The Biology Of Glycoproteins

Glycans play a vital role in modulating protein structure and function from involvement in protein folding, solubility and stability to regulation of tissue distribution, recognition specificity, and biological activity. They can act as both positive and negative regulators of protein function, providing an additional level of control with respect to genetic and environmental conditions. Due to the complexity of glycosylated protein forms, elucidating structural and functional information has been challenging task for researchers but recent development of chemical biology-based tools and techniques is bridging these knowledge gaps. This book provides a thorough review of the current state of glycoprotein chemical biology, describing the development and application of glycoprotein and glycan synthesis technologies for understanding and manipulating protein glycosylation.

This book is in many ways a sequel to The Biochemistry ojGlycoproteins and Proteoglycans. The enormous recent progress in understanding the biological roles of glycoproteins has prompted the present volume. The reasons for studying glycoproteins have multiplied, and in the present volume the roles played by glycoproteins are explored in a variety of biological situations. The first two chapters describe molecules involved in cell-substratum and cell-cell interactions in a broad sense, and also focus on recent progress in identifying specific attachment molecules. Our understanding of how normal processes, such as cellular differentiation and tissue organization, are regulated is dependent on understanding how cells interact with the extracellular matrix. When these processes go awry the consequences can be tragic, for example, when manifest as birth de fects and cancer. Our ability to devise appropriate therapies is in many cases limited by our understanding of such cell-matrix interactions. The third chapter explores the roles by glycoproteins during early mammalian development. The carbohydrate portions c1early play very important roles in presenting information during early embryogenesis, and an unusual tumor stern cell, the embryonal carcinoma, looks very promising in pro viding an experimental system for understanding how the expression of these complex carbohydrate determinants is regulated. The next three chapters explore the biology of glycoproteins in distinct situations: in the immune system, in the nervous system, and during erythropoiesis.

Sugar chains (glycans) are often attached to proteins and lipids and have multiple roles in the organization and function of all organisms. "Essentials of Glycobiology" describes their biogenesis and function and offers a useful gateway to the understanding of glycans.

Glycobiology involves studies of complex carbohydrates and posttrans- tional modifications of proteins, and has become an important interdiscip- nary field encompassing chemistry, biochemistry, biology, physiology, and pathology. Although initial research was directed toward elucidation of the different carbohydrate structures and the enzymes synthesizing them, the field has now moved toward identifying the functions of carbohydrates. The pro- cols described in Glycobiology Protocols form a solid basis for investigations of glycan functions in health and disease. The cloning of many of the genes participating in glycosylation processes has helped to enhance our knowledge of how glycosylation is controlled, but has also added another dimension of complexity to the great heterogeneous variety of the structures of the oligos- charides of glycoproteins, proteoglycans, and glycolipids. A family of similar enzyme proteins exists for each glycosylation step. Glycosyltransferases are extremely specific for both the nucleotide sugar donor and the acceptor s- strate, but many other factors control sugar transfer, including the locali- tion and topology of enzymes, cofactors, possible chaperone proteins, and the availability of sugar acceptor substrates. The analysis of the intracellular organization of glycosylation and of the factors controlling the activities of the participating enzymes in the cell are important areas that need more research efforts. Another challenge for future research is to understand the glycodynamics of a cell, that is, how the cell responds to stimuli leading to biological and pathological changes in terms of alterations in glycosylation, and how this affects the biology of the cell.

The student of biological science in his final years as an undergraduate and his first years as a graduate is expected to gain some familiarity with current research at the frontiers of his discipline. New research work is published in a perplexing diversity of publications and is inevitably concerned with the minutiae of the subject. The sheer number of research journals and papers also causes confusion and difficulties of assimilation. Review articles usually presuppose a background know ledge of the field and are inevitably rather restricted in scope. There is thus a need for short but authoritative introductions to those areas of modern biological research which are either not dealt with in standard introductory textbooks or are not dealt with in sufficient detail to enable the student to go on from them to read scholarly reviews with profit. This series of books is designed to satisfy this need. The authors have been asked to produce a brief outline of their subject assuming that their readers will have read and remembered much of a standard introductory textbook of biology. This outline then sets out to provide by building on this basis, the conceptual framework within which modern research work is progressing and aims to give the reader an indication of the problems, both conceptual and practical, which must be overcome if progress is to be maintained.

The mucins (mucus glycoproteins) have long been a complex corner of glycoprotein biology. While dramatic advances in the separation, structural an- ysis, biosynthesis, and degradation have marked the progress in general glycop- tein understanding, the mucins have lagged behind. The reasons for this lack of progress have always been clear and are only now being resolved. The mucins are very large molecules; they are difficult to separate from other molecules present in mucosal secretions or membranes; they are often degraded owing to natural protective functions or to isolation methodology and their peptide and oligos- charide structures are varied and complex. Understanding these molecules has demanded progress in several major areas. Isolation techniques that protect the intact mucins and allow dissociation from other adsorbed but discrete molecules needed to be developed and accepted by all researchers in the field. Improved methods for the study of very large molecules with regard to their aggregation and polymerization were also needed. Structural analysis of the peptide domains and the multitude of oligosaccharide chains was required for smaller sample sizes, for multiple samples, and in shorter time. In view of these problems it is perhaps not surprising that the mucins have remained a dilemma, of obvious biological importance and interest, but very difficult to analyze.

It has been predicted that nearly half of all human proteins are glycosylated indicating the significance of glycoproteins in human health and disease. For example, the glycans attached to proteins have emerged as important biomarkers in the diagnosis of diseases such as cancers and play a significant role in how pathogenic viruses gain entry into human cells. The study of glycoproteins has now become a truly proteomic science. In the last few years, technology developments including in silico methods, high throughput separation and detection techniques have accelerated the characterization of glycoproteins in cells and tissues. Glyco-engineering coupled to rapid recombinant protein production has facilitated the determination of glycoprotein structures key to exploring and exploiting their functional roles. Each chapter in this volume is written by experts in the field and together provide a review of the state of the art in the emerging field of glycoproteomics.