Glycolipid cycle

Abstract. TPA and RA have significant effects on glycolipid and glycoprotein biosynthetic enzymes in several cultured cell systems. This suggests that these compounds as well as other "tumor promoters" will be useful in further studies on the regulation and control of glycoconjugate metabolism (metabolic perturbants). Butyrate, TPA and RA appear to exert their effects at different points in the cell cycle. These results could mean that tumor promotion, differentiation and virus infection occur at discrete points in the cell cycle. Membrane glycoconjugates may participate in these processes in a dynamic time-dependent way. 

The neuraminidases together with gangliosides have been localized in the nerve ending structures (6, 7 ). Theoretically the sialylation and desialylation cycle may mediate a cyclic reaction at a very important locale in a nerve synaptic structure. This hypothetical involvement of sialic acid metabolism in synaptic transmission has gained support from several studies which have suggested a synaptic localization of the glycosyltransferases (8, 9,10,11) and from proposed theoretical models in which the sialo-glycolipids are considered an important constituent in the functional units of neuronal membranes (12,13,14). 

Figure 11.29 Viral receptors. Influenza virus targets cells by binding to sialic acid residues (purple diamonds) located at the termini of oligosaccharides present on cell-surface glycoproteins and glycolipids. These carbohydrates are bound by hemagglutinin (interaction circles), one of the major proteins expressed on the surface of the virus. The other major viral surface protein, neuraminidase, iis an enzyme that cleaves oligosaccharide chains to release the viral particle at a later stage of the viral life cycle.

Glycosphingolipids with similar structures are found in Crithidia spp, which are considered to be lower trypanosomatids as their habitat is in invertebrate and since they do not have a cycle involving vertebrates (39). Neutral glycosphingolipids were isolated and characterized in Crithidia luciliae, C.oncopelti and C.guilhermei (40). The major glycolipids of the three species of Crithidia detected by HPTLC chromatography contain... 

The organization of the glycoproteins and glycolipids on the surface of plasma membranes and their participation in cell contacts, transformation, and regulation of the cell cycle have been discussed, A model of intracellular recognition is presented and the role of the surface matrix of the cell is discussed. 

It must be remembered that glycolipids and glycoproteins might each influence the synthesis of the other and that there is some evidence that this is so. They also change during the cell cycle. Many transformed and tumour cells show the persistence of patterns of carbohydrate normally found only at mitosis throughout the cell cycle. 

The initial step in the viral life cycle is the attachment of virus particles to the cell surface. Attachment is mediated by binding of the virus to a receptor. Sometimes co-receptors are also involved that might promote post-attachment events in the entry process. Receptor molecules are crmstituents of the cell membrane, and the receptor determinant, the stmcture to which the virus binds, may be either a protein epitope or the carbohydrate of a glycoprotein or a glycolipid. Soluble proteins present in body fluids and in mucus oti respiratory and enteric epithelia may also contain such carbohydrates and therefore interfere with virus binding to the cell siuface. 

Glicosphingolipids are very important membrane components of eukaryotic cells and play a fundamental role in various stages of the cell cycle. Recently, the synthesis of triazol-hased neoglycolipids was published by Rivera and coworkers. It was based on Ugi-4CR/CuAAC combination and this type of approach was not yet explored for synthesis of triazol-linked carbohydrate-lipid conjugates. NMR analysis of the triazol-based glycolipids at room temperature showed the occurrence of two conformers and dynamic NMR experiment allowed to evaluate the free energy of activation of the isomerization of the cisitrans amide bond. 

Alternatively, native proteins such as cytochromes can be enclosed in reverse micelles of glycerides, glycolipids, phospholipids, or other surfactants capable of organizing biomolecules at the air-water interface [36-38,43-45]. Collapse of the micelles and LB formation with multiple compression-decompression cycles furnishes orientated lipid-supported protein monolayers displaying high packing densities [55],... 

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