Citric Acid Cycle Chap 18 19 I INTRO A Overall process of Cellular Respiration Acetyl coenzyme A enters the citric acid cycle But the conversion is an intermediate step 8 total steps start and end with the same product Electron Transport Chain get pyruvate into a usable form Acetyl coenzyme A is 3 carbons Conversion of NAD into NADH releases CO2 and energy releases as electrons II BEFORE CITRIC ACID CYCLE CONVERSION OF PYRUVATE TO ACETYL COA A Pyruvate Dehydrogenase Complex Rxn Pyruvate CoA NAD Acetyl CoA CO2 NADH H Pyruvate is oxidatively decarboxylated by the pyruvate dehydrogenase complex This irreversible conversion of pyruvate into acetyl conenzyme A is the link between glycolysis and the citric acid cycle This reaction is very decisive in metabolism It commits the carbon atoms of carbohydrates to oxidation by the citric acid cycle or to the synthesis of lipids Citric acid cycle takes place in the mitochondria Pyruvate must be transported into mitochondria to be aerobically metabolized NOTE Pyruvate dehydrogenase complex is a large highly integrated complex of 3 distinct enzymes Each has its own active site The PDH captures high transfer potential electrons in the form of NADH PDH pyruvate dehydrogenase E1 Decarboxylase removes CO2 Oxidative step TPP coenzyme of the pyruvate dehydrogenase component Catalyzes the Decarboxylation dihydrolipoyl transacetylase E2 2C fragment oxidized steals an electron to transfer to lipoate CoA released from the second unit Catalyzes the formation of acetyl CoA dihydrolipoyl dehydrogenase E3 Generates lipoamide Transfers FAD to NADH regulatory step Acetyl CoA is very large KNOW STRUCTURE About the size of a cell organelle 5 cofactors vitamins from diet TPP FAD Lipoate Catalytic NAD CoA Stoichiometric 1 thiamine pyrophosphate TPP thiamine 2 flavin adenine dinucleotide FAD riboflavin 3 nicatinamide dinucleotide NAD niacin 4 coenzyme A pantothenate 5 lipoate Conversion of pyruvate into acetyl CoA is 3 steps decarboxylation oxidation and the transfer of the resultant acetyl group to CoA Why are there coupled steps To preserve free energy derived from the decarboxylation step to drive the formation of NADH to acetyl CoA 1 Decarboxylation pyruvate combines with the ionized carbanion to form the decarboxylation step to drive the formation of NADH and the acetyl CoA 2 Oxidation hydroxylethyl group attached to the TPP is oxidized to form an acetyl group while being simultaneously transferred to lipoamide a derivative of lipoic acid 3 Formation of Acetyl CoA acetyllipoamide to CoA to form acetyl CoA Dihydrolipoyl transacetylase E2 Fuel for Citric acid cycle Regulation PDH the main regulation site is at E1 It is phosphorylated which render it inactive So we cannot make any more acetyl CoA Happens when we are low on pyruvate or high in Acetyl CoA The PDH is inhibited by mercury and arsenic Can go 2 directions Can go into the citric acid cycle or it can go and incorporate with fatty acids Glycolysis has both aerobic and anaerobic modes but the citric acid cycle is strictly aerobic Generic reaction 2 carbon acetyl units are accepted in the form of acetyl CoA They are introduced into the citric acid cycle by binding to a 4 carbon acceptor molecule 2 carbon units are oxidized to CO2 and the resulting high transfer potential electrons are captured The acceptor molecule is regenerated capable of processing another two carbon unit III FOCUS ON CITRIC ACID CYCLE REACTIONS follow along on handout 1 citrate synthase acetyl CoA oxaloacetate citrate Beginning of the citric acid cycle Condensation of Oxoloacetate 4 Carbons and the acetyl group of acetyl CoA 2 Carbons Oxoloacetate reacts with acetyl CoA and water to yield citrate and CoA Catalyzed enzyme by citrate synthase Synthase an enzyme that catalyzes a reation in which two units are joined without the participation of ATP Oxoloacetate and acetyl CoA condense to form citryl CoA The hydrolysis of citryl CoA thioester to citrate and CoA drives this overall reaction Thioester hydrolysis powers this synthesis and forms two new precursors This reaction indicates the Citric Acid Cycle Minimizes side reactions IMPORTANT Oxoloacetate induces a major structural rearrangement leading to the creation of a binding site for acetyl CoA The binding of oxaloacetate forces into the active site and goes from open to closed form 2 aconitase citrate aconitate isocitrate Aconitase multifunctional enzyme Very good for oxidation If aconitase does not work we will not have a lot of citrate This is bad Why If we don t have citrate we cannot go thru the citric acid cycle or glycolysis Citrate is isomerized into isocitrate Why Citrate is not optimally structured for oxidation reactions to occur Specifically the OH group is not in the right location for oxidative decarboxylation to occur How is citrate isomerized By dehydration followed by hydration Result interchange of an H and an OH Aconitase catalyzes both of these steps Cis aconitate intermediate produced in the conversion of citrate into isocitrate 3 isocitrate dehydrogenase isocitrate ketoglutarate First redox step of 4 in the cycle Isocitrate Dehydrogenase enzyme that catalyzes the redox of isocitrate to ketoglutarate Isocitrate NAD ketoglutarate CO2 NADH Intermediates oxalosuccinate The oxidation of this reaction produces the first high transfer potential electron carrier NADH 4 ketoglutarate dehydrogenase complex KG succinyl CoA ketoglutarate 5 Carbons Succinyl CoA 4 Carbons Catalyzed by ketoglutarate dehydrogenase complex Oxidative decarboxylation of ketoglutarate closely resembles pyruvate SO FAR 2 CARBON ATOMS HAVE BEEN OXIDIZED TO CO2 THE ELECTRONS FROM THE OXIDATIONS ARE CAPTURED IN TWO MOLECULES OF NADH 5 succinyl CoA synthetase succinyl CoA succinate First step in regeneration Regenerates oxaloacetate How is this accomplished Series of reactions that start and end with 4 carbon molecules Rearrangement within these reactions harvest energy in the form of high energy electron carriers and a molecule of ATP Succinyl CoA is a energy rich thioester compound How is this utilized the cleavage of the thioester bond of succinyl CoA is coupled to the phosphorylation of a GTP Catalyzed by Succinyl CoA Synthetase ONLY STEP IN CITRIC ACID CYCLE THAT DIRECTLY YIELDS A COMPOUND WITH HIGH PHOSPHORYL TRANSFER POTENTIAL 6 succinate dehydrogenase succinate fumarate Hydrogen acceptor is FAD rather than NAD in this step How does succinate dehydrogenase differ from other enzymes It is embedded