BTE 202: Metabolism Lecture-8 PDF

BTE 202: Metabolism Lecture-8 Pyruvate to Acteyl-CoA  Cellular respiration In organisms that live under aerobic conditions, pyruvate produced in glycolysis is further oxidized to H2O and CO2. This aerobic phase of catabolism is called respiration....

BTE 202: Metabolism Lecture-8 Pyruvate to Acteyl-CoA  Cellular respiration In organisms that live under aerobic conditions, pyruvate produced in glycolysis is further oxidized to H2O and CO2. This aerobic phase of catabolism is called respiration.  Cellular respiration occurs in three major stages In the first stage, oxidation of organic fuel molecules- glucose, fatty acids and some amino acids occurs to produce acetyl-coenzyme A (acetyl-CoA). In the next stage, the acetyl groups are fed into the citric acid cycle and enzymatically oxidized to CO2. The energy released during oxidation is conserved in the reduced electron carriers NADH and FADH2. The last stage is the electron transport chain where ATP is generated from the reduced electron carriers.  Cellular respiration GLYCOLYSIS is one of the pathways that provide cells with Pyruvate and this Pyruvate undergoes a decarboxylation reaction to form Acetyl-CoA. Production of Acetyl-CoA (Activated Acetate) Pyruvate, derived from glucose and other sugars by glycolysis, is oxidized to acetyl-CoA and CO2 by the pyruvate dehydrogenase (PDH) complex. PDH complex is a cluster of enzymes (multiple copies of each of the three enzymes). It is located in the mitochondria of eukaryotic cells and in the cytosol of prokaryotes. Five cofactors, four derived from vitamins, participate in the production of Acetyl-CoA.  PDH Complex The combined dehydrogenation and decarboxylation of pyruvate to the acetyl group of acetyl-CoA requires the sequential action of three different enzymes and five different coenzymes— The three enzymes are- 1. Pyruvate Dehydrogenase (E1), 2. Dihydrolipoyl Transacetylase (E2), 3. Dihydrolipoyl Dehydrogenase (E3) Each of these are present in multiple copies.  PDH PDHComplex Complex The five co-enzymes are- 1. Thiamine pyrophosphate (TPP) 2. Lipoate, 3. Coenzyme A (CoA-SH) 4. Flavin adenine dinucleotide (FAD) 5. Nicotinamide Adenine Dinucleotide (NAD) Four different vitamins required for these co-enzymes: Thiamine/Vit-B1 (in TPP) Riboflavin/Vit-B2 (in FAD) Niacin/Vit-B3 (in NAD) Pantothenate/Vit-B5 (in CoA)  PDH PDHComplex Complex Finally, two regulatory proteins are also part of this important enzyme complex. A protein kinase, Pyruvate Dehydrogenase Kinase. A phosphoprotein phosphatase. The roles of these regulatory proteins will be discussed in the last segment of this lecture. PDH Complex  The overall reaction catalyzed by the pyruvate dehydrogenase complex is an oxidative decarboxylation in which the terminal –COO- group is removed as CO2. It is an irreversible oxidation process in which the carboxyl group is removed from pyruvate as a molecule of CO2 and the two remaining carbons become the acetyl group of acetyl-CoA. PDH Complex The intermediates never leave the enzyme surface. PDH Complex  PDH Complex The intermediates never leave the enzyme surface.  PDH Complex PDH Complex Reactions The intermediates never leave the enzyme surface.  Regulation of PDH Complex PDH is regulated both by allosteric effectors and by covalent modification.  Regulation of PDH Complex by Allosteric Effectors Fructose-1,6-bisphosphate is the positive allosteric effector of the 1st enzyme of the PDH complex, Pyruvate Dehydrogenase. [Feed-forward stimulation] NADH and Acetyl-CoA are the negative allosteric regulator of Pyruvate Dehydrogenase. [Feedback inhibition]  Regulation of PDH Complex by Covalent Modification Phosphorylation of PDH is mediated by a special Protein Kinase, pyruvate dehydrogenase kinase. This enzyme is part of the PDH multi-enzyme complex. This kinase is subject to allosteric activation by NADH and Acetyl-CoA. Then phosphorylation inactivates pyruvate dehydrogenase. [Kinase shuts down the 1st enzyme of the PDH complex. So, again FEEDBACK INHIBITION!]  Regulation of PDH Complex by RegulationCovalent Modification of PDH Complex by Covalent Modification Phosphorylation is reversed, and the activity of pyruvate dehydrogenase is restored by a Protein Phosphatase, which is also associated with the pyruvate dehydrogenase complex. This Phosphoprotein Phosphatase is in turn allosterically activated by AMP and NAD+ and pyruvate. PDH phosphatase is activated by calcium ions (Ca2+). Phosphatase turns on the complex when ATP and NADH are NEEDED! and when Pyruvate is in abundance (again, feed-forward stimulation)  Regulation of PDH Complex by Covalent Modification Pyruvate need to produce High AMP No need to Dehydrogenase more ATP produce more ATP High ATP (E1) Ca2+ O (Active form- + H Dephosphorylated) Protein Phosphoprotein Kinase Phosphatase + Pyruvate High NADH, Dehydrogenase High NAD+, High Acetyl Co-A (E1) High Pyruvate, (Inactive form- P Phosphorylated)  Regulation of PDH Complex by Regulation CovalentofModification PDH Complex No need to produce more ATP Pyruvate Dehydrogenase High ATP (E1) Ca2+ OH (Active form- Dephosphorylated) + + Protein Phosphoprotein Kinase Phosphatase + + Pyruvate High NADH, Dehydrogenase High NAD+, High Acetyl Co-A (E1) High Pyruvate, High AMP P (Inactive form- Phosphorylated)