The NADH and FADH2 formed in glycolysis, fatty acid oxidation, and the citric acid cycle are energy-rich molecules because each contains a pair of electrons having a high transfer potential. When these electrons are used to reduce molecular oxygen to water, a large amount of free energy is liberated, which can be used to generate ATP. Oxidative phosphorylation is the process in which ATP is formed as a result of the transfer of electrons from NADH or FADH2 to O2 by a series of electron carriers. This process, which takes place in mitochondria, is the major source of ATP in aerobic organisms . For example, oxidative phosphorylation generates 26 of the 30 molecules of ATP that are formed when glucose is completely oxidized to CO2 and H2O.*
The flow of electrons from NADH or FADH2 to O2 through protein complexes located in the mitochondrial inner membrane leads to the pumping of protons out of the mitochondrial matrix. The resulting uneven distribution of protons generates a pH gradient and a transmembrane electrical potential that creates a proton-motive force. ATP is synthesized when protons flow back to the mitochondrial matrix through an enzyme complex.
Thus, the oxidation of fuels and the phosphorylationof ADP are coupled by a proton gradient across the inner mitochondrial membrane*
Oxidative phosphorylation is the culmination of a series of energy transformations that are called cellular respiration or simply respiration in their entirety. First, carbon fuels are oxidized in the citric acid cycle to yield electrons with high transfer potential. Then, this electron-motive force is converted into a proton-motive force Finally, the proton-motive force is converted into phosphoryl transfer potential. The conversion of electron-motive force into proton-motive force is carried out by three electron-driven proton pumps :NADH-Q oxidoreductase, Q-cytochrome c oxidoreductase, andcytochrome c oxidase
These large transmembrane complexes contain multiple oxidation-reduction centers, including :quinones, flavins, iron-sulfur clusters, hemes, and copper ionsThe final phase of oxidative phosphorylation is carried out by ATP synthase,An ATP-synthesizing assembly that is driven by the flow of protons back into the mitochondrial matrix. Components of this remarkable enzyme rotate as part of its catalytic mechanism.
Oxidative phosphorylation vividly shows that proton gradients are an interconvertible currency of free energy in biological systems.RespirationAn ATP-generating process in which an inorganic compound (such as molecular oxygen) serves as the ultimate electron acceptor. The electron donor can be either an organic compound or an inorganic one.*
Mitochondria are oval-shaped organelles, Typically about 2 um in length and 0.5 um in diameter, about the size of a bacterium. Eugene Kennedy and Albert Lehninger discovered a half-century ago that mitochondria contain :the respiratory assembly, the enzymes of the citric acid cycle, and the enzymes of fatty acid oxidation.Electron microscopic revealed that mitochondria have two membrane systems: an outer membrane and an extensive, highly folded inner membrane. The inner membrane is folded into a series of internal ridges called cristae.
There are two compartments in mitochondria: (1) the intermembrane space between the outer and the inner membranes and (2) the matrix, which is bounded by the inner membraneOxidative phosphorylation takes place in the inner mitochondrial membrane, While most of the reactions of the citric acid cycle and fatty acid oxidation take place in the matrix. The outer membrane is quite permeable to most small molecules and ions because it contains many copies of mitochondrial porin, a 30 35 kd poreforming protein also known as VDAC, for voltage-dependent anion channel. VDAC plays a role in the regulated flux of metabolites usually anionic species such as phosphate, chloride, organic anions, and the adenine nucleotides across the outer membrane. VDAC appears to form an open b -barrel structure similar to that of the bacterial porins*
In contrast, the inner membrane is intrinsically impermeable to nearly all ions and polar molecules. A large family of transporters shuttles metabolites such as ATP, pyruvate, and citrate across the inner mitochondrial membrane. The two faces of this membrane will be referred to as the: matrix side and the cytosolic side (the latter because it is freely accessible to most small molecules in the cytosol). They are also called the N and P sides, respectively, because the membrane potential is negative on the matrix side and positive on the cytosolic side.In prokaryotes, the electron-driven proton pumps and ATP-synthesizing complex are located in the cytoplasmic membrane, the inner of two membranes. The outer membrane of bacteria, like that of mitochondria, is permeable to most small metabolites because of the presence of porins.*
*Mitochondria are semiautonomous organelles that live in an endosymbiotic relation with the host cell. These organelles contain their own DNA, which encodes a variety of different proteins and RNAs. The genomes of mitochondrial range broadly in size across species. The mitochondrial genome of the protist Plasmodium falciparum consists of fewer than 6000 base pairs (6 kbp), whereas those of some land plants comprise more than 200 kbp . Human mitochondrial DNA comprises 16,569 bp and encodes 13 respiratory-chain proteins as well as the small and large ribosomal RNAs and enough tRNAs to translate all codons. However, mitochondria also contain many proteins encoded by nuclear DNA. Cells that contain mitochondria depend on these organelles for oxidative phosphorylation, and the mitochondria in turn depend on the cell for their very existence.
