12 Carbohydrate metabolism Chemistry 176

Transcript:12 Carbohydrate metabolism Chemistry 176:
12 Carbohydrate metabolism Chemistry 176 Hyperglycaemia and diabetes mellitus 183 Physiology 176 Hypoglycaemia 194 This chapter discusses carbohydrate metabolism and its synthesize glucose, abnormalities, with emphasis on diabetes mellitus and store glucose in significant amounts, hypoglycaemia. In the next decade it is predicted that metabolize substrates other than glucose and ketones there will be about 250 million people worldwide with - plasma ketone concentrations are usually very low type 2 diabetes mellitus. and ketones are of little importance as an energy source under physiological conditions, CHEMISTRY extract enough glucose from the extracellular fluid The main monosaccharide hexoses are reducing sugars. (ECF) at low concentrations for its metabolic needs, Naturally occurring polysaccharides are long-chain because entry into brain cells is not facilitated by carbohydrates composed of glucose subunits (Table insulin. 12.1): Normally the plasma glucose concentration remains Starch , found in plants, is a mixture of amylose between about 4 mmol/L and 10 mmol/L, despite the (straight chains) and amylopectin (branched intermittent load entering the body from the diet. The chains). maintenance of plasma glucose concentrations below Glycogen , found in animal tissue, is a highly branched about 10 mmol/L minimizes loss from the body as well polysaccharide. as providing the optimal supply to the tissues. Renal tubular cells reabsorb almost all the glucose ?ltered PHYSIOLOGY by the glomeruli, and urinary glucose concentration Functions of extracellular glucose is normally too low to be detected by the usual tests, The main function of glucose is as a major tissue energy Glucose source. The simplified pathways of glycolysis and the Krebs cycle [tricarboxylic acid (TCA) cycle] are shown Hexose phosphates in Figures 12.1 and 12.2. The brain is highly dependent upon the extracellular glucose concentration for its energy supply; indeed, hypoglycaemia is likely to impair Triose phosphates cerebral function or even lead to irreversible neuronal damage. This is because the brain cannot: 2-Phosphoglycerate Table 12.1 Common reducing and non-reducing sugars Phosphoenolpyruvate Reducing sugars Non-reducing sugars Monosaccharides Glucose Pyruvate Fructose Galactose Lactate Disaccharides Lactose Sucrose Figure 12.1 Simpli?cation of glycolysis pathways. (galactose + glucose) (fructose + glucose) Reproduced with kind permission from Candlish Maltose JK and Crook M. Notes on Clinical Biochemistry . (glucose + glucose) Singapore: World Scienti?c Publishing, 1993. Physiology 177 Insulin Glucose- Ribose- Glucose 6-phosphate 5-phosphate Insulin is the most important hormone controlling plasma glucose concentrations. A plasma glucose TPP Transketolase concentration of greater than about 5 mmol/L acting via the