Many different theories have been advanced concerning the biological roles of the oligosaccharide units of individual classes of glycoconjugates. Analysis of the evidence indicates that while all of these theories are correct, exceptions to each can also be found. The biological roles of oligosaccharides appear to span the spectrum from those that are trivial, to those that are crucial for the development, growth, function or survival of an organism. Some general principles emerge. First, it is difficult to predict a priori the functions a given oligosaccharide on a given glycoconjugate might be mediating, or their relative importance to the organism. Second, the same oligosaccharide sequence may mediate different functions at different locations within the same organism, or at different times in its ontogeny or life cycle. Third, the more specific and crucial biological roles of oligosaccharides are often mediated by unusual oligosaccharide sequences, unusual presentations of common terminal sequences, or by further modifications of the sugars themselves. However, such oligosaccharide sequences are also more likely to be targets for recognition by pathogenic toxins and microorganisms. As such, they are subject to more intra- and inter-species variation because of ongoing host-pathogen interactions during evolution. In the final analysis, the only common features of the varied functions of oligosaccharides are that they either mediate 'specific recognition' events or that they provide 'modulation' of biological processes. In so doing, they generate much of the functional diversity required for the development and differentiation of complex organisms, and for their interactions with other organisms in the environment.

Biological roles of oligosaccharides: all of the theories are correct

Antimicrobial host defense peptides are produced by all complex organisms as well as some microbes and have diverse and complex antimicrobial activities. Collectively these peptides demonstrate a broad range of antiviral and antibacterial activities and modes of action, and it is important to distinguish between direct microbicidal and indirect activities against such pathogens. The structural requirements of peptides for antiviral and antibacterial activities are evaluated in light of the diverse set of primary and secondary structures described for host defense peptides. Peptides with antifungal and antiparasitic activities are discussed in less detail, although the broad-spectrum activities of such peptides indicate that they are important host defense molecules. Knowledge regarding the relationship between peptide structure and function as well as their mechanism of action is being applied in the design of antimicrobial peptide variants as potential novel therapeutic agents.

Peptide Antimicrobial Agents

n, sulfation, L-iduronic acid, glycosam inoglycan-protei n in­ teractions, extracellular matrix.

Proteoglycans: structures and interactions.

Heparin, a sulfated polysaccharide belonging to the family of glycosaminoglycans, has numerous important biological activities, associated with its interaction with diverse proteins. Heparin is widely used as an anticoagulant drug based on its ability to accelerate the rate at which antithrombin inhibits serine proteases in the blood coagulation cascade. Heparin and the structurally related heparan sulfate are complex linear polymers comprised of a mixture of chains of different length, having variable sequences. Heparan sulfate is ubiquitously distributed on the surfaces of animal cells and in the extracellular matrix. It also mediates various physiologic and pathophysiologic processes. Difficulties in evaluating the role of heparin and heparan sulfate in vivo may be partly ascribed to ignorance of the detailed structure and sequence of these polysaccharides. In addition, the understanding of carbohydrate-protein interactions has lagged behind that of the more thoroughly studied protein-protein and protein-nucleic acid interactions. The recent extensive studies on the structural, kinetic, and thermodynamic aspects of the protein binding of heparin and heparan sulfate have led to an improved understanding of heparin-protein interactions. A high degree of specificity could be identified in many of these interactions. An understanding of these interactions at the molecular level is of fundamental importance in the design of new highly specific therapeutic agents. This review focuses on aspects of heparin structure and conformation, which are important for its interactions with proteins. It also describes the interaction of heparin and heparan sulfate with selected families of heparin-binding proteins.

http://www-heparin.rpi.edu/main/files/papers/4f1623c54d6ab5.21304086.pdf

Heparin-protein interactions

Simple and complex carbohydrates (glycans) have long been known to play major metabolic, structural and physical roles in biological systems. Targeted microbial binding to host glycans has also been studied for decades. But such biological roles can only explain some of the remarkable complexity and organismal diversity of glycans in nature. Reviewing the subject about two decades ago, one could find very few clear-cut instances of glycan-recognition-specific biological roles of glycans that were of intrinsic value to the organism expressing them. In striking contrast there is now a profusion of examples, such that this updated review cannot be comprehensive. Instead, a historical overview is presented, broad principles outlined and a few examples cited, representing diverse types of roles, mediated by various glycan classes, in different evolutionary lineages. What remains unchanged is the fact that while all theories regarding biological roles of glycans are supported by compelling evidence, exceptions to each can be found. In retrospect, this is not surprising. Complex and diverse glycans appear to be ubiquitous to all cells in nature, and essential to all life forms. Thus, >3 billion years of evolution consistently generated organisms that use these molecules for many key biological roles, even while sometimes coopting them for minor functions. In this respect, glycans are no different from other major macromolecular building blocks of life (nucleic acids, proteins and lipids), simply more rapidly evolving and complex. It is time for the diverse functional roles of glycans to be fully incorporated into the mainstream of biological sciences.

