Bacterial glycoproteins

Glycoproteins are a diverse group of complex macromolecules that are present in virtually all forms of life. Their presence in prokaryotes, however, has been demonstrated, and accepted, only recently. Bacterial glycoproteins have been identified in many archaeobacteria and in eubacteria. They comprise a wide range of different cell envelope components such as membrane-associated glycoproteins, surface-associated glycoproteins and crystalline surface layers (S-layers), as well as secreted glycoproteins and exoenzymes. Even their occurrence in the cytoplasm cannot yet be ruled out. This minireview tries to cover the whole subject as completely as possible and refers to available information on presence, structure, biosynthesis, and molecular biology of bacterial glycoproteins.     

Antifreeze glycoproteins

Antifreeze glycoproteins (AFGPs) are a novel class of biologically significant compounds that possess the ability to inhibit the growth of ice both in vitro and in vivo. Any organic compound that possesses the ability to inhibit the growth of ice has many potential medical, industrial, and commercial applications. In an effort to elucidate the molecular mechanism of action, various spectroscopic and physical techniques have been used to investigate the solution conformations of these glycoproteins. This review examines the characterization of AFGPs and potential biological applications relating to stabilization of lipid membranes and vitrification adjuvants.     

Tailor-made glycoproteins

Posttranslational modifications are a fundamental mechanism for the regulation of cellular physiology and function. A recent paper by Zhang et al. provides a novel strategy for the generation of homogeneous glycoproteins. The ability to install covalent modifications site-specifically into proteins holds tremendous promise for deciphering the role of posttranslational modifications and has exciting implications for the development of protein therapeutics.     

Tritiation of glycoproteins

Human caeruloplasmin and α₁-acid glycoprotein and bovine fetuin were tritiated by reductive methylation of the ε-amino group of lysine residues. Tritiated caeruloplasmin retained its oxidase activity and molecular sieve chromatography of the labelled glycoproteins showed that reductive methylation did not significantly affect their molecular size. In experiments with rats, tritiated glycoproteins exhibited plasma half-lives similar to those obtained with the same glycoproteins labelled by other methods. Further, reductive methylation did not alter the property shared by all three glycoproteins to promptly disappear from the circulation after their desialylation and injection into rats and to appear in the liver parenchymal cells. Labelling of glycoproteins by reductive methylation may be useful in metabolic studies requiring the use of both glycoproteins and their asialo-derivatives.     

Zona pellucida glycoproteins

All mammalian eggs are surrounded by a relatively thick extracellular coat, the zona pellucida, that plays vital roles during oogenesis, fertilization, and preimplantation development. The mouse zona pellucida consists of three glycoproteins that are synthesized solely by growing oocytes and assemble into long fibrils that constitute a matrix. Zona pellucida glycoproteins are responsible for species-restricted binding of sperm to unfertilized eggs, inducing sperm to undergo acrosomal exocytosis, and preventing sperm from binding to fertilized eggs. Many features of mammalian and non-mammalian egg coat polypeptides have been conserved during several hundred million years of evolution.     

Deglycosylation of glycoproteins

Glycosylation procedures and their application for the elucidation of glycoprotein structure and structure-function relations, as well as for the development of analytical systems for clinical practice are reviewed. For some common cases found in research practice, the choice of optimal deglycosylation methods is discussed. Current views on the primary structure of glycoproteins are described briefly.     

Mucin-type glycoproteins.

Considerable advances have been made in recent years in our understanding of the biochemistry of mucin-type glycoproteins. This class of compounds is characterized mainly by a high level of O-linked oligosaccharides. Initially, the glycoproteins were solely known as the major constituents of mucus. Recent studies have shown that mucins from the gastrointestinal tract, lungs, salivary glands, sweat glands, breast, and tumor cells are structurally related to high-molecular-weight glycoproteins, which are produced by epithelial cells as membrane proteins. During mucin synthesis, an orchestrated sequence of events results in giant molecules of Mr 4 to 6 x 10(6), which are stored in mucous granules until secretion. Once secreted, mucin forms a barrier, not only to protect the delicate epithelial cells against the extracellular environment, but also to select substances for binding and uptake by these epithelia. This review is designed to critically examine relations between structure and function of the different compounds categorized as mucin glycoproteins.     

Carbohydrate-peptide linkage in glycoproteins

The secondary structure of the peptide segment around the carbohydrate-peptide linkage in glycoproteins was predicted by using the Chou and Fasman determination. Such a study was carried out for 9 O-glycosidically linkages and 28 N-glycosidically linkages. In the case of O-glycosidically linkages, the residue Ser or Thr involved in the linkage always belongs to a β-turn. In the case of N-glycosidically linkages, 19 out of the 28 Asn studied belong to a β-turn. A predicted determination concerning the whole protein moiety of 9 glycoproteins in order to obtain some information concerning the spatial organization of the entire glycoprotein was carried out also. It seems that carbohydrate moiety takes place outside the glycoprotein.     

Lysosomal metabolism of glycoproteins

The lysosomal catabolism of glycoproteins is part of the normal turnover of cellular constituents and the cellular homeostasis of glycosylation. Glycoproteins are delivered to lysosomes for catabolism either by endocytosis from outside the cell or by autophagy within the cell. Once inside the lysosome, glycoproteins are broken down by a combination of proteases and glycosidases, with the characteristic properties of soluble lysosomal hydrolases. The proteases consist of a mixture of endopeptidases and exopeptidases, which act in concert to produce a mixture of amino acids and dipeptides, which are transported across the lysosomal membrane into the cytosol by a combination of diffusion and carrier-mediated transport. Although the glycans of all mature glycoproteins are probably degraded in lysosomes, the breakdown of N-linked glycans has been studied most intensively. The catabolic pathways for high-mannose, hybrid, and complex glycans have been established. They are bidirectional with concurrent sequential removal of monosaccharides from the nonreducing end by exoglycosidases and proteolysis and digestion of the carbohydrate-polypeptide linkage at the reducing end. The process is initiated by the removal of any core and peripheral fucose, which is a prerequisite for the action of the peptide N-glycanase aspartylglucosaminidase, which hydrolyzes the glycan-peptide bond. This enzyme also requires free alpha carboxyl and amino groups on the asparagine residue, implying extensive prior proteolysis. The catabolism of O-linked glycans has not been studied so intensively, but many lysosomal glycosidases appear to act on the same linkages whether they are in N- or O-linked glycans, glycosaminoglycans, or glycolipids. The monosaccharides liberated during the breakdown of N- and O-linked glycans are transported across the lysosomal membrane into the cytosol by a combination of diffusion and carrier-mediated transport. Defects in these pathways lead to lysosomal storage diseases. The structures of some of the oligosaccharides that accumulate in these diseases are not digestion intermediates in the lysosomal catabolic pathways but correspond to intermediates in the biosynthetic pathway for N-linked glycans, suggesting another route of delivery of glycans to the lysosome. Incorrectly folded or glycosylated proteins that are rejected by the quality control mechanism are broken down in the ER and cytoplasm and the end product of the cytosolic degradation of N-glycans is delivered to the lysosomes. This route is enhanced in cells actively secreting glycoproteins or producing increased amounts of aberrant glycoproteins. Thus interaction between the lysosome and proteasome is important for the regulation of the biosynthesis and distribution of N-linked glycoproteins. Another example of the extralysosomal function of lysosomal enzymes is the release of lysosomal proteases into the cytosol to initiate the lysosomal pathway of apoptosis.