Lipoproteins molecules

The Gram-negative cell envelope (Fig. 1.4) is even more complicated essentially, it contains lipoprotein molecules attached covalently to the oligosaccharide backbone and in addition, on its outer side, a layer of lipopolysaccharide (LPS) and protein attached by hydrophobic interactions and divalent metal cations, Ca and Mg. On the inner side is a layer of phospholipid (PL). 

The fatty acid chains are evidently embedded in the outer membrane as an anchor. About one-third of the lipoprotein molecules are attached covalently to the peptidoglycan through an amide linkage between the side chain amino group of the C-terminal lysine of the protein and a diaminopimelic acid residue of the peptidoglycan (Fig. 8-29). Thus, the protein replaces one of the terminal D-alanine residues of about one in ten of the murein peptides. There are 2.5 x 105 molecules of the bound form of the lipoprotein per cell spread over a surface area of peptidoglycan of 3 pm2. They appear to be associated as trimers located primarily in the periplasmic space.589... 

A similar type of interaction is thought to occur in membrane lipoprotein molecules. The problem in the latter studies is that the membrane apoproteins are not easily solubilized. If further information on the structure of biological membranes is required, then it is recommended that a recent book by Petty (1993) and one edited by Wirtz et al. (1993) be placed on a must reading list. An older, but very good, short review on lipid-protein interaction possibilities in membranes is one presented by Danielli (1982), who is widely recognized as a pioneer as well as a legend in this field. 

Fig. 6.3. Micelles, liposomes and cell membranes. Micelles are collections of lipid molecules that are relatively nonpolar internally and polar externally. This arrangement allows relative water-solubility of the micelle as a whole. Liposomes contain lipid molecules in a bilayer. They may be used as artificial vehicles for trapping and delivery of drugs to specific tissues. They are also useful as models of cell surfiice function. A real cell membrane is not only a lipid bilaycr, but also includes proteins, glycoproteins, glycolipids, and lipoprotein molecules. The glyco attachments on the outer surface may be important in labeling cells with specific cell-surfece properties.

Generalized structure of a lipoprotein molecule showing the distribution of polar components in an outer shell composed of free cholesterol, phospholipids, and amphipathic proteins and in an inner core composed of neutral triacylglycerols and cholesteryl esters. Phospholipids are oriented with polar head groups toward the aqueous environment and hydrophobic tails toward the neutral core, analogous to their positioning in the outer leaflet of the typical cell membrane. 

Snellman, Sylven and Julen isolated the heparin polypeptide and showed that this material is a potent antithrombin on thrombin with purified fibrinogen, suggesting that heparin in the mast cells is in the active anticoagulant form. Electrophoresis shows that this native heparin forms a complex compound with thrombin and also with a lipoprotein molecule. They conclude the whole heparin complex is produced in the intergranular cytoplasm of the tissue mast cells. 

No unique structural model for the plasma membrane can yet be deduced from these data (Korn, 1967) but it is now possible to consider that lipoprotein molecules may be the fundamental structural, as well as functional, components of membranes. Individual protein molecules could easily extend through the 75-100 A width of the plasma membrane. This implies that the carrier molecule and the structural molecule may be identical and that transport across the plasma membrane might be conceived as a conformational change of the membrane substructure. The membrane need no longer be viewed primarily as an inert barrier but rather as a dynamic aggregate of functional polymers (Korn, 1967). 

Figure 3.28 Diagrammatic Representation of the Insertion of Lipoprotein Into Outer Membrane. The diagram shows a bound-lipoprotein molecule linked to the diaminopimelic acid residue (DPM) of peptidoglycan by its terminal lysine. In free-lipoprotein this terminal lysine would be unsubstituted. Bound lipoprotein can be liberated by trypsin, which cleaves the polypeptide at points marked by arrows. (The symbols used for peptidoglycan structure are as in Figure 1.2.)...

Kelley (1964) found that proteolysis completely destroyed 20 percent of the lipoprotein molecules in a low density lipoprotein solution. The lipid from these molecules was transferred to the remaining lipoprotein molecules which were less stable during storage but resistant to proteolytic enzyme attack. The additional lipid associated with modified lipoproteins may explain the enhanced ether extractability observed previously. These experiments indicate that lipid-protein interactions contribute to the stability of the molecular complex. 

No consensus is available on the size and shape of high density lipoproteins. Molecular weights ranging from 165,000 to 450,000 have been reported (Oncley et al. 1947, Shore 1957, Hazelwood 1958, Shore and Shore 1962). A molecular weight of 250,000 was found for lipoprotein molecules at the center of the density continuum (Allebton 1962). 

A means of classifying the hyperlipoproteinaemias, based on the size of the lipoprotein molecules as assessed by membrane filtration and nephelometry. The hyperlipoproteinaemias are expressed in terms of an SML profile where S, M and L stand for small, medium and large particles respectively. 

In Korn s view (1966), membrane protein rather than membrane lipid is the important functional group, contrary to the implications in the paucimolecular models. Korn visualized the first step in membrane biosynthesis as the synthesis of protein, and, according to the primary structure of the protein, lipid would be bound sequentially, forming lipoprotein. The globular subunits seen in electron micrographs could then be lipoprotein molecules. 

When the lipoprotein molecules are arranged in a superhelix (Fig. 13B), a number of ionic interactions are formed between adjacent molecules stabilizing the entire assembly. As can be seen in Fig. 12, the hydrophilic bands Pa and Pb which run parallel to the hydrophobic band H are complementary to each other in terms of ionic properties when an acidic residue is located on one side, a basic residue is located on the other side. In the superhelical arrangement, as many as seven stable ionic interactions are formed between the Pa band of one a-helix, and the Pb band of the adjacent a-helix. In Fig. 12, the residues denoted by (x) indicate those residues on the Pb band of an adjacent helix (helix 2) interacting with the residues of the Pa band of helix 1. It should be noted that the superimposed residues of the Pb band of helix 2 are not plotted at the same level as those of helix 1. This is because corresponding points of adjacent helices are displaced by 5.8 A, due to the inclination of 25° between the axis of the superhelix and the axis of the a-helix, assuming that the average diameter and... 

Fig. 13. Two different cylindrical arrangements of six a-helices of the lipoprotein molecules.

The number of molecules in one superhelical assembly may be determined by maximizing the number and the stability of the ionic interactions. From such considerations, the number of lipoprotein molecules per assembly could range from 6 up to as many as 12. 

As can be seen in Fig. 14, the above assembly model provides a pore through the outer membrane. The size of the pores, or the channels, depends on the number (/i) of lipoprotein molecules per assembly. The... 

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