Carbohydrates Lecture Notes PDF

Carbohydrates  Carbohydrates General formula: Cn(H2O)n.....i.e. hydrates of carbon though some contain other elements such as nitrogen. Their chemistry is mainly that of the carbonyl (C=O) and hydroxyl (OH) groups. Alcohols tend to be very soluble in...

Carbohydrates  Carbohydrates General formula: Cn(H2O)n.....i.e. hydrates of carbon though some contain other elements such as nitrogen. Their chemistry is mainly that of the carbonyl (C=O) and hydroxyl (OH) groups. Alcohols tend to be very soluble in water due to hydrogen bonding between water molecules and the polar OH groups. 2  3 major groups Monosaccharide 1 unit e.g. ribose, glucose, fructose, Oligosaccharide 2 or more units e.g. sucrose, maltose, lactose Polysaccharide Many units e.g. cellulose, (100’s-1000’s) starch, glycogen n 3 Monosaccharides  Monosaccharides The most common monosaccharides are the pentoses (5 carbons) and the hexoses ( 6 carbons). Common names are used, ending in ‘ose’ 5  Monosaccharides Classified as aldoses or ketoses, depending on whether they contain an aldehyde or a ketone group. R R C O C O H R aldehyde ketone 6 Stereoisomers Multiple chiral carbons  Aldoses Those containing an aldehyde group are called aldoses H O C (CHOH)m (m = 1,2,3,4,5) CH2OH 8  Aldoses Those containing an aldehyde group are called aldoses 1 H O C 6 (CHOH)m (m = 1,2,3,4,5) CH2OH 9  Aldoses Those containing an aldehyde group are called aldoses 4 chiral carbons …. Therefore there are 24=16 stereoisomers , ie. 8 pairs of mirror images. H O C (CHOH)m (m = 1,2,3,4,5) CH2OH 10 3 chiral carbons …. Therefore there are 23=8 stereoisomers , ie. 4 pairs of mirror images. D/L nomenclature is based on orientation of the hydroxyl group on the C-2 of glyceraldehyde OH on the right – D OH on the CHO CHO left - L H C OH HO C H CH2OH CH2OH D-glyceraldehyde L-Glyceraldehyde 13 For larger molecules the carbon chain is numbered, starting at the C=O group end and the highest numbered chiral centre (asymmetric carbon) is used to designate D / L. 14  Optical isomers Enantiomers if non-superimposable mirror images of one another eg. D and L glucose:  Cyclic forms Cyclic forms result from the intramolecular reversible reaction of their C=O group with an OH group (usually the OH on the highest numbered chiral carbon). Alcohols can react with aldehydes and ketones to form “hemi acetyls” or “hemi ketals” 16 The pyranose form of a monosaccharide has a six- membered ring, while the furanose form has a five- membered ring. D-Glucose D-Fructose 17  Chair-type structure In reality, sugars exist in solution in the most thermodynamically favourable arrangement. For most 6-membered rings this is the chair- type structure. C1 = favoured conformation 18  Mutarotation (interconversion) When sugars such as glucose are in the ring form, carbon 1 is chiral and is called the anomeric carbon. The new stereoisomers, termed anomers, are designated  and . 19  Anomers -anomer : OH group at C-1 is on the opposite side of the ring from the CH2OH group…. shown here below the plane of the ring. -anomer : OH group at C-1 is on the same side of the ring as the CH2OH group…. D-Glucose shown here above the plane of the ring. 20 21 Most stable form of Glucose in solution C1 conformation and the  anomer is preferred for D-glucose in solution and therefore this form predominates. 22 Glycoside formation  Glycosides Cyclic forms of monosaccharides react with alcohols or with amines to form glycosides. Bonding is through the anomeric carbon. Locks ring closed Methyl--D- Glycosidic glucoside bond 24  Glycosides Many plant pigments, toxins, flavourings and steroids occur in cells as glycosides. eg. salicin - bitter tasting glycoside from willow bark, traditionally used to Note glycone (sugar) reduce fever. and aglycone (non sugar) portions glucose and salicylic alcohol 25 (metabolized to  Toxic glycosides Foxgloves (Digitalis purpurea) provide the important heart stimulant, digitalin. They become harmful when the glycon portion is stripped off during digestion. 26 Amino sugar formation  Amino Sugars Amino sugars of glucose, mannose and galactose are common in nature. Hydroxyl group on carbon-2 is replaced with an amino group (NH2) (enzymic reaction). 