Lecture 1 Glycolysis PDF

 Glycolysis -Define glycolysis -Describe reaction step of glycolysis -Calculate energetic of glycolysis  Glycolysis (Embden-Myerhof Pathway- E-M) Definition: - It is a cascade of reactions that converts glucose into two pyruvate molecules (w...

 Glycolysis -Define glycolysis -Describe reaction step of glycolysis -Calculate energetic of glycolysis  Glycolysis (Embden-Myerhof Pathway- E-M) Definition: - It is a cascade of reactions that converts glucose into two pyruvate molecules (when O2 is available) or into lactate (when O2 is not available, i.e., no mitochondria) aiming at production of ATP and other intermediates. It is also utilized in its opposite direction in gluconeogenesis.  site : - It occurs in cytoplasm. It is especially important in: 1-RBCs: are devoid of mitochondria and depend on glycolysis as the main source of energy. 2-Contracting muscles: due to occlusion of blood vessels by the muscular contraction that decreases oxygen. 3-Cornea& lens : which have a limited blood supply and lack mitochondria which if present would absorb and scatter light interfering with transparency.  Steps of glycolysis: - Glucose is transported into cells by glucose transporter that is insulin dependent in muscles and adipose tissue but not in other vital tissues, e.g., brain, heart, kidney and RBCs. - Liver is freely permeable to glucose in and out  2 phases of glycolysis a- ATP utilizing steps 1 glucose is converted into 2 glyceraldehyde 3 phosphate Utilizing 2 ATP. b-ATP producing steps 2 glyceraldehyde 3 phosphate converted into 2 pyruvate in aerobic conditions or 2 lactate in anaerobic conditions.  Bioenergetics of (or Energy yield from) glycolysis: Under anaerobic conditions: 1- Total ATP lost = 2 ATP as follows, a)One ATP in the activation of glucose to glucose-6- phosphate by hexokinase or glucokinase enzyme. b)One ATP in the activation of fructose-6-phosphate to fructose1,6-diphosphate by phosphofructokinase enzyme. 2- Total ATP gained = 4 ATP as follows, a) 2 ATP by substrate level phosphorylation (i.e., production of ATP from ADP + Pi utilizing high-energy bond in a substrate directly without intermediates.) from 1,3-diphosphoglycerate to 3 phosphoglycerate by phosphoglycerate kinase enzyme b) 2 ATP from substrate level phosphorylation from phospho- enol pyruvate to pyruvate by pyruvate kinase enzyme. 3- Net ATP gained = 4 ATP gained - 2 ATP lost = 2 ATP for the anaerobic oxidation of one mole of glucose into lactate. Under aerobic conditions: 1-Total ATP lost = 2 ATP, as under anaerobic conditions. 2-Total ATP gained = 7 or 9 ATP are generated as follows, 4 ATP (obtained by substrate level phosphorylation)= direct ATP production + 2 NADH.H+ chain (produced from oxidation of glyceraldehyde-3- phosphate)  2 X 1.5 or 2.5 ATP = 3 or, 5 ATP, after oxidation in the functioning respiratory, depending on the type of transporting shuttle used. Thus, 7 or 9 ATP are generated. Net ATP gained = 5, Or, 7 ATP as follows, 7 ATP – 2 ATP = 5, Or, 9 ATP – 2 ATP = 7 ATP for the aerobic oxidation of one mole of glucose.  Oxidation of NADH.H+: - The two NADH.H+ produced are oxidized by respiratory chain producing 3 or 5 molecules of ATP depending on the type of shuttle since mitochondria membrane is impermeable to NADH.H+. there are two types of these shuttles: A. Malate shuttle: - It is important in liver and heart = 5 ATP. B) Glycerophosphate shuttle: - It is important in muscles and nerve cells. - = 3 ATP.  