Biomolecules2 Carbohydrates Notes PDF

Carbohydrates Monosaccharides Disaccharides Polysaccharides 1 1  Carbohydrates Sugars photosynthesis 6CO2 + 6H2O + h...

Carbohydrates Monosaccharides Disaccharides Polysaccharides 1 1  Carbohydrates Sugars photosynthesis 6CO2 + 6H2O + h C6H12O6 + 6O2 Thermodynamic Stored energy stability Respiration H‐O H‐O O=O H H O=C=O O=O O=C=O H‐O O=O H O=C=O Glucose 2 Biomass contains a lot of stored energy. The energy is stored as a combination of covalently bonded biomolecules and dioxygen in the atmosphere. Carbohydrates such as the sugar glucose 2  Carbohydrates Monosaccharides H O Aldehyde C H C OH Aldose have general formula CnH2nOn Ketone HO C H In linear form have one carbonyl Ketose H C OH How many chiral centres? H C OH H C OH H Glucose Fructose 3 Monosaccharides are 3-6 carbons long with formula CnH2nOn. Despite being simple molecules, they have many different isomers, structural and optical. Their chemistry revolves around the presence of a carbonyl group and several OH groups. Complex stereochemistry requires special drawing conventions. In Nature, the n-1 carbon always has D stereochemistry (usually R configuration) 3  Carbohydrates Monosaccharides H O Aldehyde C H C OH Aldose have general formula CnH2nOn Ketone HO C H In linear form have one carbonyl Ketose H C OH How many chiral centres? H C OH H C OH H Glucose Fructose Fischer projection: CHO CHO defines each chiral centre H OH H OH R HO H HO H S horizontal bounds point outwards H OH = H OH R R H OH H OH CH2OH CH2OH D‐Glucose the mirror image L‐Glucose is not found in nature 4 Monosaccharides are 3-6 carbons long with formula CnH2nOn. Despite being simple molecules, they have many different isomers, structural and optical. Their chemistry revolves around the presence of a carbonyl group and several OH groups. Complex stereochemistry requires special drawing conventions. In Nature, the n-1 carbon always has D stereochemistry (usually R configuration) 4  Carbohydrates Monosaccharides CHO CHO CH2OH H OH H OH O HO H HO H HO H HO H H OH H OH H OH H OH H OH CH2OH CH2OH CH2OH Galactose Glucose Fructose 5 Fischer projection facilitates the comparison of different sugars – particularly in the minimal form without Hs. 5  Carbohydrates Monosaccharides Fischer Projection Haworth Projection 1 Chair representation H O Hemiacetal group C OH H OH 6 H O HO = HO HO OH OH H OH 1 OH H H CH2OH Pyranose form 6 6 Monosaccharides also cyclise to form 5 or 6 membered rings. The cyclisation process is reversible, generating an equilibrium in solution. The cyclic forms can be drawn most easily as Howarth projections, but remember that the 6 – membered rings are really in chair conformation. 6  Carbohydrates Monosaccharides Identifying a sugar: Haworth Projection HO OH H O H H OH H OH OH H 1 (Anomeric C) 7 Identifying a sugar from the molecular structure by converting to Fischer projection. 1) Find anomeric carbon. 2) follow carbon chain around ring and draw Howarth projection. 7  Carbohydrates Monosaccharides Fischer Projection Haworth Projection 1 H O C OH OH HO CH2OH 6 8 Identifying a sugar from the molecular structure by converting to Fischer projection. 1) Find anomeric carbon. 2) follow carbon chain around ring and draw Howarth projection. 3) Go around the ring again drawing Fischer projection, put down OHs on the right and up OHs on the left. 8  Carbohydrates Monosaccharides Haworth Projection Fischer Projection 1 H O C OH OH HO OH CH2OH 6 D‐Gulose 9 …… 4) Determine the stereochemistry of the ring O by breaking the anomeric bond and rotating C6 into the ring. 9  Carbohydrates Monosaccharides Example: Identify this sugar…. 10 Identifying a sugar from the molecular structure by converting to Fischer projection. 1) Find anomeric carbon. 2) follow carbon chain around ring and draw Howarth projection. 10  Carbohydrates Monosaccharides Example: Identify this sugar…. Howarth Projection 6 CH OH 2 O OH 1 OH OH OH 11 Identifying a sugar from the molecular structure by converting to Fischer projection. 1) Find anomeric carbon. 2) follow carbon chain around ring and draw Howarth projection. 11  Carbohydrates Monosaccharides Howarth Projection Fischer Projection 6 CH OH 2 O OH 1 OH OH OH 12 Identifying a sugar from the molecular structure by converting to Fischer projection. 1) Find anomeric carbon. 2) follow carbon chain around ring and draw Howarth projection. 3) Go around the ring again drawing Fischer projection, put down OHs on the right and up OHs on the left. 12  Carbohydrates Monosaccharides Howarth Projection Fischer Projection 6 CH OH 2 O OH 1 OH OH OH D‐Allose 13 Identifying a sugar from the molecular structure by converting to Fischer projection. 1) Find anomeric carbon. 2) follow carbon chain around ring and draw Howarth projection. 