Metabolism/Energy Overview Glycolysis Citric Acid Cycle  Elmhurst College Cell Structure Energy Storage - ATP FAD, CoQ, CoA  Chemistry Department Energy Overview NAD+ Electron Transport  Virtual ChemBook

Click for larger image	Electron Transport Introduction: The majority of the energy conserved during catabolism reactions occurs near the end of the metabolic series of reactions in the electron transport chain. The electron transport or respiratory chain gets its name from the fact electrons are transported to meet up with oxygen from respiration at the end of the chain. The overall electron chain transport reaction is: 2 H+ + 2 e+ + 1/2 O2 ---> H2O + energy Notice that 2 hydrogen ions, 2 electrons, and an oxygen molecule react to form as a product water with energy released in an exothermic reaction. This relatively straight forward reaction actually requires eight or more steps. The energy released is coupled with the formation of three ATP molecules per every use of the electron transport chain. Link to a complete animated version of Electron Transport - Brooks-Cole Link to: Rodney Boyer Animation of Electron Transport Pre-Initiation of Electron Transport Chain: The electron transport chain is initiated by the reaction of an organic metabolite (intermediate in metabolic reactions) with the coenzyme NAD+ (nicotinamide adenine dinucleotide). This is an oxidation reaction where 2 hydrogen atoms (or 2 hydrogen ions and 2 electrons) are removed from the organic metabolite. (The organic metabolites are usually from the citric acid cycle and the oxidation of fatty acids--details in following pages.) The reaction can be represented simply where M = any metabolite. MH2 + NAD+ -----> NADH + H+ + M: + energy One hydrogen is removed with 2 electrons as a hydride ion (H-) while the other is removed as the positive ion (H+). Usually the metabolite is some type of alcohol which is oxidized to a ketone. NAD+ is a coenzyme containing the B-vitamin, nicotinamide, shown on a previous page. The purpose of the other seven steps in the electron transport chain is threefold: 1) to pass along 2H+ ions and 2e- to eventually react with oxygen; 2) to conserve energy by forming three ATP's; and 3) to regenerate the coenzymes back to their original form as oxidizing agents. Quiz: What are the initial reactants which start the electron transport chain? Answer 2 hydrogen ions and 2 electrons start the chain. Oxygen and ADP are also reactants. What are the final products of the chain?   Answer 3 ATP and a water molecule are products.
Click for larger image	Initiation of Electron Transport Chain: Once the NADH has been made from a metabolite in the citric acid cycle inside of the mitochondria, it interacts with the first complex 1 enzyme, known as NADH reductase. This complex 1 contains a coenzyme flavin mononucleotide (FMN) which is similar to FAD. The sequence of events is that the NADH, plus another hydrogen ion enter the enzyme complex and pass along the 2 hydrogen ions, ultimately to an interspace in the mitochondria. These hydrogen ions, acting as a pump, are utilized by ATP synthetase to produce an ATP for every two hydrogen ions produced. Three complexes (1, 3, 4) act in this manner to produce 2 hydrogen ions each, and thus will produce 3 ATP for every use of the complete electron transport chain. In addition, NADH passes along 2 electrons to first FMN, then to an iron-sulfur protein (FeS), and finally to coenzyme Q. The net effect of these reactions are to regenerate coenzyme NAD+. This regeneration of reactants occurs in many of the reactions so that a cycling effect occurs. The NAD+ is ready to react further with metabolites in the citric acid cycle. Coenzyme Q, which also picks up an additional 2 hydrogen ions to make CoQH2, is soluble in the lipid membrane and can move through the membrane to come into contact with enzyme complex 3. In summary, the very first enzyme complex in the electron transport chain is coupled with the formation of ATP. The coupled reaction may be written as: a) MH2 + NAD+ ---> NADH + H+ + M + energy b) ADP + P + energy ---> ATP + H2O Quiz: What is the definition of a coupled reaction? Answer The first reaction (a) is exothermic, giving of energy, while the second reaction (b) is endothermic, requiring energy. When NADH converts back to NAD+, is this oxidation or reduction?   Answer Removal of hydrogen is oxidation. When CoQ converts to CoQH2, is this oxidation or reduction?   Answer Addition of hydrogen is reduction. Animation of Electron Transport - Thomas M. Terry, The University of Connecticut Link to Chime: Electron Transfer Flavin - Department of Biochemistry,The University of Arizona
	Electron Transport - Enzyme Complex 3: Coenzyme QH2 carrying an extra 2 electrons and 2 hydrogen ions now starts a cascade of events through enzyme complex 3, also known as cytochrome reductase bc. Cytochromes are very similar to the structure of myoglobin or hemoglobin. The significant feature is the heme structure containing the iron ions, initially in the +3 state and changed to the +2 state by the addition of an electron. The CoQH2 (yellow)passes along the 2 electrons first to cytochrome (blue) b1 heme (magenta), then b2 heme , then to an iron-sulfur protein (green), then to cytochrome c1 (red with black heme), and finally to cytochrome c (not shown). Co Q is represented by the inhibitor antimycin (yellow) in the graphic. In the meantime the 2 hydrogen ions are channeled to the interspace of the mitochondria for ultimate conversion into ATP. Cytochrome complex bc - Chime in new window Quiz: When iron +3 ions add an electron to become +2 ion, is this oxidation or reduction? Answer Change in oxidation state from +3 to +2 is a reduction. Complex 4: Refer to the middle graphic: Cytochrome c is a small molecule which is also able to move in the lipid membrane layer and diffuses toward cytochrome a complex 4. Cytochrome c - Chime in new window At this time it continues the transport of the electrons, and provides the third and final time that 2 hydrogen ions are channeled to the interspace of the mitochondria for ultimate conversion into ATP. ATP synthetase is also found at numerous locations in the bilayer membrane of the mitochondria. The function of this ATP enzyme is found in an earlier page. Three ATP are produced by the pumping action of the re-entry of the hydrogen ions through the ATP synthetase. Finally, oxygen has diffused into the cell and the mitochondria for the finally reaction of metabolism. Oxygen atom reacts with the 2 electrons and 2 hydrogens to produce a water molecule.