Sialic acid side chains

An increase in sensitivity of 30-50% can be achieved if resorcinol and Cu2+ are used.30 This increase may be by a factor of 3 to 6 if the sialic acid side-chains are oxidized by periodate prior to application of the resorcinol-Cu2+ reaction.127... 

Mild, selective oxidation of the sialic acid side-chain, followed by the addition of bisulfite and Toluidine Blue has been used for topo-optical staining of sialoglycoconjugates in erythrocyte and lymphocyte membranes. This method, which is based on a polarization-microscope technique, pennits not only specific, and very sensitive, light-microscope staining of sialic acid residues on cell surfaces but also the study of the spatial arrangement of sialoglycoconjugate molecules on cell surfaces.192,197-198... 

Knowledge concerning structural features of sialic acids necessary for their anti-recognition effect in erythrocytes is still scanty. The sialic acid side-chain seems not to be involved in this function, because shortening by periodate-borohvdride treatment does not significantly influence the viability of the erythrocytes.142 It is considered that the carboxyl group of Neu plays the main role in the protective effect of sialic acid residues. 

The complete pattern of sialic acid side-chain G-acetylation may require several sialate O-acetyltransferases each with a specificity for only one position. On the other hand, the primary insertion place for the O-acetyl function may be the H07 group alone from where the ester group migrates even under physiological conditions to the 9 position, presumably via C8 [7,23], leaving H07 ready for a new transfer. Thus, for complete side-... 

One of the main functions of sialic acid O-acetyl groups is their inhibitory effect on the action of both sialidases and sialic acid-specific lyases [5,33]. While a 4-O-acetyl group completely hinders the action of these enzymes (with the exception of a slow release by viral sialidases [252]), such ester groups at the sialic acid side chain appreciably hinder hydrolysis of the glycosidic bond of these sugars and their further breakdown by lyases. The existence of esterases acting on O-acetylated sialic acids prior to sialidase is therefore a prerequisite or at least supports the rapid turnover of O-acetylated sialoglycoconjugates. The possibility of the existence of such enzymes was raised by the observation that... 

A new colorimetric method for the measurement of free and glycosidically bound sialic acid was introduced by Durand et al. (1974) and Massamiri et al. (1978, 1979), based on the reaction of methyl-3-benzothiazolinone-2-hydrazone (MBTH) with the formaldehyde liberated on periodate oxidation of the sialic acid side chain. The molar extinction coefficient for Neu5Ac is 67,000 at 625 nm, and the method is thus similar in sensitivity to the periodic acid/thiobarbituric acid methods. 

Figure 4. Interactions at the 3 sialyUactose binding site. A. Spacefilling models of Sn and MAG showing the regions of the V-set domain of each protein that form the binding site for 3 sia-lyllactose. The model for Sn is based on the crystal structure whereas the model for MAG has been built using the Sn model as a template (A. May, personal communication), followed by changing the conformation of several amino acids to prevent clashes with other amino acid side chains. Shown are amino acids 1-5 (A strand), 40-44 (C/C strands), 97-112 (C-terminal part of F strand and N-terminal part of G strand) of Sn, shaded as indicated, and the bound 3 -sialyllactose (light). The same scheme is used for the MAG model. B. Diagram depicting the interactions between amino acids in the V-set domain of Sn and sialic acid deduced from the crystal structure of the V-set domain complexed with 3 sialyUactose. The sialic acid is shown in thick black lines.

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. 

Sesquiterpenoid. 203, 1071 Sex hormone, 1082-1083 Sharpless, K. Barry. 734 Sharpless epoxidation, 735 Shell (electron), 5 capacity of, 5 Shielding (NMR). 442 Si prochirality, 315-316 Sialic acid. 997 Side chain (amino acid), 1020 Sigma (cr) bond, 11 symmetry of, 11 Sigmatropic rearrangement, 1191-1195... 

Of the known classes of aldolase, DERA (statin side chain) and pyruvate aldolases (sialic acids) have been shown to be of particular value in API production as they use readily accessible substrates. Glycine-dependent aldolases are another valuable class that allow access to p-hydroxy amino acid derivatives. In contrast, dihydroxy acetone phosphate (DHAP) aldolases, which also access two stereogenic centres simultaneously,... 

Oral mucosae are covered with mucus which serves as a link between the adhesive and the membrane. Mucus is a glycoprotein consisting of a large peptide backbone with pendant oligosaccharide side chains. The side chains terminate in sialic or sulfonic acids, L-fucose, sulfated galactose, or A -acetylglucosamine. The glycoprotein component imparts the viscous or gel-like (non-Newtonian) nature due to its capacity... 

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