3) Go around the ring again drawing Fischer projection, put down OHs on the right and up OHs on the left. 13  Carbohydrates Glucose Howarth Projection Anomeric Carbon ‐pyranose ‐pyranose 36 % 64 % 6 H 6 HOH2C H HOH2C O OH HO 1 O OH HO OH 1 D‐glucose OH 0.25 % OH OH ‐furanose ‐furanose ~0 % ~0 % 14 Monosaccharides also cyclise to form 5 or 6 membered rings. The cyclisation process is reversible, generating an equilibrium in solution. This leads to a mixture of 5 possible forms. For glucose the b-pyranose form predominates. 14  Carbohydrates Ribose Howarth Projection 5 5 O O OH 1 1 Anomeric OH OH OH Carbon OH OH OH OH ‐pyranose ‐pyranose 20 % 60 % 5 5 CH2OH O 1 1 D‐ribose OH OH OH 0.1 % ‐furanose ‐furanose 7% 13 % 15 In Ribose – commonly found in nucleotides – the 5-membered furanose form is more stable than for glucose. 15  Carbohydrates Nucleotides Ribose (RNA) Deoxyribose (DNA) 16 Ribose is the central group in nucleotides (see later) and forms the backbones of RNA and DNA connected together by phosphate esters. 16  Carbohydrates Glycosidic Bonds 6 CH2OH D‐Glucose O OH OH 1 6 CH2OH HO OH OH O OH OH 1 D‐Glucose D‐Fructose D‐Galactose ‐ H2O OH ‐ H2O 17 Monosaccharides can join through glycosidic bonds – the most common connect C1-C2, C1-C4 and C1-C6. The simplest join 2 sugars to form disaccharides. Glyocosidic bonds can also connect sugars to bases (in nucleotides) and to proteins and peptides in glycoproteins and glycopeptides. 17  Carbohydrates Glycosidic Bonds D‐Galactose D‐Glucose D‐Glucose D‐Fructose Lactose Sucrose (1,4 linkage) (1,2 linkage) 18 Monosaccharides can join through glycosidic bonds – the most common connect C1-C2, C1-C4 and C1-C6. The simplest join 2 sugars to form disaccharides- e.g. lactose, sucrose. b-linkages occur on opposite sides of the sugar molecules, a-linkages on the same side (same or opposite stereochemistry). 18  Carbohydrates Polysaccharides ‐1,4 Amylose A polymer of D‐glucose (‐pyranose form) linked by ‐1,4 glycosidic bonds 19 Polysaccharides act as energy stores and structural biomaterials. Examples include amylose, a plant starch. Plant starches are easily digested into glucose, feeding in to our energy generation processes – glycolysis. 19  Carbohydrates Polysaccharides ‐1,4 Amylose The bend in the chain caused by ‐1,4 glycosidic bonds results in a spiral structure 20 Polysaccharides act as energy stores and structural biomaterials. Examples include amylose, a plant starch. Plant starches are easily digested into glucose, feeding in to our energy generation processes – glycolysis. 20  Amylose Carbohydrates 21 Polysaccharides act as energy stores and structural biomaterials. Examples include amylose, a plant starch. Plant starches are easily digested into glucose, feeding in to our energy generation processes – glycolysis. 21  Carbohydrates Polysaccharides Cellulose A polymer of D‐glucose (‐pyranose form) linked by ‐1,4 glycosidic bonds ‐1,4 22 Cellulose is also a straight chain polymer of glucose. It is the main structural material of plants and is used to form rigid cell walls. Unlike amylose, it is not digestible by humans and so constitutes a large proportion of dietary fibre (chain is more linear, tightly packed with H-bonding, difficult to digest) 22  Cellulose Carbohydrates 23 Cellulose is also a straight chain polymer of glucose. It is the main structural material of plants and is used to form rigid cell walls. Unlike amylose, it is not digestible by humans and so constitutes a large proportion of dietary fibre (chain is more linear, tightly packed with H-bonding, difficult to digest) 23  Carbohydrates Polysaccharides ‐1,6 ‐1,4 Glycogen/Amylopectin A polymer of D‐glucose (‐pyranose) with 2 different types of glycosidic bond to introduce branching 24 Glycogen (mammals) and Amylopectin (plant starch) are glucose polymers consisting of a-1,4 linked chains, branched by using a-1,6 glycosidic bonds. Both are easily hydrolysed to glucose. Glycogen is a main energy store in our muscles and liver. Depletion during endurance racing causes a noticeable energy crash. Amylose simply has less branching. 24  Carbohydrates Polysaccharides Chitin ‐1,4 A polymer of N‐Acetylglucosamine linked by ‐1,4 glycosidic bonds 25 Chitin has the some structure as cellulose, but has acetamide groups added to the 2-position of each glucose unit. This forms a very strong polymer, found in crab shells and fungal cell walls. 25  Carbohydrates Metabolism H O Pyruvate HO O H OH 3O2 3O2 HO H 2 O 6CO2 6CO2 6H- H OH 6H2O 6H+ H OH 2H- 2H+ H OH Oxidative H Glycolysis TCA Cycle Phosphorylation Glucose Metabolism of glucose during respiration results in the combustion products – the complicated steps above enable energy to be extracted in the form of ATP – this can directly power muscle contraction (among other things) 26 Glucose is the most abundant carbohydrate and the main source of energy in most life-forms. It is used as an energy carrier (e.g. in the blood) and an energy store in polysaccharides. The hydrides (H-) above are carried by NADH and FADH2 and their energy extracted by a chain of enzyme catalysed reactions and electron transfers. Blood glucose levels are closely regulated. Regulatory malfunction causes diabetes. 26  Carbohydrates Monosaccharides Disaccharides Polysaccharides 27 27