Overview of Carbohydrate Metabolism Introduction

Carbohydrates are carbon compounds that contain large quantities of hydroxyl groups. The presence of the hydroxyl groups allows carbohydrates to interact with the aqueous solution and to participate in hydrogen bonding All carbohydrates can be classified as either monosaccharides, oligosaccharides (from 2 – 10 monosaccharide units),or polysaccarides.

Monosaccharides The monosaccharides commonly found in humans are classified according to the number of carbons they contain in their backbone structures. The major monosaccharides contain four to six carbon atoms.

Carbohydrate Classifications Relevant examples Category Name # Carbons Glyceraldehyde, Dihydroxyacetone Triose 3 Erythrose Tetrose 4 ,Ribose, Ribulose Xylulose Pentose 5 ,Glucose, Galactose Mannose, Fructose Hexose 6 Sedoheptulose Heptose 7 Neuraminic (Sialic) acid Nonose 9

Disaccharides The glycosidic Covalent linkage of two monosaccharides result in disaccharides . Several physiogically important disaccharides are sucrose, lactose and maltose.

Sucrose : prevalent in sugar cane and sugar beets, is composed of glucose and fructose through an a-(1,2)b-glycosidic bond Sucrose

Lactose : is found exclusively in the milk of mammals and consists of galactose and glucose in a b-(1,4) glycosidic bond. Lactose

Maltose : the major degradation product of starch, is composed of glucose monomers in an a-(1,4) glycosidic bond.

Polysaccharides Most of the carbohydrates found in nature occur in the form of high molecular weight polymers called polysaccharides. The monomeric building blocks found in polysaccharides is D-glucose.

When polysaccharides are composed of a single monosaccharide building block, they are termed homopolysaccharides. Polysaccharides with more than one type of monosaccharide are termed heteropolysaccharides.

Glycogen Glycogen is the major form of stored carbohydrate in animals. This molecule is a homopolymer of glucose in a-(1,4) linkage; it is also highly branched, with a-(1,6) branch linkages occurring every 8-10 residues.

Glycogen is a very compact structure that results from the coiling of the polymer chains. This compactness allows large amounts of carbon energy to be stored in a small volume, with little effect on cellular osmolarity.

Starch Starch is the major form of stored carbohydrate in plant cells. Its structure is identical to glycogen, except for a much lower degree of branching (about every 20-30 residues). Unbranched starch is called amylose; branched starch is called amylopectin.

Digestion of Dietary Carbohydrates Dietary carbohydrate from which humans gain energy enter the body in complex forms, such as disaccharides and the polymers starch (amylose and amylopectin) and glycogen. The polymer cellulose is also consumed but not digested. (WHY)

The breakdown of polymeric sugars begins in the mouth. Saliva has a slightly acidic pH of 6.8 and contains lingual amylase that begins the digestion of carbohydrates.

The mixture of gastric secretions, saliva, and food, known collectively as chyme, moves to the small intestine. a-amylase enzyme is secreted by the pancreas and has the same activity as salivary amylase, producing disaccharides. Which converted to monosaccharides by the specific intestinal disaccharidases, sucrase, lactase, and maltase.

The resultant glucose and other simple carbohydrates are transported across the intestinal wall to the hepatic portal vein and then to liver parenchymal cells and other tissues. There they are converted to fatty acids, amino acids, and glycogen, or oxidized by the various catabolic pathways of cells.

Blood sugar levels are controlled by three hormones: insulin, glucagon, and epinephrine. If the concentration of glucose in the blood is too high, insulin is secreted by the pancreas. Insulin stimulates the transfer of glucose into the cells, especially in the liver and muscles, other organs are also metabolize glucose.

In the liver and muscles, most of the glucose is changed into glycogen by the process of glycogenesis (anabolism). Glycogen is stored in the liver and muscles until needed at some later time when glucose levels are low. If blood glucose levels are low, then epinephrine and glucogon hormones are secreted to stimulate the conversion of glycogen to glucose. This process is called glycogenolysis (catabolism).

If glucose is needed immediately when entering the cells to supply energy, it begins the metabolic process called glycoysis (catabolism). The end products of glycolysis are pyruvic acid and ATP.

Since glycolysis releases little ATP, further reactions continue to convert pyruvic acid to acetyl CoA and then citric acid in the citric acid cycle. The majority of the ATP is made from oxidations in the citric acid cycle in connection with the electron transport chain.

During exhausting muscular activity, pyruvic acid is converted into lactic acid rather than acetyl CoA. During the resting period, the lactic acid is converted back to pyruvic acid. The pyruvic acid in turn is converted back to glucose by the process called gluconeogenesis (anabolism). If the glucose is not needed at that moment, it is converted into glycogen by glycogenesis.

These processes are summarized in the Metaboism Summary in the graphic below. Each of these processes will be developed in greater detail in subsequent lectures