*An endosymbiotic event is thought to have occurred whereby a freeliving organism capable of oxidative phosphorylation was engulfed by another cell. The double membrane, circular DNA (with some exceptions), and mitochondrial-specific transcription and translation machinery all point to this conclusion. Sequence data suggest that all extant mitochondria are derived from an ancestor of Rickettsia prowazekii as the result of a single endosymbiotic event. The transient engulfment of prokaryotic cells by larger cells is not uncommon in the microbial world. In regard to mitochondria, such a transient relation became permanent as the bacterial cell lost DNA, making it incapable of independent living, and the host cell became dependent on the ATP generated by its tenant.
*High-Energy Electrons: Redox Potentials and Free-Energy ChangesHigh-energy electrons and redox potentials are of fundamental importance in oxidative phosphorylation. In oxidative phosphorylation, the electron transfer potential of NADH or FADH2 is converted into the phosphoryl transfer potential of ATP.The reduction potential is an electrochemical concept Consider a substance that can exist in an oxidized form X and a reduced form X-. Such a pair is called a redox couple.The reduction potential of this couple can be determined by measuring the electromotive force generated by a sample half-cell connected to a standard reference half-cellThe reduction potential of the X:X - couple is the observed voltage at the start of the experiment (when X, X-, and H+ are 1 M). The reduction potential of the H + :H 2 couple is defined to be 0 volts.
*Thus, a strong reducing agent (such as NADH) is poised to donate electrons and has a negative reduction potential, whereas a strong oxidizing agent (such as O2 ) is ready to accept electrons and has a positive reduction potential.
The free energy change associated with the oxidation-reduction can be measured
Where n is the number of electrons transferred, F is a proportionality constant called the faraday [23.06 kcal mol-1 V-1 (96.48 kJ mol-1 V-1)], D E 0 is in volts, and D G is in kilocalories or kilojoules per mole.
*We can utilize this equation to calculate the free energy change during an oxidation-reduction reaction i.e.
The oxidation-reduction potential of each step
*For pyruvate conversion into lactate, the free energy can be calculated with n = 2.
For NADH conversion into NAD+:
For total reaction we need to sum up these energies:
*The driving force of oxidative phosphorylation is the electron transfer potential of NADH or FADH2 relative to that of O2. How much energy is released by the reduction of O2 with NADH.The energy released in this reaction can be calculated:
This is a substantial release of free energy. Recall that G = - 7.5 kcal mol-1 ( - 31.4 kJ mol-1) for the hydrolysis of ATP. The released energy is used initially to generate a proton gradient that is then used for the synthesis of ATP and the transport of metabolites across the mitochondrial membrane
*The electron-carrying groups in the protein constituents of the electron-transport chain are flavins, iron-sulfur clusters, quinones, hemes, and copper ions. How are electrons transferred between electron-carrying groups that are frequently buried in the interior of a protein in fixed positions and are therefore not directly in contact? Electrons can move through space, even through a vacuum. However, the rate of electron transfer through space falls off rapidly as the electron donor and electron acceptor move apart from each other, decreasing by a factor of 10 for each increase in separation of 0.8 . The protein environment provides more-efficient pathways for electron conduction: typically, the rate of electron transfer decreases by a factor of 10 every 1.7 .
*For groups in contact, electron-transfer reactions can be quite fast with