Protein to lipid ratio

Tab. 1.5 Protein to lipid ratios of rat tissue membranes. (Reprinted from Table 1.5 of ref. 2, with permission from Macmillan)...

Fig. 3. The coupling efficiency of SPDP-modified human IgG to DSPC/Chol/ MPB-DSPE/jV-(2 -(co-monomethoxypolyethyleneglycol2ooo)succinoyl)-l>2-distearoyl-in-glycero-3-phosphoctiianolaminc (MePEGS-2000-DSPE) (54-n 45 1 n) liposomes. The initial protein-to-lipid ratio was 150 pg protein/pmol lipid.

In our typical coupling experiment we obtain a protein-to-lipid ratio of approx 85 pg protein/pmol lipid P. 

The plasma membrane of the brush border microvilli is characterized by certain distinctive structural features, which may be related to the specialized functional properties that distinguish it from plasma membranes of other cells. The width of the microvillus membrane (measured by electron microscopy) is 10-11 nm, whereas the average eukaryotic plasma membrane is only 7-9 nm. This is probably due to the biochemical composition of the membrane, which is characterized by a high protein to lipid ratio (1.7 1) and a unique lipid composition. The cholesterol-phospholipid ratio and the molar ratio of glycolipid to phospholipid are both about 1 1, which is consistent with the low values for membrane fluidity determined in microvillus membranes. This should be compared with the corresponding ratios... 

If a membrane contains 30% protein and 70% phospholipid by weight, with a molar protein-to-lipid ratio of 1 100, and the average molecular size of the lipid being 1 kDa, what is the average molecular size of the proteins ... 

A DEER study [141] measuring distances of up to 8.7 nm showed that the PD-linked ASYN mutations also remain capable of adopting both structures, and that the protein to lipid ratio determines whether the protein adopts the broken or extended helix conformation. 

Anacystis cells grown in the absence of nitrogen source are capable to maintain their membrane physical state at a fairly constant level the fluidizing effect of the declined protein to lipid ratio is counteracted by the microviscosity increase due to alkyl chain saturation. If the fluidity perturbation induced by p/1 fall is attained by an alternative way (CP-treatment), Anacystis cells respond by the apparently same strategy. 

A cell membrane is illustrated in Fig. 6.1. It is built from a bilayer of lipids, usually phospholipids, associated with which are membrane proteins and polysaccharides. The antiparallel orientation of lipid layers in the bilayer is maintained due to the extremely slow flip-flop rate, i.e. the rate of diffusion transverse to the bilayer. The lipid bilayer is the structural foundation and the proteins and polysaccharides provide chemical functionality. The protein to lipid ratio shows a large variation depending on the cell, but proteins make up at least half of most cell membranes. A prominent exception is mammalian nerve cells which contain only 18 % protein (here also the lipids are sphingomyelins rather than phospholipids). Here, the primary requirement is that the cell membrane should be effective as an electrical... 

In a review on membrane proteins, Guidotti (1972) has classified membranes into three types on the basis of their protein content. The first class is the simple, inert membrane represented by myelin. It consists primarily of lipid with little protein, acts as a permeability barrier and insulator, and has only three known enzymatic activities (Beck et al., 1968 Olafson et al., 1969 Kurihara and Tsukada, 1967 Gammer et al., 1976 Yandrasitz et al., 1976). The large second class of membranes which have a protein-to-lipid ratio of about 1 1 (w w) are typified by most mammalian plasma membranes. They have many enzymatic activities and sophisticated transport systems associated with them, in addition to the permeability factor. The third class of membranes has bacterial and inner mitochondrial membranes as its models. These membranes have proportionately larger amounts of protein than lipid and have added functions such as oxidative phosphorylation and nucleic acid synthesis. In general, the specialization and enzyme function of the membrane increases in proportion to its protein content. Table 4 gives the amino acid composition of some isolated membrane proteins. Total membrane protein (intrinsic + extrinsic) often has an amino acid composition which falls into the range of other nonmembrane, "soluble" proteins (Vanderkooi and Capaldi, 1972). 

We shall mainly discuss the membranes present in eukaryotic cells, although many of the principles described also apply to the membranes of prokaryotes. The various cellular membranes have different compositions, as reflected in the ratio of protein to lipid (Figure 41-1). This is not surprising, given their divergent functions. Membranes are asymmetric sheet-like enclosed structures with distinct inner and outer surfaces. 

