Lipids and Proteins Are Associated in Biological Membranes

Lipids are compounds that consist mostly of nonpolar groups. They have limited solubility in water, but dissolve freely in organic solvents. 

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Lipids are compounds that occur frequently in nature. They are found in places as diverse as egg yolks and the human nervous system and are an important component of plant, animal, and microbial membranes. The definition of a lipid is based on solubility. Lipids are marginally soluble (at best) in water but readily soluble in organic solvents, such as chloroform or acetone. 

Fats and oils are typical lipids in terms of their solubility, but that fact does not really define their chemical nature. In terms of chemistry, lipids are a mixed bag of compounds that share some properties based on structural similarities, mainly a preponderance of nonpolar groups. 

A fatty acid has a carboxyl group at the polar end and a hydrocarbon chain at the nonpolar tail. Fatty acids are amphipathic compounds because the carboxyl group is hydrophilic and the hydrocarbon tail is hydrophobic. The carboxyl group can ionize under the proper conditions. 

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Chapter 8 Lipids and Proteins Are Associated in Biological Membranes... 

There are other ways in which the lateral organization (and asymmetry) of lipids in biological membranes can be altered. Eor example, cholesterol can intercalate between the phospholipid fatty acid chains, its polar hydroxyl group associated with the polar head groups. In this manner, patches of cholesterol and phospholipids can form in an otherwise homogeneous sea of pure phospholipid. This lateral asymmetry can in turn affect the function of membrane proteins and enzymes. The lateral distribution of lipids in a membrane can also be affected by proteins in the membrane. Certain integral membrane proteins prefer associations with specific lipids. Proteins may select unsaturated lipid chains over saturated chains or may prefer a specific head group over others. 

If preparative or instrumental artifact is ruled out, the universal occurrence of red-shifted Cotton effects with a-helical character in all the membranes studied points to a common property of the proteins in biological membranes. The ORD results from lipid-free mitochondrial structural protein and erythrocyte ghost protein are consistent with assigning the red shift in these membranes to aggregated protein. It is, therefore, reasonable that similar protein-protein association may occur in all membranes. Ionic requirements for membrane stability could then reflect in part the requirements for protein-protein association. To some extent the molecular associations which stabilize membranes, therefore, may be protein-protein as well as lipid-lipid in nature. 

Studies have suggested that membrane proteins do not always undergo free diffusion in the membrane, and that they may be anchored by proteins of the cytoskelton, by the extracellular matrix or in other ways. Also, membranes are not uniform in their distribution of components, even on one side of the bilayer. They contain many specialized regions or domains, such as clathrin-coated pits, synapses in nerve cells, microvillae, and focal contacts. The lipid and protein composition of the domains are distinct and have been associated with specialized biological functions of the membrane. 

However, lipid bilayers are impermeable to ions and most polar molecules, with the exception of water, so they cannot, on their own, confer the multiple dynamic processes which we see in the function of biological membranes. All of this comes from proteins, inserted into the essentially inert backbone of the phospholipid bilayer (Figure 3.27), which mediate the multiple functions which we associate with biological membranes, such as molecular recognition by receptors, transport via pumps and channels, energy transduction, enzymes, and many more. Biomembranes are noncovalent assemblies of proteins and hpids, which can best be described as a fluid matrix, in which lipid (and protein molecules) can diffuse rapidly in the plane of the membrane, but not across it. 

The term structure, as used in chemistry and biology, relates both to molecules and to molecular aggregates. A simple example is provided by water. We can deal with the structure of the H2O molecules and also with the way in which these molecules are associated in solid ice and in liquid water. A more complicated example is a biological membrane, where we face the problem of the molecular structures of protein, lipid, and other molecules, and also of the way these molecules are aggregated in the membrane. In this chapter we are concerned with both of these problems, but more attention is given to the structures of individual molecules. We deal with some of the experimental methods used for investigating structure and with some of the structural information which has been accumulated. 

Leeder and Rippon [85] have analyzed the lipid composition of wool fibers after removing surface grease. Continued extraction with solvent removed the beta layers evidenced by electron microscopy however, the extract contained free cholesterol and free fatty acid and triglycerides but negligible quantities of phospholipid normally associated with biological membrane lipids. Koch [86], in his work with internal lipid of human hair, did not report significant quantities of phospholipid. These lipid-protein layers of hair are most likely related structurally to those of the epicuticle. 

One of the typical examples of such biomolecular interfaces is a membrane , a two-dimensional molecular array which forms the boundary of biological cells. These membranes are composed of lipids and proteins. Of these, the lipid molecules which are amphipathic (a molecule consisting of two parts, each of which has an affinity for a different phase), are considered to be responsible for maintaining the two-dimensional molecular structure the protein molecules, which perform a variety of biochemical functions, are often associated with (in/on) these lipid bilayer membranes (Figure 16). 

Surfactant molecules are amphiphilic and associate together in aqueous solution to form various structures micelles, microemulsions, vesicles, lyotropic liquid crystalline phases. In each case, their alkyl chains group together and their polar heads form a layer which separates them from the water. The laws governing this self-assembly involve subtle combinations of the two principles, order and mobility. Some fascinating illustrations are provided by the cell membrane in biological systems. In this case, order and mobility are related to the structure of functional units made up of lipids and proteins [6.10]. Such examples could only encourage chemists to carry out novel syntheses which would produce molecules capable of self-assembly. 

Biological membranes are organized structures that consist of phospholipids and proteins. They are responsible for the compartmentation of a wide variety of chemical activities in cells that are indispensable for life [1], Membrane proteins with different chemical properties and functions are associated with a double layer of phospholipids (see Fig. 1). A phospholipid molecule consists of a hydrophilic head, e.g. phosphatidylcholine, and two hydrophobic tails, which are long hydrocarbon chains. It is energetically favorable for phospholipid molecules in water to form a bilayer and hence a spherical micelle structure. The hydrophobic tails occupy the inside of the bilayer while the hydrophilic heads are directed outward and make contact with the surrounding water. Proteins associated with the outer part of a lipid bilayer are called peripheral membrane... 

The structure of the photosynthetic membrane in com m on with all biological membranes is considered to be a lipid bilayer matrix to which the various functional membrane proteins are associated. The form of the association can be intrinsic to the lipid component in which case there is presumed to be contact between the polypeptide chains of the proteins and the hydrophobic domain of the lipid. Other proteins are believed to be peripheral to the lipid bilayer and associate mainly through electrostatic interactions. 

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