Catabolism sialidases

The internal, sialic acid residue of GMj or GM2 is readily susceptible to sialidase action after enzymic release of the peripheral Gal and GalNAc residues, leading to GM3 having a terminal sialyl residue, and this represents the natural pathway of the catabolism of ganglio-sides.110 354 355... 

Fig. 13. Metabolism scheme of sialic acids. Anabolic (solid arrow) and catabolic (dashed arrow) reactions are indicated. For literature see the text. Enzymes 1, CMP-sialate synthase (EC 2.7.7.43) 2, sialyltransferases (EC 2.4.99.1.) 3, CMP-Neu5Ac hydroxylase (EC 1.14.99.18) 4, acetyl-CoA sialate 4-0-acetyltransferase (EC 2.3.1.44) 5, acetyl-CoA sialate 7(9)-0-acetyltransferase (EC 2.3.1.45) 6,. S-adenosyl-L-methionine sialate 8-O-methyltransferase (proposed EC 2.1.1.78) 7, sialate 4- or 9-0-acetylesterases (EC 3.1.1.53) 8, sialidase (EC 3.2.1.18) 9, sialate-pyruvate lyase (aldolase EC 4.1.3.3). Both Neu5Ac and Neu5Gc can be O-acetylated by the two O-acetyltransferases. There may also exist a sulfotransferase, since sulfated sialic acids have been found in e.g. echinoderms [13,577]. ( ), Sialic-acid-accepting nascent glycoconjugate.

Three enzyme systems are required for the catabolism of sialic acids 0-acetylesterases, sialidases and lyases. Furthermore, a sialic acid transporter has been described (section 10.5), which carries liberated sialic acids from lysosomes into the cytosol, where they are either degraded by the lyase or recycled after activation with CTP (section 8.3). Sialic acid permeases provide bacteria with sialic acids for nutritional purposes (section 9.4). 

It is noteworthy that some low organisms like a few bacterial and protozoan species possess sialic acids. This may help these parasites to survive in the host. For a similar reason it may be advantageous for some microorganisms to have at their disposal catabolic enzymes of sialic acid metabolism like sialidase and N-acetylneuraminate lyase. Strikingly, many of these organisms are pathogenic for mammals. It could well be that sialidase is required for a faster spreading of... 

The catabolism of the sialic acids is inseparable from the biosynthesis of the monosaccharides themselves and the various sialyltransfer reactions described in the previous subsections. Most information is available in the field of sialidase research, as will be evident below. However, the complete catabolism of the sialic acids entails several components, shown in Fig. 2, which exhibit links to all parts of the metabolic routes involving sialic acids. 

Research has also been focused on the enzymes of sialic acid catabolism, mainly the sialidases, which may initiate the degradation of sialoglycoconjugates and thus are involved in regulation of the turnover of these substances (Corfield et al., 1981a Corfield and Schauer, 1982b) (see Chapter 8). Sialidases are important enzymes, since they may destroy the sialic acid-mediated biological... 

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