Conformation of Carbohydrates

V. S. R. Rao

• Publisher: CRC Press
• Year: 2019
• ISBN:
  9781000725360
• Edition: 1
• Topic: Medicine
• Subtopic: Biochemistry in Medicine

This text will give the reader a firm understanding of all aspects of carbohydrate conformation by describing and explaining the importance of interactions between carbohydrates and interactions of carbohydrates with proteins, nucleic acids or any other macromolecule., The authors have gathered a wealth of information on carbohydrate structures, different methods of conformational analysis, the role of carbohydrates as recognition molecules in biological systems and their industrial applications., Whether you are a student, teacher or a basic researcher, this text book is a 'one-stop' source of current information on carbohydrate conformation and the potential use of conformational properties in industry and also of their crucial role in important biological events such as cell-cell interaction, cell adhesion, cellular signaling mechanism.

1. CONFIGURATION OF MONOSACCHARIDES

1.1 Introduction

Carbohydrates, as the name suggests, are molecules mainly made up of carbon, oxygen and hydrogen atoms. They are abundant in nature, constituting a significant portion of our diet, and serve mainly as energy stores, e.g., starch in plants and glycogen in animals. Carbohydrates also act as structural materials, like cellulose in plants, peptidoglycans in bacteria and chitin in the exoskeletons of arthropods. They also serve as lubricants and provide support to the fibrous and cellular elements of tissues. Specific carbohydrates serve as cell surface recognition signals for antibodies, hormones, toxins, etc. Some of the derivatives of simple carbohydrates, such as coenzymes, are key elements in the cellular metabolism. Nucleic acids, in a sense, are also derived from carbohydrates since their backbones are composed of deoxyribose in DNA and ribose in RNA. In addition to these natural functions, carbohydrates have several industrial applications, e.g., starch in the manufacture of baked goods and pastas, gums in food processing, mono- and oligosaccharides as sweeteners, cellulose in textile industries, wood pulp in paper industry, as fermentors to make alcoholic beverages and as antibiotics, anticoagulants, vitamin C, etc., in pharmaceutical industries.

The twentieth century has witnessed the greatest growth in the structure determination of biopolymers and hence increased our understanding of these biomolecules. The versatile properties of carbohydrates are fully commensurate with a diverse array of molecules that can be generated from a limited number of monosaccharides as building blocks by linking them in a variety of ways. Thus, a basic knowledge of the chemical structure of carbohydrates is essential towards the development of conformational analysis and visualization of the geometries of complex carbohydrates.

1.2 Classification

Derived from the Latin word saccharum for sugar, which itself has its origin in the Sanskrit word sarkhara, carbohydrates are commonly classified as monosaccharides, disaccharides, trisaccharides, oligosaccharides and polysaccharides. A monosaccharide is defined as that unit which cannot be further hydrolyzed into smaller carbohydrates. A disaccharide, as the name implies, can be hydrolyzed into two monosaccharides. Similarly, tri-, tetra- and penta-saccharides on hydrolysis give 3,4 and 5 monosaccharides, respectively. Generally, the term oligosaccharide (from oligos, Greek for few) is used to describe a molecule that contains 10 to 20 monosaccharides and the term polysaccharide is reserved to describe a large carbohydrate molecule.

Carbohydrates are often found covalently linked to other biomolecules such as proteins (glycoproteins and proteoglycans) and lipids (glycolipids and lipopolysaccharides). To distinguish these molecules from pure carbohydrates, they are collectively called “glycoconjugates.”

1.3 Simple Monosaccharides

Monosaccharides are generally represented by the empirical formula C_nH_2nO_n or C_n.(H₂O)_n. The latter representation led to the belief that they are hydrates of carbon, and so this family of compounds came to be called “carbohydrates.” The carbon skeleton of acyclic monosaccharides is unbranched and a hydroxyl group is attached to each carbon except for the one which carries the carbonyl oxygen (Figure 1.1). Hence, these molecules are more appropriately known as “polyhydroxyaldoses” or “polyhydroxyketoses” (or simply, aldoses or ketoses) depending on the position of the carbonyl group. In aldoses, the carbonyl group is at one end of the chain, whereas in ketoses it can be in any position except at the end. Ketoses are further classified as 2-ketoses, 3-ketoses, etc., to indicate the position of the carbonyl group. As the most commonly encountered ketoses are 2-ketoses, an unspecified ketose refers to a 2-ketose. Depending on the number of atoms in the carbon skeleton, monosaccharides are further classified as triose, tetrose, pentose, hexose, heptose, octose, nonose and decose. The position of the carbonyl group and the number of carbon atoms are both included in the general terms used for identifying the monosaccharides. For example, glucose and mannose are aldohexoses, as they are both six-carbon sugars carrying an aldehyde group. Similarly, xylose and ribose are aldopentoses as they have five carbon atoms with an aldehyde group. Ketoses are also named by adding the suffix “ulose” to the base name which indicates the number of carbon atoms. Thus, fructose, also known as laevulose, is an example of a ketohexose or a hexulose (six carbon atoms with a carbonyl group at position 2).

Figure 1.1 Examples of simple monosaccharides. The atoms are numbered such that the carbonyl group gets the lowest possible number.

The smallest of all the monosaccharides is glyceraldehyde, the aldotriose (Figure 1.2). It is “dissymmetric” i.e., one of its carbon atoms, called the “asymmetric atom” or the “chiral center”, has four nonidentical substituents. Hence, two isomeric forms of glyceraldehyde, which are mirror images of each other, are possible. These mirror images, known as “enantiomers” are designated D-glyceraldehyde and L-glyceraldehyde. All the higher aldoses derived from D-glyceraldehyde belong to the D-series and similarly, all those derived from L-glyceraldehyde belong to the L-series. Dihydroxyacetone, the simplest ketotriose, is not often considered to be a carbohydrate since it has no chiral center (Figure 1.2). Hence, erythrulose, a ketotetrose with one chiral center, is taken as the reference compound for classifying the ketoses as belonging to either the D or L-series. Still, all ketoses can be thought of as originating from dihydroxyacetone, the ketotriose. Most monosaccharides that occur in nature are D-sugars, whereas the amino acids which constitute proteins, belong to the L-series. D-Amino acids are found, however, in other biomolecules as are L-sugars such as L-arabinose, L-rhamnose, L-fucose and L-galactose.

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Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Chapter 1. CONFIGURATION OF MONOSACCHARIDES
• Chapter 2. METHODS OF CONFORMATIONAL ANALYSIS
• Chapter 3. CONFORMATION OF MONOSACCHARIDES
• Chapter 4. CONFORMATION OF DISACCHARIDES
• Chapter 5. CONFORMATION OF OLIGOSACCHARIDES
• Chapter 6. FOLDING PATTERNS OF HOMOPOLYSACCHARIDES
• Chapter 7. FIBER DIFFRACTION ANALYSIS OF POLYSACCHARIDES
• Chapter 8. STRUCTURES OF POLYOLIGOSACCHARIDES
• Chapter 9. PEPTIDOGLYCANS
• Chapter 10. CARBOHYDRATE-PROTEIN INTERACTIONS
• Index