CARBOHYDRATE METABOLISM

Carbohydrate metabolism means the various biochemical processes responsible for the formation, breakdown and interconversion of carbohydrates in living organisms. All carbohydrates share a general formula of approximately CnH2nOn; glucose is C6H12O6. Monosaccharides may be chemically bonded together to form disaccharides such as sucrose and longer polysaccharides such as starch and cellulose. Carbohydrates are a superior short-term energy reserve for organisms, because they are much simpler to metabolize than fats or proteins.

- Carbohydrates are typically stored as long polymers of glucose molecules with Glycosidic bonds for structural support (e.g. chitin, cellulose) or energy storage (e.g. glycogen, starch). - However, the strong affinity of carbohydrates for water makes storage of large quantities of carbohydrates inefficient due to the large molecular weight of the solvated water-carbohydrate complex. - In some organisms, excess carbohydrates are catabolised to form Acetyl-CoA, where they enter the fatty acid synthesis pathway. - Fatty acids, triglycerides, and other lipids are commonly used for long-term energy storage.

Dietary Carbohydrates: Monosaccharides: glucose, fructose and galactose in fruits and honey & obtained by hydrolysis of oligo- & polysacs. 2. Disaccharides: sucrose, lactose, maltose (by hydrolysis of starch). 3. Polysaccharides: starch (in potatoes, rice, corn and wheat) Cellulose (in cell wall of plants) not digested by humans due to absence of cellulase

Catabolism Carbohydrate catabolism is the breakdown of carbohydrates into smaller units. Oligo/polysaccharides are cleaved first to smaller monosaccharides by enzymes called Glycoside hydrolases.

Digestion of Carbohydrates:: 1- In the mouth: Salivary amylase hydrolyzes starch into dextrin +maltose 2- In the stomach: Due to drop of pH, salivary amylase acts for a very short time 3- In the small intestines: Pancreatic and intestinal enzymes hydrolyze the oligo- and polysaccharides as follows:

Absorption of monosaccharides: • Particularly important dietary carbohydrates include starch and disaccharides such as lactose and sucrose. None of these molecules can be absorbed for the simple reason that they cannot cross cell membranes, unlike the situation for monosaccharides, there are no transporters to carry them across. • Absorption of glucose and other monosaccharides occurs in the small intestine only by co-transport with sodium, across the epithelium and into blood.

Fate of absorbed monosaccharides: In the liver, fructose and galactose are converted to glucose. Fate of glucose: A. Uptake by different tissues. B. Utilization by the tissues: in the form of:

1. Oxidation to produce energy: - Major pathways (glycolysis & Krebs' cycle). - Minor pathways (hexose monophosphate pathway & uronic acid pathway) 2. Conversion to other substances: Carbohydrates: ribose (RNA,DNA), galactose (in milk), fructose (semen) C. Storage of excess glucose: As glycogen in liver and muscles, when these reserves are filled it is converted to TAG & deposited in adipose tissue. D. Excretion in urine If blood glucose exceeds renal threshold (180 mg/dL), it will be excreted in urine.

Oxidation of one gram of carbohydrate yields approximately 4 kcal of energy. Energy obtained from carbohydrate metabolism is usually stored in the form of ATP. The monosaccharide units can then enter into monosaccharide catabolism.

Krebs' Cycle (Citric Acid Cycle) (Tricarboxylic Acid Cycle)"TCA" • Site: mitochondria of every cell • Series of biochemical reactions that are responsible for complete oxidation of CHO, fats and Ptns to form : CO2 + H2O + Energy

Regulation of Carbohydrate Metabolism: The liver, pancreas, and other endocrine glands are all involved in controlling the blood glucose concentrations. During a brief fast, glucose is supplied to the extra cellular fluid from the liver through glycogenolysis. When fasting period is longer than 1 day, glucose is synthesized by gluconeogenesis.

Insulin: • Produced and secreted by β-cell of islets of Langerhans. • Hypoglycemic agent • Increases glycogenesis and glycolysis: Glucose glycogen pyruvate acetyl CoA. • Increases lipogenesis. • Decreases glycogenolysis. • Promotes the use of glucose by the muscles.

Glucagon: • Produced and secreted by α-cell of islets of Langerhans. • Hyperglycemic agent. • Increases glycogenolysis: Glycogen glucose. • Increases gluconeogenesis: Fatty acids acetyl CoA ketone. Proteins amino acids.

Human disease of carbohydrate metabolism are: Diabetes mellitus Lactose intolerance Fructose intolerance Galactosemia Glycogen storage disease

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