Sialic acids membrane-bound

Approximately one quarter of the total sialic acid of the L cell is bound to lipid in the form of hematoside, mono- and disialogangliosides. The remaining three-quarters of the sialic acid is bound to protein. Weinstein et al. (1970) found no significant preference of location of gangliosides and hematosides for the surface membranes of the L cell, while glycoproteins clearly concentrated in this organelle. On the other... 

In addition to binding to sialic acid residues of the carbohydrate side chains of cellular proteins that the virus exploits as receptors, hemagglutinin has a second function in the infection of host cells. Viruses, bound to the plasma membrane via their membrane receptors, are taken into the cells by endocytosis. Proton pumps in the membrane of endocytic vesicles that now contain the bound viruses cause an accumulation of protons and a consequent lowering of the pH inside the vesicles. The acidic pH (below pH 6) allows hemagglutinin to fulfill its second role, namely, to act as a membrane fusogen by inducing the fusion of the viral envelope membrane with the membrane of the endosome. This expels the viral RNA into the cytoplasm, where it can begin to replicate. 

Three different rare genetic metabolic defects in sialic acid metabolism are known, as indicated in Fig. 4.3.2 [3, 21] (1) free sialic acid storage disease (SASD Online Mendelian Inheritance in Man, OMIM 604369, 269920), a lysosomal membrane transporter defect (2) sialuria (OMIM 269921), a feedback inhibition defect in sialic acid biosynthesis (3) sialidosis (OMIM 256550), a breakdown defect of sialyloli-gosaccharides caused by a defect of lysosomal sialidase. In all these genetic defects, an increased amount of sialic acid can be found in tissues and or body fluids, either bound to OGSs as in (3), or in its free state as in (1) and (2). 

This fraction contained carbohydrates typical of membrane glycoproteins and was especially rich in galactose and sialic acid. Glycoproteins in this fraction avidly bound the lectin Concanavalin A. 

Because HA is a protein bound to viral membrane, multivalent ligands can interact with a single trimer or by spanning neighboring trimers. Molecules that can place multiple sialic acid residues in binding sites within a trimer might be expected to... 

An interesting electrochemical method for the determination of bound sialic acid has been developed, making use of a potentiometric four-channel thick-film sensor [236]. The sialidase sensor consists of a bilayer of a membrane containing Clostridium perfringens sialidase immobilized in a poly(vinyl acetate)-polyethylene copolymer, which is placed on top of an fT -selective poly(vinyl chloride)-poly(vinyl acetate) indicator membrane. The enzyme-induced release of bound sialic acid leads to a concomitant decrease in pA a of the carboxyl function of sialic acid. This decrease affords a local pH change inside the sialidase-containing sensor membrane, which is monitored by the H -selective indicator membrane. The pH optimum of the sialidase sensor was pH 4 for sialyllactose, mucin and colominic acid. 

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