Studies of membrane physical properties and their role in biological function.

Anionic Lipid Clustering Model