Sialyltransferases acceptor molecules

Figure 22 Biosynthesis of sialosides. Sialic acid aldolases catalyze the aldol condensation of W-substituted mannosamines with pyruvate to produce sialic acids. CMP-sialic acid synthetases produce CMP-sialic acid from CTP and sialic acid. Sialic acid is transferred to acceptor molecules by sialyltransferases.

Long term hormonal control functions in the expression of enzyme concentrations and de novo acceptor molecule synthesis in the case of sialyltransfer reactions. Examples in development have been cited (sections II.2, II. 3, and III. 10), and the regenerating liver has proved valuable in studies of this kind, e.g. UDP-GlcNAc 2-epimerase (Okubu et al. 1976, Okamoto and Akamatsu 1980). Elevation of enzyme concentration could be shown to be a function of de novo protein synthesis. Study of hormone effects (oestrogen, progesterone) on sialyltransferase activity and glycosidically linked sialic acid levels in the cervical cyclic phenomena have been reported (Moghissi and Syner 1976, Chantler and Debruyn 1977, Hatcher et al. 1977, Nasir-ud-Din et al. 1979, Wolf et al. 1980). Similar effects have been demonstrated in rat endometrium (Nelson et al. 1975). 

The rat mammary gland sialyltransferase is probably of more historical than biochemical interest, for it not only led to the discovery of the milk and colostrum enzymes, but also set a precedent for attention to careful chemical detail which characterized this field. It was largely through the careful analysis of products, careful chemical preparation of acceptor molecules, and use of known oligosaccharide acceptor structures that some biochemical understanding has emerged. 

The enzymes catalyzing reactions (1) and (2) both add sialic acid to j8-linked galactosyl residues however, they are distinct and separate enzymes. Again, as with the glycoprotein and mucin sialytransferases, the enzyme discerns the fine structure of the acceptor molecule. Further, these sialyltransferases will not transfer sialic acid to )8-linked galactosyl residues of glycoproteins, mucins, or oligosaccharides. The third enzyme catalyzes the addition of sialic acid to another sialic acid residue. Little is known about this enzyme since its activity is very low. It is not clear at this time whether this enzyme catalyzes the addition as shown in reaction (3) or utilizes the product of reaction (1) as its substrate. 

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