Choose the best single answer: 1-When O2 is available glucose is converted into 2 molecules of: a) Lactate b) pyruvate c) glycerate d) succinate 2- Net ATP in anaerobic glycolysis is: a) 2 b) 4 c) 5 d) 7  Regulation (or Control) of Glycolysis: A. Key regulatory enzymes: - Regulation of any biochemical cascade targets the key regulatory enzymes of it. Key regulatory enzymes are those enzymes that catalyze the irreversible steps of glycolysis :  Phosphofructokinase is the most important enzyme in glycolysis  Stimulated by : - Fructose-6- phosphate, - Fructose-2,6-diphosphate. - ADP and AMP.  Inhibited by -ATP, -low pH -and citrate.  B. Hormonal regulation: I. Insulin: stimulates the key regulatory enzyme of glycolysis II.Adrenaline and glucagon inhibit the key regulatory enzyme of glycolysis.  In vitro inhibition of glycolysis: 1-Deoxyglucose inhibits hexokinase and glucokinase. 2-Arsenate : closely resembles inorganic phosphate and replaces it in the reaction catalyzed by glyceraldehyde-3-phosphate dehydrogenase Arsenate uncouples oxidation and phosphorylation (ATP synthesis) at substrate level. 3-Iodoacetate: a SH poison inhibits glyceraldehyde-3-phosphate dehydrogenase which contains this group at its active site. 4-Fluoride inhibits enolase. So, on accurately measuring blood glucose levels, the samples should be fluorinated to stop glycolysis in RBCs  Biological importance (or Functions) of glycolysis: a. ATP formation. b. Production of 2,3-DPG that is important in tissue oxygenation. c. Reversal of glycolysis is gluconeogenesis, an important source of glucose. d. Main pathway of metabolism of fructose from the diet. Rapoport-Lubering cycle: is a side-pathway (a shunt) from glycolysis that aims at: Formation of 2,3-Diphosphoglycerate (2,3-DPG).. This cycle is a shunt from glycolysis that is formed of two steps:  Biochemical significance of 2,3- Diphosphoglycerate (2,3-DPG):  It decreases the affinity of hemoglobin (Hb) to oxygen helping oxygen dissociation.  the levels of 2,3-DPG increases markedly in hypoxic conditions (high altitudes, hypoxic hypoxia, stagnant hypoxia and anemic hypoxia).  - During storage of blood in blood banks, 2,3-DPG concentration decreases gradually to reach traces at ten days. So, Hb of this blood has a high affinity to O2 and is not suitable for blood transfusion to hypoxic patients - RBCs membrane is impermeable to 2,3-DPG. So, instead of adding 2,3-DPG, inosine is added which penetrates RBCs easily and its ribose is changed very slowly into 2,3-DPG by HMP shunt and glycolysis.  CHO CHO CH2OH Hexokinase, or H OH H OH Phosphohexose O Glucokinase HO H Isomerase HO H HO H H OH Mg +2 H OH H OH H OH H OH H OH ATP ADP+Pi O O CH2OH CH2-O P-OH CH2-O P-OH Glucose OH OH Glucose-6-Phosphate Fructose-6-Phosphate ATP Phosphofructokinase Mg+2 O ADP CH2-O P-OH O O OH CH2-O P-OH CH2OH O OH Dihydroxyacetone- HO H phosphate H OH H OH Phosphotriose O Aldolase CH2-O P-OH O O Isomerase A or B C ~ P-OH OH OH 2 NADH.H+ 2 NAD + Pi CHO Fructose-1,6- diphosphate H OH H OH O O CH2-O P-OH CH2-O P-OH Glyceraldehyde- OH OH 3-Phosphate (2) Glyceraldehyde- (2) 1,3-DiPhospho- Dehydrogenase Glycerate 3-phosphate 2 ADP Mg +2 Phosphoglycerate Kinase 2 ATP O O O 2 H2O 3-Phospho- C OH O C OH C OH O Glycerate Mg+2 O ~ P-OH H OH O H O P-OH Mutase CH2-O P-OH OH Enolase OH CH2-OH CH2 OH (2) 2-Phospho-Glycerate (2) Phosphoenol (2) 3-Phospho-Glycerate Pyruvate 2 ATP Pyruvate O O O Kinase Mg +2 C OH C OH 2 NAD 2 NADH.H+ C OH 2 ADP OH H OH O Spontaneous CH2 CH3 CH3 Lactate (2) Enol- (2) Lactate (2) Pyruvate Dehydrogenase Pyruvate  Choose the best single answer: 1- The most important enzyme in regulation of glycolysis is: a) Hexokinase b) pyruvate kinase c) phosphofructokinase d) Glucokinase 2-……… inhibits glyceraldehyde-3-phosphate dehydrogenase a) Arsenate b) iodoacetate c) fluoride d) 2-deoxy glucose