TOPICS IN MOLECULAR AND STRUCTURAL BIOLOGY. General Editors:

Transcription

1 POLYSACCHARIDES

2 TOPICS IN MOLECULAR AND STRUCTURAL BIOLOGY General Editors: Watson Fuller (University of Keele) and Stephen Neidle (University of London King's College)

3 POLYSACCHARIDES Topics in Structure and Morphology Edited by E. D. T. ATKINS Professor of Physics and Science and Engineering Research Council Senior Fellow H. H. Wills Physics Laboratory University of Bristol M MACMILLAN

4 The contributors 1985 Softcover reprint of the hardcover 1st edition 1985 All rights reserved. No reproduction, copy or transmission of this publication may be made without written permission. No paragraph of this publication may be reproduced, copied or transmitted save with written permission or in accordance with the provisions of the Copyright Act 1956 (as amended). Any person who does any unauthorised act in relation to this publication may be liable to criminal prosecution and civil claims for damages. First published 1985 Published by THE MACMILLAN PRESS LTD Houndmills, Basingstoke, Hampshire RG21 2XS and London Companies and representatives throughout the world Typeset by TecSet Ltd, Sutton, Surrey British Library Cataloguing in Publication Data Atkins, Edward Polysaccharides.-(Topics in molecular and structural biology) 1. Polysaccharides I. Title II. Series 547.7'82 QD321 ISSN ISBN ISBN (ebook) DOI /

5 The contributors Margaret Biswas Molecular Biophysics Unit Indian Institute of Science Bangalore India W. Mackie Astbury Department of Biophysics University of Leeds Leeds LS2 9JT UK Benito Casu lstituto di Chimica e Biochimica 'G. Ronzoni' Via G. Colombo 81 (Citta Studi) Milan Italy I. A. Nieduszynski Department of Biological Sciences University of Lancaster Bailrigg Lancaster LAl 4TQ UK H. Chanzy Centre de Recherches sur les Macromolecules V egetales (CNRS) 53X Grenoble Cedex France D. H. Isaac Department of Metallurgy and Materials Technology University College Swansea Singleton Park Swansea SA2 8PP UK V. S. R. Rao Molecular Biophysics Unit Indian Institute of Science Bangalore India R. Vuong Centre de Recherches sur les Macromolecules Vegetales (CNRS) 53X Grenoble Cedex France

6 Contents The contributors Preface v ix 1. NUCLEAR MAGNETIC RESONANCE STUDIES OF POLYSACCHARIDE STRUCTURE AND INTERACTIONS B. Casu 2. ULTRASTRUCTURE AND MORPHOLOGY OF CRYSTALLINE POLYSACCHARIDES H. Chanzy and R. Vuong PLANT CELL WALLS: MORPHOLOGY, BIOSYNTHESIS AND GROWTH W. Mackie CONNECTIVE TISSUE POLYSACCHARIDES l A. Nieduszynski BACTERIAL POLYSACCHARIDES D. H. Isaac CONFORMATIONS AND INTERACTIONS OF OLIGOSACCHARIDES RELATED TO THE ABH AND LEWIS BLOOD GROUPS V. S. R. Rao and Margaret Biswas 185 Index 219

7 Preface Carbohydrate molecules are ubiquitous in nature. The structures and textures of terraqueous plants are dominated by polysaccharides such as cellulose, mannan, alginate, pectin and xylans. Chitin plays a major structural role in insect cuticle, usually blending with proteins and interacting with crystalline inorganic salts in an analogous manner to the calcification of collagen in bone. In animal tissues, polysaccharides such as hyaluronate, chondroitin and dermatan sulphates function as lubricants, gels, compliant matrices and also exhibit visoelastic properties. Their polyelectrolytic character responds to different cations, changes in ionic strength and degree of hydration to produce conformational variability. Many bacteria are encapsulated with swollen polysaccharide networks with particular species often displaying numerous serotypes, each with its own signature in the form of a precisely defined covalent repeating structure. There are many instances where carbohydrates are covalently attached to proteins to form proteoglycans and glycoproteins, and which serve as vital functional operators in molecular biology. The blood group substances are delineated by their carbohydrate components, and cell surfaces are decorated with polysaccharides such as heparan sulphate, which influence cell adhesion and recognition. The polysaccharide heparin suppresses blood clotting and certain glycoproteins act at antifreeze agents in the blood of polar fish. These examples, which are part of a spectrum of occurrences, properties and functions of polysaccharides and carbohydrate polymers, highlight the importance of this group of macromolecules in the natural world. Advances in the biomolecular structure of carbohydrate polymers have been more recent and have had less popular impact than the parallel developments in the protein and nucleic acid field. However, during the last fifteen years steady progress has been made in our understanding of a wonderful world of shapes, geometries and architecture of this important group of macromolecules. This text describes the application of nuclear magnetic resonance, electron microscopy, X-ray fibre diffraction and conformational analysis to certain plant, animal and microbial polysaccharides and carbohydrate polymers. The first chapter is concerned with the application of nuclear magnetic resonance (n.m.r.) spectroscopy to polysaccharide structure and interactions. This technique is a most effective method for investigating structure, dynamics and binding properties of polysaccharides. It has the potential to follow patterns

