Biomolecules (PL1003) Notes on Carbohydrates PDF

Aldehydes or ketones i.e. carbonyl (C=O) group and alcohols i.e. hydroxyl (OH) groups. OH's are the only functional group present in all carbohydrates. Monosaccharides (one sugar unit); Oligosaccharides (2-100 sugar residues) or Polysaccharides (100 -- thousands of sugar residues). The most common...

Aldehydes or ketones i.e. carbonyl (C=O) group and alcohols i.e. hydroxyl (OH) groups. OH's are the only functional group present in all carbohydrates. Monosaccharides (one sugar unit); Oligosaccharides (2-100 sugar residues) or Polysaccharides (100 -- thousands of sugar residues). The most common are the pentoses (5 carbons) and the hexoses (6 carbons). Classed as aldoses (aldehyde group) or ketoses (ketone group). Sugars contain chiral carbons therefore optical isomerism exists. The two enantiomers are given either an "L" or a "D" prefix to designate which is which. By convention the enantiomer with the OH on the highest numbered chiral carbon shown on the **right** is "D" and On the **left** is "L". It is the D form which occurs in nature. Alcohols can react with aldehydes and ketones to form "hemi acetyls" or "hemi ketals*"*. Cyclic forms of sugars result from the intra-molecular reversible reaction of their C=O group with an OH group (usually the OH on the highest numbered chiral carbon). See (b) and (c) above. In solution there is a continuous opening and closing of the ring. The "OH" group on carbon 1 may be above or below the plane of the ring when it closes. The new stereoisomers, termed anomers, are designated  or . This continuous inter-conversion between the form,  form and the open chain form is called "mutarotation". **C1** **chair** conformation and the  **anomer** is preferred and therefore this form predominates in solution. This conformation minimises steric hindrance between the large OH groups. A weak oxidising reagent such as Fehling's solution oxidises the aldehyde group of the monosaccharide (CHO) to a carboxylic acid group (COOH). The copper of the Fehling's reagent is reduced to Cu~2~O in the process which precipitates as a brick red solid. +-----------------------------------+-----------------------------------+ | | Glucose Gluconic acid | +===================================+===================================+ | | A strong oxidising agent such as | | | dilute nitric acid, can oxidise | | | both ends of a monosaccharide | | | producing a dicarboxylic acid. | +-----------------------------------+-----------------------------------+ | | Specific | | | enzymes can oxidise the primary | | | alcohol end of an aldose such as | | | glucose, without oxidising the | | | aldehyde group to give uronic | | | acids. These commonly occur as | | | components of polysaccharides | | | e.g. | | | | | | hyaluronic acid, found in the | | | vitreous humour of the eye | +-----------------------------------+-----------------------------------+ | | The aldehyde group (CHO) can be | | | reduced to an hydroxyl group | | | (OH), using hydrogen as the | | | reducing agent to produce a poly | | | hydroxy alcohol (sugar alcohol). | | | Used as bulk sweeteners in food | | | and as a moisturising agent in | | | products such as cosmetics. | +-----------------------------------+-----------------------------------+ | | In nature, | | | many plant pigments, toxins, | | | flavourings and steroids occur as | | | glycosides. eg. salicin & | | | digitalin. These molecules have | | | glycone (sugar) and aglycone (non | | | sugar) portions. | +-----------------------------------+-----------------------------------+ | | Amino sugars of glucose, mannose | | | and galactose, and their acetyl | | | derivatives, are common in nature | | | e.g. present in the | | | oligosaccharide biological | | | markers on red blood cells -- | | | blood group. | +-----------------------------------+-----------------------------------+ | | The cyclic | | | form of one monosaccharide can | | | react with an alcohol group from | | | another to form a disaccharide | | | (enzyme catalysed condensation | | | reaction). The glycosidic bond is | | | strong, stable under normal | | | conditions and to break it | | | requires hydrolyses with acid + | | | heat [or] specific | | | enzymes. | +-----------------------------------+-----------------------------------+ | | Free oligosaccharides other than | | | disaccharides, are rarely found | | | in biological systems and are | | | usually associated with proteins | | | or lipids | +-----------------------------------+-----------------------------------+ +-----------------------------------+-----------------------------------+ | | Among these common | | | | | | disaccharides sucrose is unique | | | in that the anomeric carbons | | | (shown by ) of both sugars are | | | involved in forming the | | | glycosidic bond. closed -- non | | | reducing. | | | | | | This effectively locks both rings | | | | | | Human enzymes can hydrolyse the | | | linkage in sucrose, lactose and | | | maltose but not the 1-4 linkage | | | in cellobiose | +===================================+===================================+ | | Lactose has a β1-4 glycosidic | | | linkage -- indicates that C1 | | | (anomeric carbon) of galactose is | | | linked to C4 of the glucose. The | | | OH at C1 which reacted was above | | | the plane of the ring ie β. | +-----------------------------------+-----------------------------------+ | | Hydrolysed sucrose ie an equal | | | mixture of glucose and fructose. | +-----------------------------------+-----------------------------------+ | | Each sugar addition to the chain | | | carried out by a specific enzyme; | | | may be linear or branched; may be | | | homogeneous or heterogeneous; are | | | commonly food reserves and | | | structural components of cells. | +-----------------------------------+-----------------------------------+ | | **homopolysaccharides** - replace | | | "ose" of sugar with "an" eg. | | | glucan. **heteropolysaccharides** | | | - use structural unit of main | | | chain, prefixed with names of | | | other units. eg. | | | xylogalactomannan -- mannose | | | chain with xylose and galactose | | | present. | +-----------------------------------+-----------------------------------+ | | Osmotic pressure is proportional | | | to the number of solute molecules | | | \... So 1000 glc molecules has a | | | 1000 x higher osmotic pressure | | | than 1 molecule of glucose | | | polymer with 1000 units. | +-----------------------------------+-----------------------------------+ | | Polyglucan. Is the main food | | | reserve in plants eg. cereals, | | | potatoes, rice etc. (granules) | +-----------------------------------+-----------------------------------+ | | **Amylose:** **Linear** with all | | | the glucose units linked via | | | α**(1-4)** linkages \[200-2000 | | | glc units long\]; polysaccharides | | | with α**(1-4)** linkages tend to | | | form a helix -- iodine forms a | | | blue complex with this. | | | | | | **Amylopectin: Branched**, having | | | a small no. of α**(1-6)** | | | linkages at various points along | | | an α**(1-4)** chain \[up to | | | 100,000 units\]. | +-----------------------------------+-----------------------------------+ | | - Polyglucan | | | | | | - 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. | +-----------------------------------+-----------------------------------+ | | - Polyglucan | | | | | | - Structural component of | | | plants cell walls. | | | | | | - Composed of linear chains of | | | 2000-3000 glucose units, | | | joined by β**(1- 4)** | | | linkages. | | | | | | - 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). | +-----------------------------------+-----------------------------------+