Carbohydrate metabolism Dr Almoeiz Yousif Entry of glucose
Carbohydrate metabolism Dr. Almoeiz Yousif
Entry of glucose into cells: • Intracellular glucose concentration is very low compared to blood (for human < 100 mg/dl). However, glucose does not enter the cells by simple diffusion. Two specific transport systems are recognized for entry of glucose into cells: Tuesday, November 24, 2020 Almoeiz Yousif , Department of Biochemistery -SIU 2
• Insulin independent transport system: • This is a carrier mediated uptake of glucose which is not dependent on the insulin hormone. This is operative in hepatocytes, erythrocytes and brain. • Insulin dependent transport system: • This occurs in muscles and adipose tissues. Tuesday, November 24, 2020 Almoeiz Yousif , Department of Biochemistery -SIU 3
Glucose Cellular Uptake • Glucose transporters ( GLUT) • GLUT 1 • GLUT 2 • GLUT 3 • GLUT 4 in muscle and adipose tissue • GLUT 5 Insulin helps GLUT 4 to take glucose into cells. Tuesday, November 24, 2020 Almoeiz Yousif , Department of Biochemistery -SIU 4
Tuesday, November 24, 2020 Almoeiz Yousif , Department of Biochemistery -SIU 5
GLYCOLYSIS Glycys = sweet Lysis = breakdown
Definition of Glycolysis • first pathway for the breakdown of carbohydrates Tuesday, November 24, 2020 Almoeiz Yousif , Department of Biochemistery -SIU 7
Features of glycolysis: • Glycolysis takes place in all cells of the body. • The enzymes of this pathway are present in the cytosol. • Energy-yielding pathway for cells in the absence of oxygen. • Is a major pathway for ATP synthesis in tissues lacking mitochondria e. g. erythrocytes, cornea and lens. • Aerobic & Anaerobic oxidation of glucose. Tuesday, November 24, 2020 Almoeiz Yousif , Department of Biochemistery -SIU 8
Importance of glycolysis: 1. Producing energy. 2. Give intermediates metabolites. e. g. pyruvate—alanine. 3. Give 2, 3 bisphoglycerate which help in O 2 delivery. Tuesday, November 24, 2020 Almoeiz Yousif , Department of Biochemistery -SIU 9
Glycolysis can be divided into two phases: A) Energy-requiring phase B) Energy-yielding phase Tuesday, November 24, 2020 Almoeiz Yousif , Department of Biochemistery -SIU 10
10 Steps of Glycolysis Tuesday, November 24, 2020 Almoeiz Yousif , Department of Biochemistery -SIU 11
A)The Energy-requiring Phase • These are five reactions. • Two ATPs are used • Glucose is broken down into two glyceraldehyde-3 -phosphate (G-3 -P). Tuesday, November 24, 2020 Almoeiz Yousif , Department of Biochemistery -SIU 12
Hexokinase Phosphohexose isomerase Phospho. Fructosekinase-1 Aldolase Triose phosphate isomerase Tuesday, November 24, 2020 Almoeiz Yousif , Department of Biochemistery -SIU 13
Reaction 1: Phosphorylation of glucose Irreversible reaction Tuesday, November 24, 2020 Almoeiz Yousif , Department of Biochemistery -SIU 14
• Kinases: (transferases): catalyze the transfer of a phosphoryl group from ATP to some molecule. • Liver cells also contain an isozyme of hexokinase called glucokinase • Isozymes : Two enzymes that catalyze the same reaction but differ in chemical and physical properties Tuesday, November 24, 2020 Almoeiz Yousif , Department of Biochemistery -SIU 15
