Water interactions with lipids

The intracellular and plasma membranes have a complex structure. The main components of a membrane are lipids (or phospholipids) and different proteins. Lipids are fatlike substances representing the esters of one di- or trivalent alcohol and two aliphatic fatty acid molecules (with 14 to 24 carbon atoms). In phospholipids, phosphoric acid residues, -0-P0(0 )-O-, are located close to the ester links, -C0-0-. The lipid or phospholipid molecules have the form of a compact polar head (the ester and phosphate groups) and two parallel, long nonpolar tails (the hydrocarbon chains of the fatty acids). The polar head is hydrophihc and readily interacts with water the hydrocarbon tails to the... 

To move through the membrane (change sides or transverse diffusion), a molecule must be able to pass through the hydrophobic portion of the lipid bilayer. For ions and proteins, this means that they must lose their interactions with water (desolvation). Because this is extremely difficult, ions and proteins do not move through membranes by themselves. Small molecules such as C02, NH3 (but not NH ). and water can diffuse through membranes however, most other small molecules pass through the lipid bilayer very slowly, if at all. This permeability barrier means that cells must develop mechanisms to move molecules from one side of the membrane to the other. 

The one and only distinctive feature of lipids is that they are all totally hydrophobic (water fearing), or nearly so. They generally will not chemically interact with water and therefore will not dissolve in water. Chemically, lipids fit into several categories, each of which is structurally unique. Common types of lipids include triacylglycerols, phospholipids, and steroids. 

The bilayer is composed of amphipathic lipid molecules oriented according to their preferences for interaction with water. 

Nonpolar (hydrophobic) compounds dissolve poorly in water they cannot hydrogen-bond with the solvent, and their presence forces an energetically unfavorable ordering of water molecules at their hydrophobic surfaces. To minimize the surface exposed to water, nonpolar compounds such as lipids form aggregates (micelles) in which the hydrophobic moieties are sequestered in the interior, associating through hydrophobic interactions, and only the more polar moieties interact with water. 

The structures of the various lipoproteins appear to be similar (figs. 20.11 and 20.12). Each of the lipoprotein classes contains a neutral lipid core composed of triacylglycerol and/or cholesteryl ester. Around this core is a coat of protein, phospholipid, and cholesterol, with the polar portions oriented toward the surface of the lipoprotein and the hydro-phobic parts associated with the neutral lipid core. The hydrophilic surface interacts with water in plasma, promoting the solubility of the lipoprotein. 

The majority of publications on cuticular lipids involve analyses of lipid composition. Which compounds are present, and what is their function Correlations between lipid composition and water-loss have provided indirect tests of the phase transition hypothesis, under the assumption that changes in lipid composition predictably affect lipid properties. In this section, we summarize available information on how specific structural changes affect the physical properties of pure surface lipids, as well as how different lipids interact with each other. 

A cell membrane is a fluid mosaic of lipids and proteins. Phosphoglycerides are the major membrane lipids that form a bilayer with their hydrophilic head groups interacting with water on both the extracellular and intracellular surfaces, and their hydrophobic fatty acyl chains in the central and hydrophobic regions of the membrane. Peripheral proteins are embedded at the periphery, while integral proteins span from one side to the other. Biomembranes separate the contents of the cell from the external environment. 

It is natural to classify lipids as polar or non-polar according to their interaction with water. Non-polar lipids, for example triglyceride oils, do not form aqueous phases, whereas polar lipids do. Except for cholesterol, membrane-forming lipids form aqueous phases and have polar head groups. Within membranes there are also trace amounts of lipids in membranes that do not interact with water, for example diacylglycerols. The structural formulae of two common membrane lipids, phosphatidylcholine (PC) and phosphatidylethanolamine (PE) are shown above. 

I. Lipid molecules have no functional groups, so they do not interact with water. 

The most important emulsifiers are polar lipids which interact with water to form liquid crystalline structures. These are often three dimensional stmctures which can assume different geometric configurations. The most active is referred to as the layered or lamellar phase. Figure 3.43 shows the lamellar form as a two dimensional structure. 

FIGURE 7. Illustration of the floating peroxyl radical theory, where the lipid chain (LH = dihneloyl phosphatidyl chohne) bearing the polar peroxyl radical migrates towards the polar surface of the liposome, where it can interact with water-soluble antioxidants like Trolox... 

Interactions of phospholipids in an aqueous medium and the formation of liposome vesicle. Phospholipids spontaneously form lipid bilayers in which the polar head groups interact with water, whereas the hydrophobic tails interact among themselves to form an environment that excludes water. The lipid bilayers are stabilized by noncovalent interactions. 

Insoluble nonswelling amphiphiles (H20). Lipids in this group have sufficient polarity to orient themselves at an air-water or air-oil interface, but do not interact with water sufficiently to dissolve in aqueous solutions. Examples of interest are cholesterol, triglyceride, the fat-soluble vitamins, and un-ionized fatty acids. 

The main stabilizing feature of biological membranes is hydrophobic interactions among the molecules in the lipid bilayer. The phospholipids in the lipid bilayer orient themselves so that their polar head groups interact with water. Proteins in the lipid bilayer interact favorably in their hydrophobic milieu because they typically have hydrophobic amino acid residues on their outer surfaces. 

Cholesterol and its derivatives constitute the third important class of membrane lipids, the steroids. The basic structure of steroids is a four-ring hydrocarbon. Cholesterol, the major steroidal constituent of animal tissues, has a hydroxyl substituent on one ring (Figure 5-5c). Although cholesterol is almost entirely hydrocarbon in composition, it is amphlpathlc because its hydroxyl group can interact with water. Cholesterol is especially abundant in the plasma membranes of mammalian cells but is absent from most prokaryotic cells. As much as 30-50 percent of the lipids in plant plasma membranes consist of certain steroids unique to plants. 

Biological membranes are lipid bilayers in which the hydrophobic hydrocarbon tails are packed in the center of the bilayer and the ionic head groups are exposed on the surface to interact with water (Figure 18.11). The hydrocarbon tails of membrane phospholipids provide a thin shell of nonpolar material that prevents mixing of molecules on either side. The nonpolar tails of membrane phospholipids thus provide a barrier between the interior of the cell and its surroundings. The polar heads of lipids are exposed to water, and they are highly solvated. Little exchange, known colloquially as "flip-flop," occurs between lipids on the outer and... 

The matrix that defines a biological membrane is a lipid bilayer composed of a hydrophobic core excluded from water and an ionic surface that interacts with water and defines the... 

Polar lipids are amphipathic in nature containing both hydrophobic domains, which do not interact with water, and hydrophilic domains, which readily interact with water. The basic... 

A specific phospholipid requirement has been determined for optimum in vitro reconstitution of function for more than 50 membrane proteins. If one considers specific lipid requirements for membrane association and activation of amphitropic proteins, the number is in hundreds. Integral membrane proteins fold and exist in a very complex environment and have three modes of interaction with their environment. The extramembrane domains are exposed to the water milieu, where they interact with water, solutes, ions, and water-soluble proteins. Part of the protein is exposed to the hydrophobic-aqueous interface region (Fig. 9). The remainder of the protein is buried within the approximately 30-A thick hydrophobic interior of the membrane. Amphitropic proteins may spend part of their time completely in the cytosol and are recruited to the membrane surface, or even partially inserted into the membrane, in response to various signals. 

---