Renewable Resources for Functional Polymers and Biomaterials

Polysaccharides, Proteins and Polyesters

Peter Williams

• Publisher: Royal Society of Chemistry
• Year: 2015
• eBook ISBN: 9781782625841
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Medical Theory, Practice & Reference
• Index: Biological Sciences

This book details polysaccharides and other important biomacromolecules covering their source, production, structures, properties, and current and potential application in the fields of biotechnology and medicine. It includes a systematic discussion on the general strategies of isolation, separation and characterization of polysaccharides and proteins. Subsequent chapters are devoted to polysaccharides obtained from various sources, including botanical, algal, animal and microbial.

In the area of botanical polysaccharides, separate chapters are devoted to the sources, structure, properties and medical applications of cellulose and its derivatives, starch and its derivatives, pectins, and exudate gums, notably gum arabic. Another chapter discusses the potential of hemicelluloses (xylans and xylan derivatives) as a new source of functional biopolymers for biomedical and industrial applications. The algal polysaccharide, alginate, has significant application in food, pharmaceuticals and the medical field, all of which are reviewed in a separate chapter. Polysaccharides of animal origin are included with separate chapters on the sources, production, biocompatibility, biodegradability and biomedical applications of chitin (chitosan) and hyaluronan. With the increasing knowledge and applications of genetic engineering there is also an introduction in the book to nucleic acid polymers, the genome research and genetic engineering.

Proteins and protein conjugates are covered, with one chapter providing a general review of structural glycoproteins, fibronectin and laminin, together with their role in the promotion of cell adhesion in vascular grafts, implants and tissue engineering. Another chapter discusses general aspects of a number of industrial proteins, including casein, caseinates, whey protein, gluten and soy proteins, with emphasis on their medical applications, and with reference to the potential of bacterial proteins. Another natural polymer resource, microbial polyesters, although small compared with polysaccharides and proteins, is also gaining increasing interest in biomedical technology and other industrial sectors. One chapter, therefore, is devoted to microbial polyesters, with comprehensive coverage of their biosynthesis, properties, enzymic degradation and applications.

By dealing with biopolymers at the molecular level, the book is aimed at the biomedical and wider materials science communities and provides an advanced overview of biopolymers at the graduate and postgraduate level. In addition it will appeal to both academic and industrial life scientists who are involved in research and development activities in the medical and biotechnology field.

CHAPTER 1

Natural Polymers: Introduction and Overview

PETER A. WILLIAMS

Glyndwr University, Plas Coch, Mold Road, Wrexham LL11 2AW, UK

This chapter introduces the aim and scope of the book and presents an overview of the subject of natural polymers, mainly polysaccharides and proteins, but also microbial polyesters and nucleic acids. Details of the types, names and functions of individual polysaccharides and proteins are provided together with examples of their structural and physicochemical characteristics. It also highlights the main functional properties of these polymers including their rheological behavior, mechanisms of gelation, emulsification characteristics and film formation. The main applications of polysaccharides and proteins in medical and industrial sectors are also outlined with market size and trends.

1.1 Introduction to Biopolymers

Proteins and polysaccharides are found abundantly in nature and are major constituents of plants, animals and micro-organisms serving a number of important functions.^1,2 Proteins are composed of amino acids and although there are hundreds of different proteins they all consist of linear chains of the same twenty L-α-amino acids which are linked through peptide bonds formed by a condensation reaction. Each amino acid contains an amino group and carboxylic acid group with the general formula:

The R group differs for the various amino acids and can impart polar, non-polar, anionic or cationic characteristics. The amino acid units are linked together through a peptide bond to form a polypeptide chain as illustrated below:

Proteins consisting of between 15 and 10 000 amino acids are known. Since proteins contain both cationic and anionic charges due to the presence of ionisable groups, notably amine and carboxyl, they have a characteristic isoelectric point which corresponds to the pH at which the molecules have a net zero charge.

There are various levels of protein structure. The protein primary structure is defined by the characteristic sequence of amino acids of the polypeptide chain. Certain amino acids within the chain give rise to local secondary structures such as the α helix and the pleated sheet. There is a tendency for the more hydrophobic amino acids present to reside within the core of the molecule so that they are less exposed to the aqueous environment, and the overall shape of the protein that is formed (the tertiary structure) is stabilised by a range of interactions including hydrogen bonds, disulfide bonds and salt bridges. The protein molecules may self-associate to create a larger assembly referred to as quaternary structures.

