Carbohydrate Digestion and Absorption PDF

In the diet carbohydrates are present as complex polysaccharides (starch, glycogen), and to a minor extent, as disaccharides (sucrose and lactose). They are hydrolyzed to monosaccharide units in the gastro- intestinal tract.  Digestion of carbohydrates 1-In the mouth: -Salivary al...

In the diet carbohydrates are present as complex polysaccharides (starch, glycogen), and to a minor extent, as disaccharides (sucrose and lactose). They are hydrolyzed to monosaccharide units in the gastro- intestinal tract.  Digestion of carbohydrates 1-In the mouth: -Salivary alpha amylase (Ptyalin) acts on starch and glycogen. - hydrolysis of α-1,4- glycosidic linkages. 2- In the stomach: - The salivary amylase continues for 2-3 min. due to unsuitable PH. 3- In the small intestine: Digestion is carried out by 2 enzymes: I. The pancreatic amylase. II. Small intestinal enzymes. 2 1-The pancreatic amylase: acts on starch, glycogen and dextrin which escape digestion in the mouth. 2- Small intestinal enzymes: a) Maltase: hydrolyses maltose into two molecules of glucose. b) Lactase: hydrolyses lactose into glucose and galactose. c) Sucrase: hydrolyses sucrose into glucose and fructose. So, the digestion of carbohydrates results in production of monosaccharides like: Glucose, fructose, galactose, mannose and pentoses. 3  Absorption of carbohydrates Mechanisms of absorption: (1)Passive diffusion: -Depend on concentration gradient of sugar -No energy is required for such transport. 4 (2) Active transport: -Protein transporters bind both Na+ and glucose at separate sites - Then transport them through the plasma membrane of intestinal cells. -Na+ is transported down its concentration gradient and causes the transport of glucose against its concentration gradient. -The free energy is obtained from Na+ - K+ pump Glucose is the preferred source of energy for most of the body tissues. Brain cells derive the energy mainly from glucose. Glycolysis (Embeden Meyerhof`s pathway) Definition: In glycolytic pathway glucose is converted to pyruvate (aerobic condition) or lactate (anaerobic condition). Site of reactions: All the reaction steps take place in the cytoplasm. Significance of Glycolysis Pathway 1. It is the only pathway that is taking place in all the cells of the body. 2. Glycolysis is the only source of energy in erythrocytes. 3. In strenuous exercise, when muscle tissue lacks enough oxygen, anaerobic glycolysis forms the major source of energy for muscles. 4. Most of the reactions of the glycolytic pathway are reversible, which are also used for gluconeogenesis Different Sizes The -oses 3 Carbons 4 Carbons 5 Carbons 6 Carbons D-Glyceraldehyde Triose D-Threose Tetrose D-Ribose Pentose D-Glucose Hexose Common Sugars Aldose and Ketose Aldehyde Ketone { D-Glucose } D-Fructose Aldose Ketose  1 2 3 4 2 2 5 2 7 6 8 2 9 2 10 12 2 2 2 11 13 Steps 1,2  Step 1 of Glycolysis i. Glucose is phosphorylated to glucose-6-phosphate ii. The enzyme is Hexokinase. iii. Hexokinase is a key glycolytic enzyme. The reaction is allosterically inhibited by glucose-6- phosphate -The reaction is irreversible. Hexokinase and Compare? glucokinase may be Hexokinase Glucokinase considered as iso-enzymes The site All tissues except liver. Only in liver specificity Act on glucose, fructose and Only glucose mannose. Affinity to substrate High / low km Low / high km Inhibition by glucose-6- Inhibited Not inhibited phosphate induction Not induced Induced by insulin and glucagon function Even when blood sugar level -Acts only when blood is low, glucose is utilized by glucose level is more body cells than 100 mg/dL; then  Step 2 of Glycolysis Glucose-6-phosphate is isomerized to fructose-6- phosphate by phosphohexose isomerase. This is readily reversible  Step 3 of Glycolysis A-Fructose-6-phosphate is further phosphorylated to fructose1,6- bisphosphate. B- The enzyme is phosphofructokinase. C-Phosphofructokinase (PFK) is the rate limiting enzyme of glycolysis. It is an important key enzyme of this pathway. D- This reaction is an irreversible E-Is the pace maker of glycolysis. Short note on pace maker of glycolysis? Its allosteric inhibitors are ATP and citrate. Its allosteric stimulators AMP and fructose 2,6 diphosphate 1)- ATP lost under both aerobic and anaerobic condition: One ATP in reaction (1) activated by hexokinase or glucokinase. One ATP in reaction (3) activated by phosphofruktokinase. So 2 ATP are lost.  Step 4 of Glycolysis The 6 carbon fructose-1,6-bisphosphate is cleaved into two 3 carbon units; one glyceraldehyde-3- phosphate and another molecule of dihydroxy acetone phosphate (DHAP) the enzyme is called aldolase. This reaction is reversible.  