Enzymic Modifications of Sialic Acids in the Course of Glycoconjugate Biosynthesis

Abstract

This chapter explores the enzymic modifications of sialic acids in the biosynthesis of glycoconjugate. Sialic acids occur widely in animals and a few bacteria. Many physical and biological functions of the neuraminic acid-containing macromolecules occurring in the body fluids and in the structural elements of cells can be attributed to their sialic acid components. While N-acetylneuraminic acid is the most common sialic acid, occurring at least in small amounts in tissues synthesizing sialic acids, the pattern of occurrence of the remaining neuraminic acid derivatives is species and tissue-specific. An example of the gas–liquid chromatograms of the sialic acid mixtures isolated from bovine and equine submandibular glands. In contrast, the biosynthetic pathways of the other neuraminic acid derivatives isolated from biological materials are unknown. Different methods for assay of these O -acetyltransferase activities have been described. Free sialic acids (both N -acetyl- and N -glycolylneuraminic acid), sialic acids bound to nascent glycoprotein molecules of the Golgi membranes, and sialic acids as components of glycoprotein's present in the cytosol of bovine submandibular gland. While the mode and subcellular site of the modification reactions producing N -glycolylneuraminic acid and O -acetylated sialic acids have been elucidated and knowledge of the degradation of these sialic acids has also been obtained, the biological significance of the N -glycolyl group or of the different O -acetyl groups in sialic acids is almost unknown, and no definite function has been ascribed to them. It is only known that the O -acetyl groups have a great influence on the activity of neuraminidase and acylneuraminate pyruvate-lyase.