Proteins tend to have either a linear or globular conformation and have a unique characteristic molecular mass. Linear proteins function as structural elements in the connective tissue of animals. The polypeptide chains are arranged in parallel forming long fibres. Fibrous proteins are insoluble in aqueous environments and are mechanically strong. Examples include collagen, found in tendons, cartilage and bone, and keratin, found in hair, skin and nails. Other proteins tend to fold into compact spherical or globular conformations. Globular proteins tend to be soluble in aqueous environments and are involved in transport processes or in dynamic functions in the cell. There are also special classes of proteins such as enzymes which are able to selectively cleave covalent bonds in biomacromolecules and antibodies which can attach to specific binding sites. Table 1.1 provides a list of common proteins together with their biological functions.

Table 1.1 List of common proteins and their biological function.

Protein	Biological function
Collagen	Fibrous connective tissue (tendons, cartilage, bone)
Elastin	Elastic connective tissue (ligaments)
Keratin	Hair, skin, nails
Sclerotin	Exoskeletons of insects
Fibroin	Spiders web
Myosin	Thick filaments in microfibril
Actin	Thin fibrils in microfibril
Haemoglobin	Transports oxygen in blood
Myoglobin	Transports oxygen in muscle cells
Serum albumin	Transports fatty acids in blood
Ovalbumin	Egg-white protein
Casein	Milk protein
Ferritin	Stores iron in the spleen
Gliadin	Seed protein of wheat
Soy	Seed protein of soya bean
DNA polymerase (enzyme)	Replicates and repairs DNA
Galactosidase (enzyme)	Cleave galactose glycosidic bonds

Proteins are sometimes referred to as ‘simple’ and ‘conjugated’. Simple proteins consist purely of linear chains of amino acids while conjugated proteins have organic or inorganic components linked to the amino acid chain. Examples of the latter include lipoproteins, nucleoproteins and glycoproteins which have lipids, nucleic acids and carbohydrate groups attached respectively. The chapters by Holt and Pham and Mann and Turner review nucleic acid polymers and glycoproteins respectively.

Polysaccharides are composed of sugar units which are connected through so-called glycosidic bonds. The bond occurs by a condensation reaction between the anomeric carbon (C1) of one sugar residue and an oxygen atom from a hydroxyl group on another sugar residue. Since there are hydroxyls present on the C2, C3, C4 and C6 carbons the bonds formed can be 1 → 2, 1 → 3, 1 → 4, or 1 → 6 linked. Figure 1.1 shows disaccharide residues which are linked 1 → 4 and 1 → 6.

Figure 1.1 Disaccharide repeat units for glucopyranose units linked β-(1 → 4) (top) and β-(1 → 6) (bottom). ϕ, ψ and ω define the angles of rotation.

The rotation around the glycosidic bond and can be defined by the torsion angles ϕ, ψ, and ω. Since the hydroxyl groups on C1 can be above or below the plane of the ring the linkages are referred to as α or β. The nature of the glycosidic bond leads to a variety of secondary structures. For example, β-(1 → 4) linkages give rise to extended ribbon-like polysaccharide chains which are relatively stiff and such molecules commonly hav...

Table of contents

• Cover
• Title
• Copyright
• Preface
• Contents
• Chapter 1 Natural Polymers: Introduction and Overview
• Chapter 2 Natural Polymer Resources: Isolation, Separation and Characterization
• Chapter 3 Cellulose and Its Derivatives in Medical Use
• Chapter 4 Xylan and Xylan Derivatives – Basis of Functional Polymers for the Future
• Chapter 5 Starch and its Derived Products: Biotechnological and Biomedical Applications
• Chapter 6 Gum Arabic and other Exudate Gums
• Chapter 7 Alginates: Existing and Potential Biotechnological and Medical Applications
• Chapter 8 Pectins: Production, Properties and Applications
• Chapter 9 Hyaluronan: a Simple Molecule with Complex Character
• Chapter 10 Chitin and Chitosan: Sources, Production and Medical Applications
• Chapter 11 β-Glucans
• Chapter 12 Microbial Polyesters: Biosynthesis, Properties, Biodegradation and Applications
• Chapter 13 Glycoproteins and Adhesion Ligands: Properties and Biomedical Applications
• Chapter 14 Nucleic Acid Polymers and Applications of Recombinant DNA Technology
• Subject Index