Protein-phospholipid complex, physical

Lipids have several important functions in animal cells, which include serving as structural components of membranes and as a stored source of metabolic fuel (Griner et al., 1993). Eukaryotic cell membranes are composed of a complex array of proteins, phospholipids, sphingolipids, and cholesterol. The relative proportions and fatty acid composition of these components dictate the physical properties of membranes, such as fluidity, surface potential, microdomain structure, and permeability. This in turn regulates the localization and activity of membrane-associated proteins. Assembly of membranes necessitates the coordinate synthesis and catabolism of phospholipids, sterols, and sphingolipids to create the unique properties of a given cellular membrane. This must be an extremely complex process that requires coordination of multiple biosynthetic and degradative enzymes and lipid transport activities. 

Natural biological membranes consist of lipid bilayers, which typically comprise a complex mixture of phospholipids and sterol, along with embedded or surface associated proteins. The sterol cholesterol is an important component of animal cell membranes, which may consist of up to 50 mol% cholesterol. As cholesterol can significantly modify the bilayer physical properties, such as acyl-chain orientational order, model membranes containing cholesterol have been studied extensively. Spectroscopic and diffraction experiments reveal that cholesterol in a lipid-crystalline bilayer increases the orientational order of the lipid acyl-chains without substantially restricting the mobility of the lipid molecules. Cholesterol thickens a liquid-crystalline bilayer and increases the packing density of lipid acyl-chains in the plane of the bilayer in a way that has been referred to as a condensing effect. 

Although DSC and other physical techniques have made considerable contributions to the elucidation of the nature of lipid-protein interactions, several outstanding questions remain. For example, it remains to be dehnitively determined whether some integral, transmembrane proteins completely abolish the cooperative gel-to-liquid-crystalline phase transition of lipids with which they are in direct contact or whether only a partial abolition of this transition occurs, as is suggested by the studies of the interactions of the model transmembrane peptides with phospholipids bilayers (see above). The mechanism by which some integral, transmembrane proteins perturb the phase behavior of very large numbers of phospholipids also remains to be determined. Finally, the molecular basis of the complex and unusual behavior of proteins such as the concanavalin A receptor and the Acholeplasma laidlawii B ATPase is still obscure (see Reference 17). 

The structure, building up, and functions of diverse membranes of cells, organelles, and vesicles of all organisms concern purely physical-chemical issues, although they are quite complex and diverse. The essential structural components of membranes are the lipids in most cases, phospholipids usually contain proteins... 

At higher temperatures, the protein moiety of hpoproteins undergoes denaturation (Anton et al., 2000), which affects the structure and the surface properties. Lipids and phospholipids are liberated from their complexes with proteins so that they become better available for further reactions (e.g., extraction or oxidation), hi technological and cuhnary operations, fats and oils or their emulsions are often added to the material, they are mechanically dispersed, and hydrogen bonds between hpid and protein molecules are formed or rearranged. Other weak physical forces also play a role. The final effect depends on the particular technology employed and the composition of the food. 

A specific type of interaction between lipids and proteins is found in lipoproteins which transport triglycerides and cholesteryl esters in the plasma of mammalians. The largest lipoproteins, chylomicrons with a diameter between 800 A and 5000 A, and very-low-density lipoproteins (VLDL), with a diameter of 300-800 A, resemble emulsion droplets with a core of non-polar lipid and a surface coat of phospholipids and proteins (cf. Brown et ai, 1981). A physical characterization of chylomicrons has been reported (Parks et al.y 1981). Most of the plasma cholesterol occurs in low-density lipoprotein (LDL) which is a particle with a diameter of 200 A. The core consists of almost pure cholesteryl esters and a surface coat of a phospholipid monolayer and four tetrahedrally arranged apoproteins (Gulik-Krzywicki et aly 1979). The smallest particle, high-density lipoprotein (HDL), is a kind of molecular lipid-protein complex. 

Lipids exist in most foods as multiphased colloidal systems bound by surface-active phospholipids, proteins and emulsifiers. The oxidative stability of food lipids is greatly affected by the partitioning of the lipid substrates, metal initiators and antioxidants, which is complex and depends on the physical properties of the food. We may consider three types of food systems (see Chapter 10) ... 

Special fat-carrying proteins which are produced in the liver. Beta-lipoproteins are also called low density lipoproteins (LDL). Normally, they contain from 10 to 50% triglycerides, 22 to 45% cholesterol, 18 to 22% phospholipid, and 9 to 21% proteins. It is not desirable for one to have a high blood level of beta-lipoproteins because the cholesterol they carry may be deposited in the blood vessels. However, alpha-lipoproteins (HDL) convey cholesterol in a much more stable complex, so that there is little risk of cholesterol deposits when the blood levels of these proteins are elevated. It appears that the levels of beta- lipoproteins may be reduced, and those of alpha-lipoproteins raised by (1) loss of excess body weight, (2) strenuous physical exercise, (3) a diet low in animal fats and cholesterol, (4) drugs such as clofibrate and niacin (this vitamin must be administered in large doses to achieve the desired effects), and (5) moderate amounts of alcohol. 

The oligomycin-sensitive ATPase complex, a major enzyme of the mitochondrial inner membrane, can serve to illustrate some of these principles. The ATPase complex is a water-insoluble enzyme which spontaneously associates into vesicular membranes in the presence of phospholipids. The ultrastructural appearance of the reconstituted ATPase membranes is similar to that of the native inner mitochondrial membrane. The complex consists of at least 10 different subunit polypeptides which have been resolved into 3 components, each with a measurable function (1) Five of the polypeptides are part of a catalytic unit called Fi, a water-soluble ATPase which neither requires phospholipids for activity nor has any detectable capacity for binding phospholipids. (2) Four other polypeptides form a unit which has no presently known enzymatic function, but when combined with Fi modifies its physical and catalytic properties. The proteins of this unit are extremely insoluble in water and can combine with phospholipids to form membranes. One of the proteins of the membrane unit is characterized by an unusually large proportion of nonpolar amino acids and a high affinity for phospholipids. (3) The third component (OSCP) is a single polypeptide whose function is to link F to the hydrophobic membrane unit. In the purified state, this protein is completely water soluble. ... 

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