5,6,7 steps  Step 5 of Glycolysis I. Dihydroxyacetone phosphate is isomerized to glyceraldehyde-3-phosphate by the enzyme phosphotriose isomerase. II. Thus net result is that glucose is now cleaved into 2 molecules of glyceraldehyde-3-phosphate  Step 6 of Glycolysis Glyceraldehyde-3-phosphate is dehydrogenated and simultaneously phosphorylated to 1,3-bisphosphoglycerate (1,3- BPG) with the help of NAD+. The enzyme is glyceraldehyde-3-phosphate dehydrogenase. The product contains a high energy bond. This is a reversible reaction Step 7 of Glycolysis i. The energy of 1,3-BPG is trapped to synthesize one ATP molecule with the help of bisphosphoglycerate kinase. ii. ii. This is an example of substrate level phosphorylation, where energy is trapped directly from the substrate, without the help of the complicated electron transport chain reactions. When energy is trapped by oxidation of reducing equivalents such as NADH, it is called oxidative phosphorylation. Step 6 is reversible Step 8 of Glycolysis 3-phosphoglycerate is isomerized to 2- phosphoglycerate by shifting the phosphate group from 3rd to 2nd carbon atom. 8 The enzyme is phosphoglyceromutase. This is a readily reversible reaction. Step 8 of Glycolysis 2-phosphoglycerate is converted to phosphoenol pyruvate by the enzyme enolase. One water molecule is removed A high energy phosphate bond is produced. The reaction is reversible. Enolase requires Mg++, and by 9 removing magnesium ions, fluoride will irreversibly inhibit this enzyme. Thus, fluoride will stop the whole glycolysis. So when collecting blood for sugar estimation, fluoride is added to blood. If not, glucose is metabolized by the blood cells, so that lower blood sugar values are obtained  Step 10 of Glycolysis phosphoenol pyruvate (PEP) is first converted to the enol intermediate and then spontaneously isomerized to keto pyruvate, the stable form. One mole of ATP is generated during this reaction. This is again an example of substrate level phosphorylation. The pyruvate kinase is a key glycolytic enzyme. 10  The pyruvate kinase is a key glycolytic enzyme. Inhibited by ATP Phosphorylation-dephosphorylation mechanism. Phoshorylation inactivates this enzyme while, dephosphorylation activates it. This step is irreversible. The reversal, however, can be brought about in the body with the help of two enzymes (pyruvate carboxylase and phosphoenol pyruvate carboxy kinase) and hydrolysis of 2 ATP molecules (see 10 gluconeogenesis). Step 12 of Glycolysis In anaerobic condition, pyruvate is reduced to lactate by lactate dehydrogenase (LDH). Energy produced from glycolysis: (1)- ATP lost under both aerobic and anaerobic condition: One ATP in reaction (1) activated by hexokinase or glucokinase. One ATP in reaction (3) activated by phosphofruktokinase. So 2 ATP are lost. (2)- ATP gained: A- Under anaerobic condition: 2 ATP in reaction (7) activated by phosphoglycerate kinase. 2 ATP in reaction (10) activated by pyruvate kinase. *So two ATP are gained anaerobic glycolysis. B- Under aerobic condition: In reaction (6) NADH+H+ is re-oxidized in respiratory chain and 3ATP are gained. 2 NADH+H+ are produced in this reaction so 6 ATP are produced. *So 8 ATP molecules are gained aerobic glycolysis. 31  Control of glycolysis? IS through three irreversible steps activated by: 1- Hexokinase enzyme: The reaction is allosterically inhibited by glucose-6-phosphate. 2- Phosphofructokinase enzyme: Is the pace maker of glycolysis. Its allosteric inhibitors are ATP and citrate. Its allosteric stimulators AMP and fructose 2,6 diphosphate 3- Pyruvate kinase enzyme: This enzyme is inhibited by ATP Phosphorylation-dephosphorylation mechanism. Phoshorylation inactivates this enzyme while, dephosphorylation activates it. 32 Glycolysis Compare? (Embeden Meyerhof`s pathway) Aerobic Anaerobic presence of oxygen Absence of oxygen End product: pyruvic acid enters the lactic acid which enters the Cori's citric acid cycle cycle pyruvic acid enters the lactic acid enters gluconeogenesis. mitochondrion and complete in citric acid cycle location : in cytoplasm need shuttle system for oxidation not need The oxidative phosphorylation is but in anerobic on substrate level only produced on respiratory chain and substrate levels 8 ATP 2 ATP 33  Pyruvate is transported into the mitochondria via a special pyruvate transporter Pyruvate enters the mitochondria to be oxidized into CO2 and H2O in the form of Acetyl coA  Oxidative decarboxylation?  