This review deals with the stages of synthesis and processing of asparagine-linked oligosaccharides occurring in the lumen of the endoplasmic reticulum and their relation with the acquisition by glycoproteins of their proper tertiary structures. Special emphasis is put on reactions taking place in trypanosomatid protozoa as their study allowed detection of the transient glucosylation of glycoproteins, catalyzed by the UDP-Glc: glycoprotein glucosyltransferase and glucosidase II. The former enzyme has the unique property of covalently tagging not properly folded conformations by catalyzing the formation of protein-linked Glc1Man7GlcNAc2, Glc1Man8GlcNac2 and Glc1Man9GlcNA2 from the unglucosylated compounds. The enzyme is a soluble protein of t that recognizes protein domains expohe endoplasmic reticulumsed in denatured but not in native conformations (probably hydrophobic amino acids) and the innermost N-acetylglucosamine unit that is hidden from macromolecular probes in most native glycoproteins. In vivo, the glucose units are removed by glucosidase II. The influence of oligosaccharides in glycoprotein folding is reviewed as well as the participation of endoplasmic reticulum chaperones (calnexin and calreticulin) that recognize monoglucosylated species in the same process. Glycoproteins that are not properly folded are retained in the endoplasmic reticulum where they are proteolytically degraded. A model for the quality control of glycoprotein folding in the endoplasmic reticulum in which calnexin (and calreticulin) and the UDP-Glc: glycoprotein glucosyltransferase are the main elements is reviewed.
The connective tissue polysaccharide hyaluronan (hyaluronic acid, HA) is cleared extremely efficiently from the circulation via receptor mediated endocytosis by endothelial cells of the liver (LEC). In liver disease, such as cirrhosis, the reduced uptake of HA by LEC results in incresed serum concentrations of HA. At least two different receptor activities exist on the surface of LEC, one is calcium dependent and seems not to be directly involved in endocytosis while another is calcium independent and mediates endocytosis. The receptors recognise other ligands besides HA, such as chondroitin sulphate and dextran sulphate, and thereby have a similarity to the scavenger receptors. High Mw HA binds with higher affinity to the receptors than low Mw species, and the smallest oligosaccharide recognized by the receptors is a hexasaccharide. The endocytic receptors recirculate and mediate transport of the polysaccharide to lysosomes where it is degraded to N-acetylglucosamine and glucuronic acid. The monosaccharides are transferred to the cytosol and ultimately broken down to water, carbon dioxide and urea in the hepatocytes. The molecular nature of the HA receptors on LEC is not completely clear. One tentative receptor is characterized as a 340kDa protein by crosslinking with a HA photoaffinity probe but has not yet been sequensed. Another tentative receptor has been characterized as a 95kDa protein by affinity chromatography of solubilized LEC on HA coupled to agarose and specifically eluted with HA oligosaccharides. This latter protein was recently partly sequensed and found to have identical sequences to intercellular adhesion molecule 1 (ICAM-1). The localisation of ICAM-1 corresponds well with tissues and cells involved in HA binding and uptake. Based on inhibition studies with ligands for the scavenger receptors, members of this type of molecules are suggested to bind HA as well. The binding of HA to ICAM-1 and/or to scavenger receptors can explain several phenomena in HA metabolism found in physiological and pathological conditions.
Gelatinase A is a member of the Matrix Metalloproteinase (MMP) family. These enzymes are usually secreted as latent pro-enzymes, are activated by proteolytic cleavage of an amino terminal domain, and are inhibited by tissue inhibitor of metalloproteinases (TIMPs). They are involved in extracellular matrix remodeling, both in normal processes of growth and development, and in pathological processes such as tumor invasion and metastasis. Gelatinase A has a number of distinctive characteristics, that suggest it may play a unique role in these processes. Unlike most MMPs it is constitutively expressed by many cells and has a ubiquitous tissue distribution. Another unique feature is that this protease is activated on the cell surface by the recently discovered membrane-type MMP (MTMMP). Expression of gelatinase A correlates with the aggressiveness of many tumors, which suggests that it may be a useful prognostic indicator, as well as a suitable target for anti-cancer therapies.
The glycosyltransferase-mediated synthesis of oligosaccharide analogs is reviewed, with emphasis on the use of modified sugar-nucleotide donors or modified acceptors as substrates. A survey of the most studied enzymes: β(1→4)-galactosyltransferase, N-Acetylglucosaminyltransferase I, α(1→3/4)-fucosyltransferases and α(2→3 or 2→6)-sialyltransferases, shows them to be very useful for the rapid production of chemically modified di-, tri- and tetrasaccharide analogs.