To introduce the complexes that will be discussed, Fig. The mitochondrial DNA encodes for all the RNA involved in protein synthesis in the matrix but also for 13 proteins that are subunits of the ETC complexes: seven subunits in Complex I, one in Complex III, three in Complex IV, and one in the ATP synthase (sometimes called complex V). Thought to originate from endosymbiosis, each mitochondrion has 5–10 copies of circular DNA and a complete protein translation system in the matrix. Mitochondria are central to energy integration in eukaryotic cells. The gradient is then dissipated through the ATP synthase complex, driving conformational changes that are transmitted through the protein to the catalytic trimer of this molecular machine. Figure 1b shows the general scheme with a proton pumping apparatus generating a hydrogen ion gradient across a membrane impermeable to protons. Later work showed that movement of organic ions such as succinate could also convert the pH gradient to a membrane potential, and using a bacterial system, that the membrane potential influenced the kinetics of ATP synthase. Just equilibrating isolated mitochondria to pH 8.8 then adding acid to the external medium to give a pH of about 4.3 can generate ATP. Illuminating the bacteriorhodopsin enabled its proton pumping, and ATP was synthesized from adenosine diphosphate (ADP) and phosphate.
1, a key experiment proving that a hydrogen ion gradient can drive ATP synthesis was the incorporation of isolated ATP synthase and isolated bacteriorhodopsin into artificial vesicles. Ultimately, Peter Mitchell was awarded a Nobel Prize in 1978 for his innovative work.Īs shown in Fig. Although controversial when first described in 1961 and recently republished, chemiosmosis was finally accepted as the mechanism enabling oxidative phosphorylation and generation of ATP. It is the same in chloroplasts using energy from sunlight and in mitochondria using the chemical energy from the breakdown of sugars, proteins, and fats. įigure 1 shows the underlying principle of the chemiosmotic mechanism for energy conversion from its original form to a hydrogen ion gradient that drives the ATP synthase molecular machine. This article focuses on the components and mechanism of the electron transport chain (ETC) that supports oxidative phosphorylation in mammalian mitochondria, a process described in all biochemistry textbooks, and in more advanced detail in the book Bioenergetics 4 by Nicholls and Ferguson. Bacteria, chloroplasts, and mitochondria transport systems use the energy that is released as electrons are passed to progressively higher redox potential electron carriers to generate proton gradients across membranes that can drive ATP synthesis or transport systems. These systems not only convert energy from one form (chemical or light) to another (ion gradient across an impermeable membrane and subsequently back to chemical energy in the form of ATP) but also allow the energy to be conserved rather than lost as heat. There is a wide diversity of electron transport chains across the range of lifeforms, using either light or metabolic energy as the input, with not only oxygen but also other final electron acceptors. The chemiosmotic mechanism for ATP synthesis is key to aerobic energy conversion in all cells, supplying the majority of the energy required for survival, repair, growth, and reproduction of the organism.
Mitochondria and their proteins play roles not only in the production of ATP but also in cell survival, for which energy supply is the key. The three processes of proton pumping are now known after the successful determination of the structures of the large membrane protein complexes involved. The electron carriers include flavins, iron–sulfur centers, heme groups, and copper to divide the redox change from reduced nicotinamide adenine dinucleotide (NADH) at −320 mV to oxygen at +800 mV into steps that allow conversion and conservation of the energy released in three major complexes (Complexes I, III, and IV) by moving protons across the mitochondrial inner membrane. The electron transport chain converts the energy that is released as electrons are passed to carriers of progressively higher redox potential into a proton gradient across the membrane that drives adenosine triphosphate (ATP) synthesis. It gives references chosen to reflect the history of the field and to highlight some of the recent advances in bioenergetics.
This summary of four lectures on the electron transport system in mitochondria is an introduction to the mammalian electron transport chain for those unfamiliar with mitochondrial oxidative phosphorylation. The chemical system for the transformation of energy in eukaryotic mitochondria has engaged researchers for almost a century.
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