Figure 41-1. Ratio of protein to lipid in different membranes. Proteins equal or exceed the quantity of lipid in nearly all membranes. The outstanding exception is myelin, an electrical insulator found on many nerve fibers.

The ratio of protein to lipid in the cell membrane of M. luteus is about 7 to 3 (wt/wt), and the lipids comprise up to 70% negatively charged PG. This lipid is predominantly located in the outer monolayer, and the remaining 30% dimannosyldiacyl-glycerol (DMDG) are symmetrically distributed between both leaflets [103, 133]. 

Myelin in situ has a water content of about 40%. The dry mass of both CNS and PNS myelin is characterized by a high proportion of lipid (70-85%) and, consequently, a low proportion of protein (15-30%). By comparison, most biological membranes have a higher ratio of proteins to lipids. The currently accepted view of membrane structure is that of a lipid bilayer with integral membrane proteins embedded in the bilayer and other extrinsic proteins attached to one surface or the other by weaker linkages. Proteins and lipids are asymmetrically distributed in this bilayer, with only partial asymmetry of the lipids. The proposed molecular architecture of the layered membranes of compact myelin fits such a concept (Fig. 4-11). Models of compact myelin are based on data from electron microscopy, immunostaining, X-ray diffraction, surface probes studies, structural abnormalities in mutant mice, correlations between structure and composition in various species, and predictions of protein structure from sequencing information [4]. 

The particular proteins and phospholipids incorporated into the membrane, the proportions, and their arrangement vary depending on the cell type in which the membrane is located and also the part of the membrane. For example, the ratio of protein to lipid varies from 0.25 1 in the myelin membrane to 4.6 1 in the intestinal epithelial cell. Furthermore, the... 

Membranes consist largely of protein and lipid. The ratio (by weight) of protein to lipid varies from 0.25 in myelin to 3.0 in bacterial membranes. In membranes of erythrocytes it is about 1.2 and a ratio of about 1.0 may be regarded as typical for animal cells. Small amounts of carbohydrates (<5%) are present, as are traces of RNA (<0.1%). 

Over a long life span, the ratio of protein to lipid in the body shifts towards lipid. The phenomenon is controlled by a change of activity of somatotrophin... 

Using the total mass of lipid in the membrane solution (calculated at the end of Day 1), what is the mass ratio of protein to lipid in the erythrocyte membrane ... 

A steady flow of metabolites both in and out of the mitochondrial matrix space is necessary for mitochondria to perform functions which involve the participation of enzymes inside the membrane permeability barrier. These functions include oxidative phosphorylation and therefore O2, ADP, phosphate and electron-rich substrates such as pyruvate, fatty acids and ketone bodies must enter the mitochondria, and the products, HjO, CO2 and ATP must leave. Although Oj, HjO and CO2 are permeable to the inner mitochondrial membrane [1,2], most metabolites are not, because of their highly hydrophiUc nature. The outer mitochondrial membrane does not present a significant barrier to hydrophilic metabolites because of the presence of large unregulated channels composed of the membrane protein, porin [3]. The inner mitochondrial membrane has a much larger surface area [4] than the outer membrane and a much higher ratio of protein to lipid [5]. It is composed not only of proteins involved in electron transport and oxidative phosphorylation but also specialized proteins which facilitate, and in many cases provide, directionality to the transport of metabolites [6]. 

Weigh 2-4 mg of mPEG-DSPE-maleimide, and dissolve it in DMSO to make a final concentration of 10 mg/mL (2.9 mM). Add mPEG-DSPE-maleimide directly to Trout s reaction for a final protein-to-lipid molar ratio of 1 2. Incubate the reaction at room temperatnre for 1 h. 

For example, it is known that proteins are important membrane constituents the ratio by weight of proteins to lipids varies between 1.5 1 and 4 1 for different membranes. Since the bulk of the membrane capacitance is associated with the lipid phase, the constancy of the membrane capacitance during the passage of an electrical impulse may be interpreted assuming that only a minute fraction of these proteins (a few molecules scattered throughout within the nerve membrane) underwent transitions inducing dramatic changes in conductivity. 

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