Krebs Cycle PDF

KREBS CYCLE Presented by: 1 Prof. (Mrs.) Linda A. Fondjo Department of Molecular Medicine, SMS, KNUST  Learning Objectives What is the Krebs cycle? What is Aerobic or Cellular Respiration? What is the link betwe...

KREBS CYCLE Presented by: 1 Prof. (Mrs.) Linda A. Fondjo Department of Molecular Medicine, SMS, KNUST  Learning Objectives What is the Krebs cycle? What is Aerobic or Cellular Respiration? What is the link between Glycolysis & Krebs cycle? Reactions of the Krebs cycle Regulation of Krebs cycle Amphibolic nature of Krebs cycle Replenishing pathways of Krebs cycle L.A FONDJO 2  KREBS CYCLE ¡Krebs Cycle ¡Citric Acid Cycle ¡Tricarboxylic Acid 3 Hans Adolf Krebs- 1900-1981  KREBS CYCLE is a series of enzymatic reactions used by all aerobic living organisms to generate cellular energy through the oxidization of acetyl units derived from carbohydrates, fats and proteins into carbon dioxide and water 4 L.A FONDJO 5  KREBS CYCLE Krebs cycle is the central metabolic pathway connecting almost all other individual metabolic pathways. Krebs cycle is the final oxidative pathway for the metabolism of Carbohydrates, Proteins and Fats Krebs cycle involves the oxidation of acetyl 6 CoA to CO2 and H2O KREBS CYCLE 7  KREBS CYCLE ¡ Supplies energy (ATP) & Reduced coenzymes for ETC eg. FADH, NADH ¡ Provides intermediates for synthesis of biomolecules- Heme (porphyrins), nucleotide base, amino acids. 8  KREBS CYCLE ¡ What is Cellular Respiration? ¡ Cellular Respiration is simply the process by which biochemical energy from nutrients is converted into Energy (ATP). ¡ Process of energy generation from the food we eat. L.A FONDJO 9 Cellular Respiration 10 The 3 stages of Cellular Respiration 11  KREBS CYCLE Aerobic Reaction 12  KREBS CYCLE ¡Mitochondria- Matrix 13 L.A FONDJO Link Between Glycolysis & Krebs Cycle 14  Pyruvate- Production & Entry Into TCA ¡ Pyruvate is translocated from the cytosol to the mitochondria Pyruvate is oxidatively decarboxylated to form Acetyl-CoA Pyruvate Dehydrogenase complex uses TPP, CoASH, Lipoic acid, FAD and NAD+ Acetyl-CoA formed is irreversible & enters 15 TCA cycle L.A FONDJO TRANSLOCATION OF PYRUVATE 16 17  PYRUVATE DEHYDROGENASE COMPLEX Composed of 3 enzymes & 5 cofactors: ¡ Pyruvate dehydrogenase (E1) (cofactor - TPP) ¡ Dihydrolipoamide acetyltransferase (E2) (cofactor - Lipoamide, CoASH) ¡ Dihydrolipoamide dehydrogenase (E3) (cofactor - FAD, NAD+) 18 L.A FONDJO COFACTORS & VITAMINS ¡ Thiamine pyrophosphate (TPP)- Thiamine ¡ Nicotinamide Adenine Dinucleotide (NAD)- Niacin ¡ Flavin Adenine Dinucleotide (FAD)- Riboflavin ¡ Lipoic Acid (CoA/ COASH)- Pantothenic acid 19 L.A FONDJO PYRUVATE DEHYDROGENASE DEFICIENCY ¡ Pyruvate dehydrogenase deficiency is a rare disorder. ¡ Can result in elevated serum levels of lactate, pyruvate. Can cause acidosis and can be fatal in severe cases. ¡ Leads to neurological disorders 20 L.A FONDJO  THIAMINE DEFICIENCY ¡ Beriberi- dietary deficiency of thiamine causes neurologic & cardiac disorder. e.g. Alcoholics-malnourished, polished rice –husk ¡ Thiamine-deficient individuals are unable to oxidize pyruvate. ¡ High levels of pyruvate in the blood ¡ In thiamin deficiency the activity of the 2 dehydrogenases in Krebs cycle is decreased 21 ¡ Decreased production of ATP and impaired cellular function L.A FONDJO Reactions of the TCA cycle LA FONDJO 22 STEP 1: Formation of Citrate Condensation Reaction 23 STEP 2: Formation of Isocitrate Isomerization Reaction 24 Step 3: Formation of A-KG 25  STEP 4: Formation of SuccinylCoA A second oxidative decarboxylation a-KG is almost identical to pyruvate dehydrogenase –in terms of structure and mechanism. 