8 X PREFACE of interaction from solution into the condensed phase and offers exciting prospects for future advances in our understanding of polysaccharide behaviour. A comprehensive review and literature survey are given of the application of n.m.r. to many aspects of polysaccharide structure and interaction. The next chapter focuses attention on the texture, morphology and ultrastructure of crystalline polysaccharides using electron microscopy and electron diffraction. Delightful textures in the form of extended-chain crystals overlaid with lamellar platelets to form a 'shish-kebab' morphology, are convincingly visualised. Biological polymers are extremely sensitive to electron-beam damage and crystallinity is easily destroyed. Methods of examining hydrated specimens using quench-freeze techniques are described and numerous high-quality patterns illustrated. Progress in the visualisation of polysaccharide hydrogels is discussed and examples of the results obtained are shown in order to provide the reader with a feel for the current state of the art. Chapter 3 reviews the morphology, biosynthesis and growth of plant cell walls with emphasis on the polysaccharide components. Much of the plant world is dominated by composite structures consisting of cellulose microfibrils interacting with less crystalline non-cellulosic polysaccharides capable of fabricating a variety of molecular architectures. In algae, mannan and xylan can replace cellulose as the microfibrillar component. The development and growth of plant cell walls is an intriguing subject and this chapter reviews the current level of our understanding. Chapter 4 reviews the molecular shapes, and the interactions of the connective tissue polysaccharides or glycosaminoglycans. These polysaccharides were first crystallised in a tangible form in the early 1970s and the results obtained have encouraged considerable interest and activity in the molecular biology of polysaccharides. Their polyelectrolyte character has added a further dimension to their versatility in conformation and interactions. X-ray diffraction, and more recently n.m.r. spectroscopy, have provided important insights into their design and patterns of behaviour. Changes in puckered-ring structures occur as a function of hydration and ionic environment. They bind to proteins to create exceedingly complex and hierarchical proteoglycan structures. Chapter 5 reviews the polysaccharides produced by bacteria. These biopolymers have quite complex repeating sequences (up to six or so different saccharide residues per repeat) and glycosidic linkage geometries: some are linear and others have side branches. It is only in recent years that the commercial potential of these polysaccharides has been appreciated and that the exploitation of micro-organisms as polysaccharide producers using continuous fermentation techniques has been given serious consideration. Most of the conformations described in this chapter have only been investigated within the last ten years. The Klebsiella serotypes have so far provided the most comprehensive glimpse of variations in polysaccharide conformation using X-ray fibre diffraction and computer modelling procedures. These polysaccharides highlight the variety of geometries generated by variation of glycosidic linkage.

9 PREFACE xi The final chapter discusses the shapes and interactions of the blood-grouprelated oligosaccharides. It has not yet been possible to crystallise these substances but the preferred shapes can be generated using computer modelling procedures and energy minimisation. The oligosaccharides are usually bifurcated and attached to a polypeptide chain at a serine or threonine residue. The threedimensional shapes generated are introduced into the known active site regions of antibodies. Changes in specific saccharide units can have important consequences for binding. I sincerely hope that this selection of topics, covering many aspects of the structural molecular biology of polysaccharides, will encourage others to interest themselves in carbohydrate polymers. I thank the contributors for providing concise and authoritative reviews of their subject matter. Bristol, July 1985 E.D.T.A.