Chapter 14 - Carbohydrate metabolism

Abstract

Carbohydrates are key components of the human diet. Glucose is the main monosaccharide absorbed in the intestine and used by cells of the body. Glucose is first phosphorylated to produce glucose-6-phosphate (G-6-P) by hexokinases (I–IV). Glycogenesis is the process by which glycogen is formed from glucose and it occurs mainly in the liver and muscle. It involves the conversion of G-6-P into glucose-1-phosphate (G-1-P) by phosphoglucomutase, formation of uridine diphosphate glucose, and the addition of glucose to glycogen. Glycogenolysis is very active in the liver and uses pathways different from those of glycogenesis. Glycolysis is the main pathway for glucose catabolism in tissues. It involves the conversion of G-6-P into fructose-6-phosphate (F-6-P) by phosphoglucoisomerase. F-6-P is further phosphorylated and then cleaved into two triose-phosphate molecules, glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP), by aldolase. After oxidation and phosphorylation, phosphoglycerate is produced and converted into pyruvate. Under anaerobic conditions, pyruvate is reduced to lactate. Under aerobic conditions, pyruvate undergoes oxidative decarboxylation in the mitochondria. Pyruvate is first converted into CO2 and acetate. Acetate, linked to CoA, enters the Krebs or citric acid cycle, where it reacts with oxaloacetate. The different steps of the cycle include the formation of citric acid, isocitrate, α-ketoglutarate, succinate, fumarate, and malate. The net energy yield from the total oxidation of glucose in the cell is 36–38mol of ATP per mole of glucose. An alternative route for glucose metabolism is the pentose phosphate pathway. Gluconeogenesis involves the formation of glucose and glycogen from noncarbohydrate compounds. Fructose and galactose metabolism involve their phosphorylation and subsequent cleavage. Glycoprotein synthesis occurs by the addition of carbohydrates to a protein through O-glycosidic and N-glycosidic bonds.