/pdf/biological-roles-of-glycans-zowb8sw07y.pdf

Biological Roles of Glycans

With the possible exception of hyaluronic acid, the connective tissue polysaccharides are all synthesized by their parent cells as components of proteoglycans. In these substances, a number of polysaccharide chains are covalently linked to a protein core; e.g., in the proteoglycan of bovine nasal cartilage, which is the prototype of molecules of this kind, close to 100 chondroitin sulfate chains, with a molecular weight of approximately 20,000, and slightly fewer keratan sulfate chains are linked to a core protein (mol. wt. 200,000) which constitutes 7–8% of the entire molecule. In many respects, the proteoglycans are similar to other protein-bound complex carbohydrates, and the conspicuous polysaccharide component per se does not distinguish the proteoglycans from the class of glycoproteins; e.g., there are members of the glycoprotein class, such as the blood group substances, which have a high relative content of carbohydrate consisting of a substantial number of monosaccharide units. Rather, the segregation of the proteoglycans into a separate category is based on a few specific characteristics: (1) each polysaccharide consists of repeating disaccharide units in which a hexosamine, d-glucosamine, or d-galactosamine is always present; (2) all connective tissue polysaccharides except keratan sulfate contain a uronic acid, either d-glucuronic acid or its 5-epimer, l-iduronic acid, or both; (3) ester sulfate groups are present in all members of the group except in hyaluronic acid; in addition, N-sulfate groups are found in heparin and heparan sulfate. Although certain other bipolymers are known to contain ester sulfate, e.g., some epithelial mucins (Horowitz, 1977), these compounds are clearly distinguishable from the connective tissue polysaccharides by the other criteria indicated above. It may also be mentioned that the d-glucuronic-acid-containing repeating disaccharide of chondroitin, N-acetylchondrosine, has recently been identified as a component of thyroglobulin (Spiro, 1977); however, since the disaccharide is present as a single unit, thyroglobulin may not be considered a proteoglycan.

Structure and Metabolism of Connective Tissue Proteoglycans

[7] Isolation and characterization of connective tissue polysaccharides

Biosynthesis of heparin

The formation of labeled heparin-precursor polysaccharide (N-acetylheparosan) from the nucleotide sugars, UDP-[14C]glucuronic acid and UDP-N-acetylglucosamine, in a mouse mastocytoma microsomal fraction was abolished by the addition of 1% Triton X-100. In contrast, the detergent-treated microsomal preparation retained the ability to convert such preformed polysaccharide into sulfated products during incubation with 3′-phosphoadenylylsulfate (PAPS). However, as shown by ion-exchange chromatography of these products, the detegent treatment changed the kinetics of sulfation from the rapid, repetivive process characteristic of the unperturbed system to a slow, progressive sulfation, which involved all polysarccharide molecules simultaneously and yielded, ultimately, a more highly sulfated product. The detergent effect was attributed to solubilization of sulfotransferases from the microsomal membranes, along with other polymer-modifying enzymes and the polysaccharide substrate. The resulting product showed an apparently random distribution ofN-acetyl andN-sulfate groups, instead of the predominantly block-wise arrangement achieved through membrane-associated biosynthesis.O-Sulfation occurred mainly at C2 of the iduronic acid units in the membrane-bound polysaccharide but at C6 of the glucosamine residues in the presence of detergent. A capsular polysaccharide fromEscherichia coli K5, previously found to have a structure identical to that of the nonsulfated heparin-precursor polysaccharide, was sulfated in the solubilized system in a fashion similar to that of the endogenous substrate, but was not accessible to the membrane-bound enzymes. These findings suggest that the regulation of the polymer-modification process, and hence the structure of the final polysaccharide product, depends heavily on the organization of the enzymes and their proteoglycan substrate in the endoplasmic membranes of the cell.

Lennart Rodén

Papers

Structure and Metabolism of Connective Tissue Proteoglycans

[7] Isolation and characterization of connective tissue polysaccharides

Biosynthesis of heparin

Biosynthesis of heparin

Inhibition of chondroitin and heparan sulfate biosynthesis in Chinese hamster ovary cell mutants defective in galactosyltransferase I.