Membrane bound carbohydrates

The membrane-bound carbohydrates exist as glycoproteins and glycolipids. Although the functional importance of these substances is far from proven they appear to be essential parts in phenomena such as cellular adhesion, control of differentiation and cell growth, and the binding by cells of enzymes, hormones and toxins. 

Membranes of animal plasma cells have large numbers of relatively small carbohydrates bound to them. In fact, the outsides of most plasma cell membranes are literally sugarcoated. These membrane-bound carbohydrates are part of the mechanism by which the different types of cells recognize each other in effect, the carbohydrates act as biochemical markers (anti-... 

Among the first discovered and best understood of these membrane-bound carbohydrates are those of the ABO blood-group system, discovered in 1900 by Karl Landsteiner (1868-1943). Whether an individual has type A, B, AB, or O blood is genetically determined and depends on the type of... 

Many proteins found in nature are glycoproteins because they contain covalently linked oligo- and polysaccharide groups. The list of known glycoproteins includes structural proteins, enzymes, membrane receptors, transport proteins, and immunoglobulins, among others. In most cases, the precise function of the bound carbohydrate moiety is not understood. 

Abstract To understand how membrane-active peptides (MAPs) function in vivo, it is essential to obtain structural information about them in their membrane-bound state. Most biophysical approaches rely on the use of bilayers prepared from synthetic phospholipids, i.e. artificial model membranes. A particularly successful structural method is solid-state NMR, which makes use of macroscopically oriented lipid bilayers to study selectively isotope-labelled peptides. Native biomembranes, however, have a far more complex lipid composition and a significant non-lipidic content (protein and carbohydrate). Model membranes, therefore, are not really adequate to address questions concerning for example the selectivity of these membranolytic peptides against prokaryotic vs eukaryotic cells, their varying activities against different bacterial strains, or other related biological issues. 

Eukaryotic cells have evolved a complex, intracellular membrane organization. This organization is partially achieved by compartmentalization of cellular processes within specialized membrane-bounded organelles. Each organelle has a unique protein and lipid composition. This internal membrane system allows cells to perform two essential functions to sort and deliver fully processed membrane proteins, lipids and carbohydrates to specific intracellular compartments, the plasma membrane and the cell exterior, and to uptake macromolecules from the cell exterior (reviewed in [1,2]). Both processes are highly developed in cells of the nervous system, playing critical roles in the function and even survival of neurons and glia. 

This experimental setup was applied for characterization of the interaction between WGA as a targeter and isolated Caco-2 cells, as well as monolayers. Accordingly, all the carbohydrate-combining sites accessible at the cell membrane were occupied within 10 min. This step was followed by internalization of 80% of membrane-bound lectin after 90 min in the case of isolated cells, and 240 min in the case of monolayers [25], The principle of this assay design also works for characterization of the interaction between targeted nanoparticles and monolayers (unpublished data). 

Phosphatidylinositol Synthesis. The symptoms of inositol depletion include a decreased hyphal extension rate, increased branching and cell lysis. Other changes occur in protein, carbohydrate and nucleic acid metabolism, and reduced activity levels of membrane-bound wall biosynthetic enzymes.7... 

In nature, mammalian antibodies occur in five distinct classes IgG, IgA, IgM, IgD, and IgE. These differ in structure, size, amino acid composition, charge, and carbohydrate components. The basic structure of each of the classes of immunoglobulins consists of two identical polypeptide chains linked by disulfide bonds to two identical heavy chains. Differences between classes and subclasses are determined by the makeup of the respective heavy chains. IgG is the major serum immunoglobulin and occurs as a single molecule IgA also occurs as a single molecule but also polymerizes, primarily as a dimer and also associates with a separate protein when secreted. IgM occurs in the serum as a pentamer, with monomers linked by disulfide bonds and the inclusion of an additional polypeptide component, the J-chain. IgD and IgE occur primarily as membrane-bound monomers on -cells, or basophils and mast cells, respectively. 

Plants must be especially versatile in their handling of carbohydrates, for several reasons. First, plants are autotrophs, able to convert inorganic carbon (as C02) into organic compounds. Second, biosynthesis occurs primarily in plastids, membrane-bounded organelles unique to plants, and the movement of intermediates between cellular compartments is an important aspect of metabolism. Third, plants are not motile they cannot move to find better supplies of water, sunlight, or nutrients. They must have sufficient metabolic flexibility to allow them to adapt to changing conditions in the place where they are rooted. Finally, plants have thick cell walls made of carbohydrate polymers, which must be assembled outside the plasma membrane and which constitute a significant proportion of the cell s carbohydrate. 

Role of PI in membrane protein anchoring Specific proteins can be covalently attached via a carbohydrate bridge to membrane-bound PI (Figure 17.9). [Note Examples of such proteins include alkaline phosphatase (a digestive enzyme found on the surface of the small intestine that attacks organic phosphates), and acetylcholine esterase (an enzyme of the postsynaptic membrane that... 

Both protein- and lipid-bound carbohydrates are present in milk lipid globule membrane (Table 10.2). Lipid-bound carbohydrates, present predominantly in glucosyl and lactosyl ceramides and in gangliosides,... 

As already mentioned, formation of glycosidic linkages between monomeric units of the carbohydrate-containing polymers of the bacterial cell-surface is catalyzed by membrane-bound glycosyltransferases, and glycosyl nucleotides are the usual glycosyl donors in the reaction. 

The membrane-bound enzymes participating in the assembly of the pep-tidoglycan carbohydrate chains were solubilized, and partially purified. The... 

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