Oxidative decarboxylation occurs in the mitochondria of animal cells in five successive steps.  Pyruvate derived from glycolysis is oxidatively decarboxylated to acetyl CoA by the pyruvate dehydrogenase  It needs five coenzymes which are TPP , lipoic acid, CoA-SH, NAD and FAD.  Mg2+ helps the reaction. Regulation of pyruvate dehydrogenase complex? 1- Products of reaction: Acetyl-CoA, ATP and NADH which are the products of oxidative decarboxylation of pyruvic acid. These are allosteric inhibitors. 2- Phosphorylation-dephosphorylation mechanism: The phosphrylated enzyme complex remains inactive until the phosphate group is removed from the serine residue by a specific phosphatase (dephosphorylation). Fermentation Fermentation is an anaerobic process in which energy can be released from glucose even if oxygen is not available.”. Fermentation - produced by yeast and bacteria. - In fermentation pyruvic acid is produced and is then converted to CO2 and ethyl alcohol. - This is done by pyruvate decarboxylase and ethanol dehydrogenase. 40 Anaerobic fermentation glycolysis  Compare? An. Glycolysis Fermentation End product Lactic acid Ethyl alcohol + CO2 Acceptor of 2(H) from NADH+H. Pyruvate Acetaldhyde Not produced Produced CO2 Lactate dehydrogenase Required Not required Pyruvate decarboxylase Not required Required Alcohol dehydrogenase Not required Required Coenzymes NAD NAD and TPP 42  Glycolysis in RBCs (Rapoport Luebering cycle or R-L cycle) Erythrocytes, which lack mitochondria, are completely depend on glucose as source of energy. Glycolysis in erythrocytes, even under aerobic conditions, always terminates in lactate. Glycolysis in RBCs (Rapoport Luebering cycle or R-L cycle  In erythrocytes of many mammalian species, the step catalyzed by phosphoglycerate kinase may be bypassed by mutase and phosphatase enzymes steps. 44 Importance of R-L cycle:  ATP is lost in the formation of 2,3-BPG from 1,3- BPG  It serves as a mechanism of dissipation (wasting) the excess of energy not need by RBCs.  Production of 2,3 Bisphoshospho glycerate (2,3 BPG) which decreases the affinity of hemoglobin to oxygen.  2,3-BPG facilitates release of oxygen to the tissues Metabolic Pathways of the Red blood Cell  Embden-Meyerhof Pathway.  90% of glucose is oxidized by glycolysis.  Rapoport- Luebering Cycle. The Luebering-Rapoport shunt is part of the Embden-Meyerhof pathway  HMP-shunt.  10% of glucose enters the HMP shunt  Kerbs cycle Tricarboxylic acid cycle Citric acid cycle Catabolism of acetyl CoA Mitochondrial pathway of glucose oxidation Kerbs cycle Carbohydrates Fatty Amino Ketone acids acids bodies It is a final common pathway for oxidation of major nutrients. Acetyl coA TCA CO2+ H2O The link between catabolic and anabolic pathways (amphibolic role).  10 1 9 2 3 8 Citric acid cycle 7 6 4 5  Generation of ATP by citric acid cycle? Reaction number The enzyme Method of ATP production ATP formed Reaction (4) Isocitrate Respiratory chain 3 ATP dehydrogenase oxidation of NADH Reaction (6) α-Ketoglutarate Respiratory chain 3 ATP dehydrogenase oxidation of NADH Reaction (7) Succinate Phosphorylation at 1 ATP thiokinase substrate level Reaction (8) Succinate Respiratory chain 2 ATP dehydrogenase oxidation of FADH2 Reaction (10) Malate Respiratory chain 3 ATP dehydrogenase oxidation of NADH The net energy produced from one molecule of Acetyle-CoA 12 ATP Total energy from one glucose molecule Net generation in glycolytic pathway 8 ATP molecules Generation in pyruvate dehydrogenation 3 ATP produced from oxidation of NADH Generation in citric acid cycle 12 ATP So one molecule of pyruvate when oxidized aerobically gives 15 ATP. Since one molecule of glucose when oxidized to CO2 + H2O gives two molecules of pyruvate Net generation of ATP from one glucose mol 8 ATP + (2x15) ATP (from oxidation of two molecules of pyruvate) = 38 ATP. Inhibitors of TCA cycle? 1- Flouroacetate: Inhibits aconitase enzyme causes accumulation of citric acid. 2- Arsenite: Inhibits oxidative decarboxylation of α-Ketoglutarate dehy. 3-Malonic acid: It acts as competitive inhibitors for succiniate dehydrogenase a enzyme. It competes with succinic acid for the active site of enzyme. Regulation of citric acid cycle Citric acid cycle is controlled by energy state of the cell. Sites of control of citric acid cycle?  Step 1 Citrate synthetase: ATP is an allosteric inhibitor.  Step 4 Isocitrate dehydrogenase:  NADH and ATP is an allosteric inhibitor  while, ADP is allosteric stimulator.  Step 6 α-Ketoglutarate dehydrogenase:  It is inhibited by the end products succinyl CoA and NADH.