The struction and function of Carbohydrates by David Matongwe from Leihgninger principles of biochemistry.

Biochemistry of carbohydrates

1. CARBOHYDRATES BMS2110

2. Introduction Classification Structure Function

3. INTRODUCTION A carbohydrate is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms. usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula Cm(H2O)n.

4. carbohydrates are divided into four chemical groups: monosaccharides , disaccharides, oligosaccharides, and polysaccharides. Monosaccharides and disaccharides, the smallest (lower molecular weight) carbohydrates commonly referred to as sugars . Eg fructose and glucose.

5. CLASSIFICATION Carbohydrates are classified into three groups: monosacharides , oligosaccharides and polysaccharides Monosacharides are the simplest and contain free eldehyde (-CHO) and ketones(> C=O) groups

6. Aldehydes contain the carbonyl group bonded to at least one hydrogen atom while Ketones contain the carbonyl group bonded to two carbon atoms In an aldehyde compound, the carbonyl group is attached to a carbon atom at the end of a carbon chain while in a ketone compound, the carbonyl group is attached to a carbon atom within the carbon chain

7. Monosaccharides cannot be hydrolyzed into simpler carbohydrates and may be classified as trioses , tetroses , pentoses, hexoses, or heptoses, depending upon the number of carbon atoms Classification of Important Sugars Aldoses Ketoses Trioses (C 3 H 6 O 3 ) Glycerose (glyceraldehyde) Dihydroxyacetone Tetroses (C 4 H 8 O 4 ) Erythrose Erythrulose Pentoses (C 5 H 10 O 5 ) Ribose Ribulose Hexoses (C 6 H 12 O 6 ) Glucose Fructose Heptoses (C 7 H 14 O 7 ) — Sedoheptulose

8. Oligosaccharides consist of short chains of monosaccharide units , or residues, joined by characteristic linkages called glycosidic bonds The most abundant are the disaccharides , with two monosaccharide units, E.g. sucrose All common monosaccharides and disaccharides have names ending with the suffix “- ose .” In cells, most oligosaccharides consisting of three or more units do not occur as free entities but are joined to non-sugar molecules (lipids or proteins) in glycoconjugates

9. The polysaccharides are sugar polymers containing more than 10 monosaccharide units; some have hundreds or thousands of units. Some polysaccharides, such as cellulose, are linear chains; others , such as glycogen, are branched Both cellulose and glycogen consist of recurring units of D-glucose, but differ in the type of glycosidic linkages that results in different properties and biological roles Polysaccharides are sometimes classified as hexosans or pentosans , depending on the identity of the constituent monosaccharides (hexoses and pentoses , respectively)

10. Structure of carbohydrates Carbohydrates are hydrates of carbon and have the generic structure of C(n)H(2n)O(n). C hiral carbon centers   are also found at carbon atoms that are attached to four different substituents. they are placed at the corners of a tetrahedron, c hiral carbon atoms are also referred to as ' stereogenic carbons' or 'asymmetrical carbon atoms'

11. The chiral centers give rise to the many sugar stereoisomers found in nature and stereoisomerism in sugars is biologically significant. enzymes that act on sugars are strictly stereospecific, typically preferring one stereoisomer to another by three or more orders of magnitude, as reflected in binding constant values stereoisomers are molecules that have the same numbers of the same kinds of atoms have the same formula but differ in chemical and physical properties.

12. All the monosaccharides except dihydroxyacetone contain one or more asymmetric (chiral) carbon atoms and thus occur in optically active isomeric forms The simplest aldose, glyceraldehyde , contains one chiral center (the middle carbon atom ) and therefore has two different optical isomers, or enantiomers. en antiomers are pair of molecules that exist in two forms that are mirror images of one another but cannot be superimposed one upon the other .

