The manyfold structural modifications of N-acetylneuraminic acid originating from the activities of various enzymes result in a large family of different sialic acids. Many of these sugars have been preserved in evolution since the echinoderms. Modifications, such as N-acetyl hydroxylation and O-acetylation, increase the structural diversity of sialoglycoconjugates and contribute to their variety of biological and pathophysiological roles. Out of a wealth of data a few examples of the functional highlights in sialobiology and sialopathology have been taken to illustrate our understanding and mirror the many unanswered questions, such as our incomplete knowledge about the distribution of the different sialic acids in microbial and animal species, as well as in developing, adult, ageing and malignantly transformed cells and tissues of animals and man. To fully understand the sialic acid modifications the enzymes involved in these processes have to be characterized and investigations of their regulation on the gene level are necessary.
The genes encoding the homologs of alphaherpesviral glycoprotein G (gG) are located within the unique short segment of the viral genome. Analysis and comparison of the aminoacid sequences deduced from the open reading frames (ORFs) encoding gG of bovine herpesvirus 1 (BHV-1) and bovine herpesvirus 5 (BHV-5) revealed the structure predicted for type I glycoproteins and showed that the homologs share 75% identical and 85% similar amino acids. Polyvalent antisera, raised in rabbits against recombinant vaccinia viruses expressing the respective ORFs precipitated secreted glycoproteins of 65kDa from BHV-1 and BHV-1 infected cell culture media, designated gG, and a high a diffusely migrating protein species with apparent molecular mass between 90 and >240kDa. The last was shown to be an chondroitin sulfate containing isoform of gG and was designated glycoproteoglycan G (gpgG). For BHV-1 it was demonstrated that gG and gpgG are not essential for repliction in cell culture and in bovines. Both gG and gpgG could not be found in purified virions, indicating that they are not involved in the entry process into target cells and in the target cell tropism of BHV-1. No role of gG and gpgG could be found for replication in cell culture and also the function of these proteins in vivo awaits identification.
Many exoglycosidases have been shown to have transglycosylation activity in addition to hydrolytic activity. The transglycosylation activities of endoglycosidases acting on complex carbohydrates have not been studied in detail, the first report being on endo-β-N-acetylglucosaminidase from Flavobacterium meningosepticum (Endo-F). This enzyme transfers high-mannose oligosaccharide (Man6GlcNAc) to glycerol during cleavage of the chitobiose linkage in GlcNAc2Man6Asn, as judged from the observation that the Endo-F released oligosaccharide had no reducing end in the presence of glycerol. Endo-β-N-acetylglucosaminidase from Arthrobacter protophormiae (Endo-A) was also found to have transglycosylation activity. Digestion of Man6GlcNAc2Asn with Endo-A in the presence of 4-L-aspartylglycosylamine (GlcNAc-Asn) gave a mixture of hydrolytic (Man6GlcNAc) and transglycosylic (Man6GlcNAc2Asn) products. By means of translycosylation, Man6GlcNAc was transferred en bloc to partially deglycosylated ribonuclease B having a GlcNAc-Asn residue, concomitant with the hydrolysis of Man6GlcNAcAsn. Thus, N-linked oligosaccharides in the native ribonuclease were converted into Man6GlcNAc, and the neoribonuclease containing homogeneous oligosaccharide was synthesized through the transglycosylation activity of Endo-A. This enzyme exhibited exclusive transglycosylation activity in the presence of acetone. The novel endo-β-N-acetylglucosaminidase from Mucor hiemalis (Endo-M), which can cleave not only the high-man-nose type of N-linked oligosaccharide but also the complex type of oligosaccharide, has transglycosylation activity. This enzyme transfers the sialo complex type oligosaccharide from the human transferrin glycopeptide to an appropriate acceptor having a GlcNAcAsn residue. Using the transglycosylation reaction of Endo-M, a sialo complex type oligosaccharide was attached to a synthetic peptide containing GlcNAc, such as peptide T-GlcNAc. The transglycosylation product was ascertained to have more resistance against proteolysis. Endo-α-N-acetylgalactosaminidase from Diplococcus pneumoniae exhibits transglycosylation and transfer reaction (reversed hydrolysis) activities. Treatment of asialoglycoproteins having Galβ1→3GalNAcα1→Ser/Thr linkages with the enzyme in the presence of glycerol resulted in the formation of Galβ1→3GalNAcα1→1(3)-glycerol. Moreover, D-glucose, D-galactose, p-nitophenol, threonine, etc. were good substrates (acceptors) for the transfer reaction (reverse hydrolysis). The endoglycoceramidases (ceramide glycanase) from leech and Corynebacterium sp. exhibit transglycosylation activity that transfers the oligosaccharides en bloc from various glycosphingolipids to suitable acceptors. They transfers the intact oligosaccharide from GM1 to various 1-alkanols having a long carbon chain. The transglycosylation activity of these enzymes will be useful for synthesizing neoglycolipids and novel alkylglycosides.