Carbohydrate Metabolism Prepared and compiled by Shuaib A

Carbohydrate Metabolism Prepared and compiled by: Shuaib A.

Digestion of Carbohydrates: In the mouth: Salivary amylase hydrolyzes starch into dextrin +maltose In the stomach: due to drop of p. H salivary amylase acts for a very short time In the small intestines: Pancreatic and intestinal enzymes hydrolyze the oligo- and polysaccharides as follows: Pancreatic amylase Starch maltose + isomaltose Maltase Maltose 2 glucose Lactase Lactose glucose + galactose Sucrase Sucrose glucose + fructose 2

Absorption of monosaccharides: 1. Simple diffusion: Depending on the concn gradient of sugars e. g. Fructose and pentose 2. Facilitated transport: It requires a transporter. e. g. Glucose, Fructose and galactose 3. Active transport : It needs energy derived from the hydrolysis of ATP. glucose & galactose are actively transported Against their concentration gradients by this mechanism. 3

Fate of absorbed monosaccharides: In the liver, fructose and galactose are converted to glucose. Fate of glucose: A. Uptake by different tissues (by facilitated diffusion) B. Utilization by the tissues: in the form of: 1. Oxidation to produce energy: - Major pathways (glycolysis & Krebs' cycle). 2. Conversion to other substances: Carbohydrates: ribose (RNA, DNA), galactose (in milk), fructose (semen) Lipids: Glycerol-3 P formation of triacylglycerols. Proteins: Non-essential amino acids which enter in formation of proteins. 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/d. L), it will be excreted in urine. 4

Glucose Oxidation Extracting Energy from Glucose: There are 2 major biochemical processes that occur in cells to progressively breakdown glucose with the release of various packets of energy: Glycolysis (occurs in the cytoplasm and is only moderately efficient). Krebs' cycle (takes place in the matrix of the mitochondria and results in a great release of energy). 5

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Glycolysis Embden-Meyerhof pathway (or) E. M. Pathway Definition: Glycolysis is defined as the sequence of reactions converting glucose (or glycogen) to pyruvate or lactate, with the production of ATP 7

GLYCOLYSIS Series of biochemical reactions by which glucose is converted to: -Pyruvate (in aerobic conditions)or -Lactate (in anaerobic conditions). Site: cytosol of every cell. Physiologically it occurs in: -muscles during exercise (lack of oxygen) -RBCs (no mitochondria). 8

Metabolism Thousands of chemical reactions are taking place inside a cell in an organized, well co-ordinated and purposeful manner; all these reactions are called as METABOLISM. 9

Food molecules Simpler molecules major pathways Anabolic Proteins, carbohydrates, lipids, nucleic acids etc. Catabolic CO 2+H 2 O 10

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Energy production of glycolysis: ATP production = ATP produced - ATP utilized ATP produced In absence of oxygen (anaerobic glycolysis) 4 ATP (Substrate level phosphorylation) ATP utilized Net energy 2 ATP From glucose to glucose 6 -p. From fructose -6 -p to fructose 1, 6 p. 2 ATP 4 ATP (substrate level phosphorylation) 2 ATP -From glucose to glucose 6 -p. 2 ATP from 1, 3 BPG. 2 ATP from phosphoenol pyruvate. From fructose -6 -p to fructose 1, 6 p. 8 ATP / 6 ATP (Pyruvate dehydrogenase 2 NADH, ETC, Oxidative phosphorylation) 2 ATP from 1, 3 DPG. 2 ATP from phosphoenol pyruvate In presence of oxygen (aerobic glycolysis) + 4 ATP or 6 ATP (from oxidation of 2 NADH + H in mitochondria). 13

Importance of Glycolysis: 1. Glycolysis provides mitochondria with pyruvic a oxaloacetate which is the primer of the Krebs' cycle. 2. Glycolysis provides dihydroxyacetone P glycerol 3 -P that is important for lipogenesis (TAG synthesis) 3. Energy production: Glycolysis liberates only a small part of energy from glucose 14

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 : CO 2 + H 2 O + Energy 15

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Energy yield of Krebs' cycle: • • • • 1 mole of acetyl Co. A through Krebs' cycle produces 12 ATPs: 1 ATP (substrate level oxidative phosphorylation). 1 FADH 2 → 2 ATP (respiratory chain oxidative phosphorylation). 3 NADH+H+→ 9 ATP(respiratory chain oxidative phosphorylation) oxidative decarboxylation of pyruvate gives 1 NADH+H+ → 3 ATP Thus net ATP gain is: 12 + 3 = 15 ATP Since 1 glucose molecule by undergoing glycolysis gives 2 pyruvate Thus 1 glucose molecule yields 15 × 2 = 30 ATP. 17

Importance of Krebs' cycle • • • 1. Energy production: 1 acetyl Co. A yields 12 ATP. 2. It is the final common metabolic pathway for complete oxidation of acetyl Co. A which results from the partial oxidation of CHO, fats and proteins (amino acids). 3. Interconversion of carbohydrates, fats and proteins (gluconeogenesis, lipogenesis, and formation of nonessential amino acids). 18