5 coenzymes used - TPP, CoASH, Lipoic acid, NAD+, FAD 2nd NADH is generated 26 L.A FONDJO 27  A-KETOGLUTARATE DEHYDROGENASE COMPLEX ¡ The a-ketoglutarate dehydrogenase consists of Ø transsuccinylase (E1) Ø α -ketoglutarate dehydrogenase (E2) Ø dihydrolipoyl dehydrogenase (E3) 28 L.A FONDJO  STEP 5: Formation of Succinate A substrate-level phosphorylation A nucleoside triphosphate is made - (GTP / ATP) 29 STEP 6: Formation of Fumarate Inhibited by malonate 30  Strong competitive inhibitor of succinate dehydrogenase. In its presence succinate accumulates and respiration is blocked since FADH2 is not produced for the respiratory chain 31  STEP 7: Formation of Malate Hydration across the double bond Stereospecific reaction Fumarase is a highly stereospecific enzyme 32 L.A FONDJO 33 34  Step 8: Regeneration of Oxaloacetate Regeneration of Oxaloacetate 35 Summary of Products Per Turn of Krebs Cycle C6 C4 C6 C4 C5 C4 C4 C4 L.A FONDJO 36 KREBS CYCLE 37  REDUCED COENZYMES FUEL ATP PRODUCTION IN THE ELECTRON TRANSPORT CHAIN ¡ Overall reaction: Acetyl-CoA + 3 NAD+ + FAD + GDP + Pi + 2 H20 CoASH+ 3NADH + FADH2 + GTP + 2 CO2 + 2 H+ ¡ Assuming: Isocitrate Dehydrogenase 1 NADH= 2.5 ATP ¡ α-ketoglutarate Dehydrogenase 1 NADH= 2.5 ATP ¡ Succinyl-CoA synthetase 1 GTP = 1.0 ATP ¡ Succinate Dehydrogenase 1 FADH2 = 1.5 ATP ¡ Malate Dehydrogenase 1 NADH = 2.5 ATP ¡ ¡ Total of 10 ATPs gained from oxidation of 1 Acetyl-CoA L.A FONDJO 38 Regulation of the Krebs cycle L.A FONDJO 39  KREBS CYCLE- REGULATION ¡ 3 factors: ¡ Availability of Substrate (Acetyl-CoA & OAA) ¡ Inhibition by accumulating products ¡ Allosteric Feedback Inhibition (high ratio of NADH/NAD+). 40 L.A FONDJO 41 Covalent Modification of Pyruvate Dehydrogenase Complex 42  PYRUVATE DEHYDROGENASE PHOSPHATASE DEFICIENCY Ø Some individuals are deficient of pyruvate dehydrogenase phosphatase. Ø Pyruvate dehydrogenase is always phosphorylated and thus inactive, glucose is converted to lactic acid. Ø resulting in lactic acidosis which leading to disruption in tissue function -especially the central nervous system. 43 L.A FONDJO  Krebs cycle is Amphibolic (It serves in both catabolic and anabolic processes) 44  THE ANAPLEROTIC REACTIONS The Replenishing OR "filling up" reactions Reactions that replenish intermediates in the TCA cycle. These reactions enable the catabolic TCA cycle to continue when anabolic reactions draws off its intermediates L.A FONDJO 45 REPLENISHING REACTIONS malate TAKE HOME LESSON SUGGESTED TEXTBOOKS/READING MATERIALS ¡ LEHNINGER PRINCIPLES OF BIOCHEMISTRY-4th Ed or beyond ¡ LUBERT STRYER–BIOCHEMISTRY ¡ BIOCHEMISTRY FOR SPORTS AND EXERCICE SCIENCE- Wiley By Don McLaren & James Morton. ¡ Online Videos (can be very useful too!) 48 L.A FONDJO LAF DISCLAIMER ¡ All images used in this presentation have been downloaded from the internet. ¡ I am grateful to all who have made these images available for teaching purposes. LA FONDJO 51