28  eg. Blood groups Present in the biological markers on red blood cells, which distinguish blood type e.g.: N acetyl glucosamine 29 Oligosaccharides  The glycosidic linkage/bond. The cyclic form of one monosaccharid e can react with an alcohol group from another to form a disaccharide. 31  Properties of glycosidic bonds Stable under normal conditions Hydrolysed by acid + heat or specific enzymes 32  Occurrence of free oligosaccharides Free oligosaccharides other than disaccharides, are rarely found in biological systems and are usually associated with proteins or lipids. 33 Disaccharides  Cellobiose: From Sucrose: Main Lactose: 5-8% in cellulose hydrolysis. food sweetener. milk. Digestible by Does not occur Digestible by many humans. naturally. Not humans. digestible by humans. Maltose: From starch hydrolysis, used in food fermentations. Digestible by humans. Common ingredient in baby food 36  A closer look at lactose: Lactose consists of β-D-galactose and D- glucose molecules bonded through a β1-4 glycosidic linkage. The C1 on the glucose unit is not involved in bonding 37  A closer look at maltose Maltose is a disaccharide formed by the linkage of two monosaccharides with the elimination of water. OH O OH Maltose HO HO C H O + HO-H OH + HO H OH HO OH HO C O O α-D-glucose HO HO O HO OH C HO OH C HO H HO H β-D-glucose α-glycoside bond Two glucose units joined C1 to C4  lactose, cellobiose and maltose The glucose unit, shown on the right, in lactose, cellobiose and maltose can exist in the open chain form in solution and therefore these disaccharides possess a free aldehyde group. They are therefore reducing and can exist in both  and  anomeric forms in solution. 39  A closer look at sucrose The glycosidic bond in sucrose is an , (1-2) linkage and involves the anomeric carbons of both its constituent sugar residues. 40  The rings of glucose and fructose are effectively locked closed and therefore the disaccharide does not possess a free aldehyde group in solution and sucrose is non reducing. 41  Enzyme hydrolysis Human enzymes can hydrolyse the linkage in sucrose, lactose and maltose but not the  1-4 linkage in cellobiose (a derivative of cellulose). Enzymes are specific in their actions. 42 Polysaccharides  General properties Contain many, often tens of thousands of monosaccharide units joined by glycosidic linkages. Each addition carried out by a specific enzyme. Linear or branched. May be homogeneous or heterogeneous. Commonly food reserves and structural components of cells. Often insoluble and difficult to purify. 44  Naming polysaccharides Originally named according to their source, properties or function. Now named on the basis of their monosaccharide units: homopolysaccharides - replace “ose” of sugar with “an” eg. glucan. heteropolysaccharides - use structural unit of main chain, prefixed with names of other units. eg. galactomannan 45 Some common glucans Starch Glycogen Cellulose  Amylose Linear with all the glucose units linked via (1-4) linkages [200-2000 glc units long]. 47 Amylose tends to form a helix. The polysaccharide forms a blue complex with iodine  Amylopectin Branched, having a small no. of (1-6) linkages at various points along an (1- 4) chain [up to 100,000 units]. Branching is similar to the veins of a leaf, starting with a single strand which branches out every 20-25 units. 49  Starch Is the main food reserve in plants eg. Cereals, potatoes, rice etc. (present as granules) Composed of two soluble polysaccharides.  Starch The proportion of amylose and amylopectin varies depending on plant source. Most starches (maize, potato, rice) contain 20 - 30% amylose. 51  Glycogen Energy reserve polysaccharide, abundant in the liver and in muscle cells. Similar to amylopectin in structure but more highly branched, with shorter branches (average of 12 glucose units). Around 60,000 glucose units. Built up and broken down by enzymic reactions. 52  Cellulose Structural component of plants cell walls. Composed of linear chains of 2000- 3000 glucose units, joined by (1 → 4) linkages. 53  Cellulose in plants Linearity - allows polymers to line up in fibres with a great deal of hydrogen bonding between adjacent chains (strong) and little interaction with water (insoluble). 54  Cellulose fibre Cellulose in Plants Macrofibril Microfibril Chains of glucose 55