Hexokinase & Glucokinase • Hexokinase: • Glucokinase: • Km 0. 1 m. M • So hexokinase is normally active! • Found in most cells • Works with all hexoses • Inhibited by G 6 P Tuesday, November 24, 2020 • Km 10 m. M • only works when glucose concentration is high • Found in liver, pancreas • Works ONLY with glucose • Not inhibited by G 6 P Almoeiz Yousif , Department of Biochemistery -SIU 16
B) The Energy-yielding Phase • These are five reactions. • 4 ATP and 2 NADH are produced. • Glyceraldehyde-3 -phosphate is oxidized to pyruvate Tuesday, November 24, 2020 Almoeiz Yousif , Department of Biochemistery -SIU 17
Tuesday, November 24, 2020 Almoeiz Yousif , Department of Biochemistery -SIU 18
ATPs produced from glycolysis Tuesday, November 24, 2020 Almoeiz Yousif , Department of Biochemistery -SIU 19
End-products of Glycolysis Pyruvate Glucose Net: • 2 ATP • 2 NADH • 2 Pyruvate molecules Tuesday, November 24, 2020 Almoeiz Yousif , Department of Biochemistery -SIU 20
Anaerobic Glycolysis • If O 2 is absent, pyruvate cannot be oxidized further to give energy. • Pyruvate is reduced to lactic acid so that NAD+ will be still available for further glycolysis Tuesday, November 24, 2020 Almoeiz Yousif , Department of Biochemistery -SIU 21
Anaerobic Glycolysis Tuesday, November 24, 2020 Almoeiz Yousif , Department of Biochemistery -SIU 22
Tuesday, November 24, 2020 Almoeiz Yousif , Department of Biochemistery -SIU 23
GLYCOLYSIS IN RBCs • Has no nucleus nor mitochondria so glycolysis is anaerobic yield two ATP for cell membrane integrity. • Any defect in glycolytic pathway specially enzymes(pyruvate kinase)will lead to hemolytic anemia. Tuesday, November 24, 2020 Almoeiz Yousif , Department of Biochemistery -SIU 24
Regulation of glycolysis • 1. level of ATP, ADP • 2. level of NADH&NAD+ • 3. key enzymes: • Hexokinase. Glucokinase, PFK-1, pyruvate kinase. • 4. Hormonal control • 5. inhibitors-flouride, CN. Tuesday, November 24, 2020 Almoeiz Yousif , Department of Biochemistery -SIU 25
The three enzymes catalyzing the irreversible reactions are regulating glycolysis. • Hexokinase: is inhibited by G 6 -P (feedback inhibition). Glucokinase is an inducible enzyme. • PFK: catalyses the rate limiting committed step. PFK is an allosteric enzyme; ATP and citrate are allosteric inhibitors whereas AMP and Pi are the allosteric activators. • Pyruvate kinase: inhibited by ATP and activated by F 1. 6 bisphosphate. Also pyruvate kinase is active in dephosphorylated state and inactive in phosphorylated state. Tuesday, November 24, 2020 Almoeiz Yousif , Department of Biochemistery -SIU 26
Citric acid cycle (tricarboxylic acid cycle) (kreb’s cycle)
Other Names of Citric Acid Cycle • Krebs Cycle • Tricarboxylic acid (TCA) cycle Tuesday, November 24, 2020 Almoeiz Yousif 28
Definition of Citric Acid Cycle • It is the final common pathway for the oxidation of fats, proteins and carbohydrates. Tuesday, November 24, 2020 Almoeiz Yousif 29
Catabolism of Food to Energy Tuesday, November 24, 2020 30
General Properties of Citric Acid Cycle • It takes place in the mitochondrial matrix • It is amphibolic ( both catabolic and anabolic) • Is an eight-step cycle Tuesday, November 24, 2020 Almoeiz Yousif 31
In Mitochondrial Matrix Tuesday, November 24, 2020 Almoeiz Yousif 32
Citric Acid Cycle is amphibolic • Contains both catabolism and anabolism Besides release of energy, it takes part in the synthesis of glucose, some amino acids and lipids Tuesday, November 24, 2020 Almoeiz Yousif 33
