Carbohydrates

Functions Carbohydrates serve as: Carbohydrates serve as: Energy stores Energy stores Liver and muscle glycogen Liver and muscle glycogen Fuels Fuels Catabolised in glycolysis to produce ATP Catabolised in glycolysis to produce ATP Structural molecules Structural molecules eg. Ribose forms part of structure of ATP and nucleic acids (DNA and RNA) eg. Ribose forms part of structure of ATP and nucleic acids (DNA and RNA) Molecules on surface of cell membranes involved in: Molecules on surface of cell membranes involved in: –Cell recognition –Cell adherence –Hydrophilic properties attract water layer as lubricant From: Elliott WH & Elliott DC. (1997) Biochemistry and Molecular Biology. New York: Oxford University Press. p170 Adapted from: Bettelheim FA and March J (1990) Introduction to Organic and Biochemistry (International Edition). Philadelphia: Saunders College Publishing p383. From: Tortora GJ & Grabowski SR (2000) Principles of Anatomy and Physiology (9th Edition). New York: John Wiley & Sons, p62

Structures Carbohydrates exist as: Carbohydrates exist as: Monosaccharides Monosaccharides Single structure which can contain up to 8 carbon atoms Single structure which can contain up to 8 carbon atoms Not degraded by hydrolysis Not degraded by hydrolysis Disaccharides Disaccharides Two monosaccharides bonded together Two monosaccharides bonded together Polysaccharides Polysaccharides More than two monosaccharides bonded together More than two monosaccharides bonded together

Monosaccharides Empirical formula (CH 2 0) n Empirical formula (CH 2 0) n –n indicates number of carbons in structure Trioses (3 carbons) Trioses (3 carbons) –All CHO degraded to trioses during metabolism Tetroses (4 carbons) Tetroses (4 carbons) –Intermediaries during metabolism Pentoses (5 carbons) Pentoses (5 carbons) –Form ring structures –Most important is ribose Hexoses (6 carbons) Hexoses (6 carbons) –Most common monosaccharides –Glucose (Dextrose) most important –All CHO converted to glucose during digestion Heptoses (7 carbons) Heptoses (7 carbons) Octoses (8 carbons) Octoses (8 carbons) From: Tortora GJ & Grabowski SR (2000) Principles of Anatomy and Physiology (9th Edition). New York: John Wiley & Sons, p43

Disaccharides Composed of two monosaccharides joined by glycosidic bond Composed of two monosaccharides joined by glycosidic bond Produces water Produces water Degraded to monosaccharides by hydrolysis Degraded to monosaccharides by hydrolysis Uses water Uses water Formation of a glycosidic bond. From: Summerlin LR (1981) Chemistry for the Life Sciences. New York: Random House, p395.

Disaccharides Only important disaccharides are: Only important disaccharides are: Maltose Maltose Two glucose units Two glucose units Sucrose Sucrose Glucose and fructose Glucose and fructose Lactose Lactose Glucose and galactose Glucose and galactose Maltase, sucrase, lactase in brush border of GI tract hydrolyse disaccharides Maltase, sucrase, lactase in brush border of GI tract hydrolyse disaccharides Converted to glucose and absorbed Converted to glucose and absorbed Lactose intolerance - lack of lactase Lactose intolerance - lack of lactase From: Summerlin LR (1981) Chemistry for the Life Sciences. New York: Random House, p339.

Polysaccharides Large complex CHO that liberate many monosaccharies on hydrolysis Large complex CHO that liberate many monosaccharies on hydrolysis Glycogen one of most important Glycogen one of most important Storage form of CHO in animals Storage form of CHO in animals Liver and muscle Liver and muscle Formed by strings of glucose (1-4 bonds) Formed by strings of glucose (1-4 bonds) Strings joined approx. every 10 glucose units (1-6 bonds) Strings joined approx. every 10 glucose units (1-6 bonds) Glycogenesis - formation of glycogen Glycogenesis - formation of glycogen Glycogenolysis - hydrolysis of glycogen Glycogenolysis - hydrolysis of glycogen From: Summerlin LR (1981) Chemistry for the Life Sciences. New York: Random House, p339.

Phosphorylated sugars Important class of CHO due to charge of phosphate groups Important class of CHO due to charge of phosphate groups ATP ATP Phosphorylation gives high potential energy Phosphorylation gives high potential energy Glucose-6-phosphate Glucose-6-phosphate Phosphorylation by hexokinase traps glucose in skeletal muscle Phosphorylation by hexokinase traps glucose in skeletal muscle From: Stryer LS (1988) Biochemistry (3rd Ed.). New York: WH Freeman & Co, p338.