Electron Transport Chain lecture

• The molecule which continues to diffuse that barrier to another produces a concentration gradient
• NADH and FADH2 transfer electrons to a series of electron carriers to O2, and everytime it transfers, protons are pumped in the intermembrane space, which then creates a proton gradient across the inner mitochondrial membrane
• The proton gradient would create a negative transmembrane potential leading to a proton motive force
• Proton motive force is the driving force behind ATP synthesis which couples ETC and oxidative phosphorylation
• Electron transport chain and oxidative phosphorylation occurs in mitochondria, and 26 out of 32 ATP is produced in ETC and oxidative phosphorylation
• The complete oxidation of glucose is C6H12O6 + 6 O2 -> 6 CO2 + 6 H2O and it can be broken down to two half reactions
• The two half reactions include 1.) C6H12O6 + 6H2O -> 6CO2 + 24 H+ + 24e- and 2.) 6O2 + 24H+ + 24e- -> 12H2O
• The 24e- are produced from GAPDH, pyruvate dehydrogenase, isocitrate, alpha ketoglutarate, succinate, and malate dehydrogenases. Reactions that are oxidized produces an accompanying electron
• Oxidation of fatty acids can produce NADH and these can lead to the production of ATP
• During the electron transport process, NADH and FADH2 are reoxidized to NAD+ and FAD, a path used to participate to glycolysis for redox balance
• During the electron transport process, the electrons participate in sequential redox of multiple redox centers in four enzyme complexes before reducing O2 to H2O
• Each enzyme complex is made of several proteins and redox centers. The redox centers can be a cytochrome which is hydrophobic and soluble in membrane; heme which has a iron sulfur complex and copper ion
• During the electron transport process, the free energy stored in the proton gradient during the electron transport chain is used to drive ATP synthesis from ADP and Pi by oxidative phosphorylation
• Oxidative phosphorylation is a process where ATP is formed via the transfer of electrons from reduced coenzymes to O2, by a series of electron carriers as it is to be released from ATP synthase
• A highly folded, protein rich inner membrane separates the mitochondrial matrix from the outer membrane
• The invagination of the inner membrane is called cristae, a part that reflects the respiratory activity of the cell. While the matrix is a gel like solution that contains DNA and several metabolites and enzymes for oxidizing phosphorylation, substrate, and inorganic ions
• The shape and length of the cristae vary, and that it prevents migration of metabolites across the inner membrane
• The ions and metabolites enter mitochondria by transporters. The intermembrane space is equivalent to cytosol in its concentration of metabolites and ions
• The outer membrane allow diffusion of some molecules, but the inner membrane is not permeable. Which is why metabolites are contained in the matrix
• The controlled impermeability of the inner mitochondrial membrane to most ions and metabolites generates ion gradients across the barrier resulting to a division of metabolic functions between cytosol and mitochondria
• The cytosolic reducing equivalents such as NADH are transported into the mitochondria
• The two mechanisms that moves electrons from cytoplasmic NADH into the mitochondrial ETC are glycerol phosphate shuttle and malate aspartate shuttle
• The glycerol phosphate shuttle is active in insect, plant, muscles and is expressed in varying amounts in different tissues
• In the malate-aspartate shuttle, oxaloacetate is reduced by NADH to form malate which then enters the mitochondrion. When malate is reoxidized in the matrix, it gives up the reducing equivalents that originated in the cytosol
• In glycerophosphate, the NADH produced in glycolysis is oxidized by the 3 phosphoglycerol dehydrogenase which can reduce DHAP to 3 phosphoglycerol.
• The electrons in 3 phosphoglycerol is oxidized with flavoprotein dehydrogenase with the cofactor FAD and the electrons in FADH2 can be released in complex II of the electron transport chain across the inner mitochondrial membrane
• The electrons in glycerol phosphate shuttle resolve in 1.5 ATP, which is lesser compared to malate aspartate shuttle which produces 2.5 ATP
• The entry of ADP into the mitochondrial matrix is coupled to the exit of ATP by ATP-ADP translocase
• ATP from matrix is transported out in exchange for ADP to enter in the matrix
• Bovine ATP ADP translocator/Translocase is a dimer of identical 300 residue subunits. Each subunit has 1 binding site for which ATP and ADP compete
• There are two types of Bovine ATP-ADP Translocator, one faces in the matrix and the other faces the intermembrane space
• The Bovine ATP-ADP translocator when faced in the matrix has a deep cavity that is positively charged, where the ATP can bind to the central cavity of the translocator
• The ADP ATP translocase catalyzes the coupled entry of ADP and exit of ATP
• The binding of ADP from the cytoplasm favors reversal of the transporter. The ADP is released from the transporter, and subsequent binding of ATP from matrix in the cavity is added which reverses back to its original position while releasing ATP into the cytosol
• The ATP ADP translocator inhibitors such as atractyloside and its derivative carboxyatractyloside and Bongkrekic acid is used to elucidate mechanism that binds to the central cavity
• The Pi is returned to the mitochondrion by the phosphate carrier, an electroneutral Pi- H+ symport is driven by a change in pH. The import of phosphate entering the mitochondria to the matrix is by proton gradient
• There is 1 complex embedded in the inner mitochondrial membrane, and 3 electron carriers are mobile outside the inner mitochondrial membrane which can serve as the connection point between electrons
• Oxidation of NADH and FADH2 is carried by the ETC, a set of protein complexes containing redox centers with progressively increasing affinities for electrons
• The respiratory chain consist of four complexes: Three proton pumps and a physical link to the citric acid cycle
• NADH released would be oxidized and passes its electrons to FMN, a redox center part of complex I. FADH2 and FMN would pass electrons to Coenzyme Q, an mobile carrier