13. A molecule with n chiral centers can have 2 n stereoisomers . Glyceraldehyde has 2 1 = 2; the aldohexoses , with four chiral centers, have 2 4 = 16 The stereoisomers of monosaccharides of each carbon-chain length can be divided into two groups that differ in the configuration about the chiral center most distant from the carbonyl carbon Those in which the configuration at this reference carbon is the same as that of D-glyceraldehyde are designated D isomers, and those with the same configuration as L-glyceraldehyde are L isomers 1

14. Two sugars that differ only in the configuration around one carbon atom are called epimers ; D-glucose and D-mannose, which differ only in the stereochemistry at C-2, are epimers , as are D-glucose and D- galactose (which differ at C-4 )

15. Some sugars occur naturally in their L form; examples are L-arabinose and the L isomers of some sugar derivatives that are common components of glycoconjugates

16. FUNCTIONS OF CARBOHYDRATES Source of energy Synthesis of Ribose from Glucose Conversion to Fat Promotes growth of desirable bacteria Contribution of Dietary Fibre Biosynthesis of Amino acids Structural component

17. Protein Sparing Action Oxidation of Fats Indispensability for Nervous System Role in Muscle Role in Liver

18. Source of energy Carbohydrates are least expensive source of energy to the body. Every gram of Carbohydrate, when oxidized yields on an average 4 kilo calories.

19. Protein Sparing Action The body uses carbohydrates as a source of energy , when they are adequately supplied in the diet , sparing protein for tissue building If diet does not adequate calories from carbohydrate , the dietary protein is oxidised as a source of energy There is also breakdown of tissue proteins to a greater extent This function of carbohydrate of serving as a source of energy and preventing dietary protein from being oxidised is called Protein Sparing Action .

20. Oxidation of fats In oxidation of fats, the acetyl CoA formed from the oxidation of fatty acids reacts with oxaloacetic acid from carbohydrate and amino acid metabolism to form citric acid which is oxidised through the TCA cycle back to oxaloacetic acid through a series of reactions Hence , for β-oxidation of fats, carbohydrate is essential If adequate amount of carbohydrate is not consumed , intermediary products of fat oxidation are accumulated Acetone , acetoacetic acid and β- hydroxy butyric acid accumulate and produce ketosis

21. Indispensability for Nervous System The main source of energy for central nervous system is glucose Prolonged hypoglycemia can lead to irreversible damage to the brain tissue

22. Role in musle Carbohydrates are the major source of energy for muscular work During muscular contraction, glycogen is broken down to lactic acid through glycolysis During the recovery period, lactic acid is first oxidised to pyruvic acid and then to acetyl CoA which is then oxidised to C02 and H2O, thus producing energy for muscular work

23. Role in liver These include detoxifying action and regulating influence of protein and fat metabolism Liver is rich in glycogen and is more resistant to certain poison such as carbon tetrachloride , alcohol, arsenic and toxins of bacteria The rate of oxidation of amino acids in liver is diminished if abundant supply of carbohydrates are available

24. Source of E nergy for Heart M uscle The heart muscle mainly uses glucose as source of energy In hypoglycemia, a definite adverse change in the working of the heart has been observed

25. Synthesis of Ribose from Glucose The pentose, ribose is present in RNA and in many nucleotides It is formed in the body from glucose by Hexose Mono Phosphate pathway

26. Conversion to Fat Excess of calories fed in diet in the form of carbohydrate is stored as fat in adipose tissue When the body is in need of energy, it can be released from the adipose tissue

27. Promotes the Growth of Desirable Bacteria Lactose has several functions in the gastrointestinal tract It promotes the growth of desirable bacteria, some of which are useful in the synthesis of B-complex vitamins Lactose also enhances the absorption of calcium

28. Contribute to Dietary Fibre A small amount of energy is produced from the release of short chain fatty acids in the large intestine Fibre stimulates the peristalic movement It helps in preventing many degenerative diseases

29. Biosynthesis of Amino Acids The carbon skeleton for the synthesis of alanine , aspartic acid and glutamic acid are provided by glucose during its oxidation, from pyruvic acid, oxalo acetic acid and β - ketoglutaric acid respectively

30. Structural Component Carbohydrates are precursors for many organic compounds As glycoproteins and glycolipids participate in the structure of cell membrane and cellular functions such as cell growth adhesion and fertilisation Carbohydrates are structural components of many organisms These include the fibre of plants, exoskeleton of some insects and cell wall of microorganisms