Citric Acid Cycle is amphibolic Tuesday, November 24, 2020 Almoeiz Yousif 34
End-products of Glycolysis Pyruvate Glucose Tuesday, November 24, 2020 Net: • 2 ATP • 2 NADH • 2 Pyruvate molecules 35
The Fate of Pyruvate In the PRESENCE of oxygen, Pyruvate; • enters the mitochondria • Is then oxidized to Acetyl Co. A before it enters TCA cycle Tuesday, November 24, 2020 Almoeiz Yousif 36
Oxidative Decarboxylation of Pyruvate Tuesday, November 24, 2020 Almoeiz Yousif 37
Pyruvate to Acetyl Co. A, irreversible step Tuesday, November 24, 2020 Almoeiz Yousif 38
The PDH complex 1. pyruvate dehydrogenase • Oxidative decarboxylation of pyruvate 2. dihydrolipoyl transacetylase • Transfers acetyl group from enzymebound TDP to lipoic acid. • Transfers acetyl group to Co. A, 3. dihydrolipoyl dehydrogenase • transfers electrons from reduced lipoic acid to produce NADH Tuesday, November 24, 2020 Almoeiz Yousif 39
Biochemical importance of PDH • deficiency of thiamine inhibits PDH activity resulting in the accumulation of pyruvate lead to abnormalities of the nervous system and elevated levels of lactate (lactic acidosis) • In the thiamine deficient alcoholics, pyruvate is rapidly converted to lactate, resulting in lactic acidosis. Tuesday, November 24, 2020 Almoeiz Yousif 40
• Acetyl Co. A moves on to the Krebs Cycle Acetyl Co. A is the fuel for the Krebs cycle Tuesday, November 24, 2020 Almoeiz Yousif 41
Krebs Cycle • Each acetyl Co. A is decarboxylated and oxidized, giving: • Three molecules of NADH + H+ • One molecule of FADH 2 • One molecule of ATP • Two molecules of CO 2 Tuesday, November 24, 2020 Almoeiz Yousif 42
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The citric acid cycle Tuesday, November 24, 2020 Almoeiz Yousif 44
Succinate Dehydrogenase • Succinate dehydrogenase is embedded in inner mitochondrial membrane. • All other Krebs cycle enzymes are located in matrix. * *Succinate dehydrogenase Tuesday, November 24, 2020 Almoeiz Yousif 45
Energy produced by TCA
Remember that NADH+H =3 ATP FADH 2=2 ATP GTP =ATP
ATP Energy from Citric Acid Cycle One turn of the citric acid cycle 3 NADH x 3 ATP = 9 ATP 1 FADH 2 x 2 ATP = 2 ATP 1 GTP x 1 ATP= 1 ATP Total = 12 ATP Glucose provides two acetyl COA molecules for two turns of citric acid cycle 2 acetyl Co. A 24 ATP + 4 CO 2 Tuesday, November 24, 2020 Almoeiz Yousif 48
Control of Citric Acid Cycle Regulatory enzymes: 1. Citrate synthase 2. isocitrate dehydrogenase 3. - ketoglutarate dehydrogenase Tuesday, November 24, 2020 Almoeiz Yousif 49
Regulation of citric acid cycle: 1. citrate synthase. 2. isocitrate dehydrogenase 4. α ketoglutarate dehydrogenase Tuesday, November 24, 2020 Almoeiz Yousif 50
REGULATION OF TCA CYCLE Tuesday, November 24, 2020 Almoeiz Yousif 51
REGULATION OF TCA CYCLE Tuesday, November 24, 2020 Almoeiz Yousif 52
INHIBITOR OF TCA CYCLE 1. Fluoroacetate • Condensation with Co. A Fluoroacetyl-Co. A • Condensation Fluoroacetyl Co. A with Oxaloacetate Fluorocitrate inhibit Aconitase enzyme accumulation of citrate • Fluoroacetate pesticide 2. Malonate succinat dehydrogenase enzyme 3. Arsenite α-ketoglutarate dehydrogenase enzyme. Tuesday, November 24, 2020 Almoeiz Yousif 53
Home work What is the amount of ATP produced by complete oxidation of one molecule of glucose Almoeiz Yousif
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