Lecture 36Outline of Last Lecture I. Mitochondrial Electron Transport ChainII. Reduction PotentialIII. The Four ComplexesIV. Flow of Electrons Outline of Current Lecture I. MitCurrent Lecture This lecture will continue on with the mitochondrial electron transport chain. First we will look at each of the complexes individually within the electron transport chain. Complex I is also called the NADH: ubiquinone oxidoreductase. Complex I is made up of multiple proteins. Some of them include flavoprotein, which contains FMN. One specific example of this would be riboflavin, also known as vitamin B2. Other proteins contain iron-sulfur centers. This complex of proteins contains two main centers for the oxidation-reduction reactions to take place. One is the FMN group which is reduced to FMNH2. The other is Fe3+ which is reduced to Fe2+. Only one electron is passed through the complex at a time. Complex II is also called succinate dehydrogenase. This is also a component of the citric acid cycle. This protein includes flavoprotein that contains FAD and iron-sulfur centers. This complex also has two centers for oxidation-reduction reactions to take place. One is FAD which is reduced to FADH2. The other is Fe3+ which is reduced to Fe2+. Both complex I and complex II, though separate parts of the path, come into coenzyme Q. Coenzyme Q is not a protein. It is also called ubiquinone. This is a membrane soluble, low molecular weight, compound. Its purpose is to shuttle electrons and protons from complex I and II to complex III. Ubiquinone has a very long hydrophobic tail. This tail helps to keep it anchored to the mitochondrial inner membrane. Ubiquinone can be reduced to ubiquinol, an alcohol. Complex III is the next step in the transport chain. This contains iron-sulfur centered proteins. It also contains cytochromes, which are proteins that contain hemes. This also contains the oxidation reduction center of Fe3+, which is reduced to Fe2+. This complex contains what is called the Q cycle. This is a mini cycle that contains multiple cytochromes, which the electrons are shuttled through. Cytochrome C is a specific cytochrome. It contains iron. It is a peripheral membrane protein. Its purpose is to shuttle electrons and protons from complex III to complex IV. Complex IV is the final stop in the electron transport chain. This is another complex of proteins that contains 2 cytochromes and some proteins with copper centers. There are two main centers for oxidation-reduction reactions to take place in this complex. One is Fe3+, which is reduced to Fe2+. The other is Cu2+, which is reduced to Cu1+. This final step reduces oxygen to water. BCHM 307 1nd EditionWe are now going to focus on oxidative phosphorylation, which takes the energy from the electron transport chain to make ATP. Complex I, III, and IV pump protons across the inner mitochondrial membrane. Only completely integral membrane proteins can do this. The pumping uses energy harnessed from the oxidation of NADH and FADH2. This pumping generates what is called a membrane potential, also known as an electrochemical gradient. This gradient create a negative and alkaline environment inside of the membrane and a positive and acidic environment outside the membrane. Complex I pumps out 4 protons, complex III pumps out 4 protons, and complex IV pumps out 2 protons. The ATP synthase complex is a large transmembrane complex. It faces into the mitochondrial matrix. This is what generates ATP. It has a proton pore part and an ATP synthesizing subunit. The general guideline is that 1 unit of ATP is generated for every three protons that pass through the complex. The oxidation of NADH generates 10 protons, and therefore approximately 3 ATP are made. The oxidation of FADH2 pumps 6 protons through and generates 2
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