Proton transport, mitochondrial

This is a crucial point because (as we will see) proton transport is coupled with ATP synthesis. Oxidation of one FADHg in the electron transport chain results in synthesis of approximately two molecules of ATP, compared with the approximately three ATPs produced by the oxidation of one NADH. Other enzymes can also supply electrons to UQ, including mitochondrial 5w-glyc-erophosphate dehydrogenase, an inner membrane-bound shuttle enzyme, and the fatty acyl-CoA dehydrogenases, three soluble matrix enzymes involved in fatty acid oxidation (Figure 21.7 also see Chapter 24). The path of electrons from succinate to UQ is shown in Figure 21.8. 

As with Complex 1, passage of electrons through the Q cycle of Complex 111 is accompanied by proton transport across the inner mitochondrial membrane. The postulated pathway for electrons in this system is shown in Figure 21.12. A large pool of UQ and UQHg exists in the inner mitochondrial membrane. The Q cycle is initiated when a molecule of UQHg from this pool diffuses to a site (called Q, ) on Complex 111 near the cytosolic face of the membrane. 

The reduction of oxygen in Complex IV is accompanied by transport of protons across the inner mitochondrial membrane. Transfer of four electrons through this complex drives the transport of approximately four protons. The mechanism of proton transport is unknown but is thought to involve the steps from state P to state O (Figure 21.20). Four protons are taken up on the matrix side for every two protons transported to the cytoplasm (see Figure 21.17). 

FIGURE 21.21 A model for the electron transport pathway in the mitochondrial inner membrane. UQ/UQH9 and cytochrome e are mobile electron carriers and function by transferring electrons between the complexes. The proton transport driven by Complexes I, III, and IV is indicated. 

The oxidation of reduced jS-nicotinamide adenine dinucleotide (NADH) by quinone derivatives (Q) by has been investigated extensively, since the reaction was considered to be essential in the proton transport and the energy accumulation occurring at the mitochondrial inner membrane [2]. However, most of fundamental work in this field has been done in homogeneous solutions [48-52] though the reaction in living bodies has been believed to proceed at the solution membrane interface. 

These studies demonstrate the general mechanism of synchronization of biochemical systems, which I expect to be operative in even more complex systems, such as the mitochondrial respiration or the periodic activity of the slime mold Dictyostelium discoideum. As shown in a number of laboratories under suitable conditions mitochondrial respiration can break into self-sustained oscillations of ATP and ADP, NADH, cytochromes, and oxygen uptake as well as various ion transport and proton transport functions. It is important to note that mitochondrial respiration and oxidative phosphorylation under conditions of oscillations is open for the source, namely, oxygen, as well as with respect to a number of sink reactions producing water, carbon dioxide, and heat. 

The energy must be in A/ZII form, equivalent to Ai//and ApH. The mitochondrial reaction mechanism (3.54) involving H+-ATP-synthase is illustrated [22] in Figure 3.3a. The advantage of this scheme is that it indicates ways of consumption of substrates and products in A/ZII generation and utilization, i.e. protons transported by means of the F0 factor to the water molecule are included, produced in the reaction (3.54). 

Electron transport through oxidases in the plasma membrane contributes to, or controls, part of the proton release from the cell. The details of oxidase function and the mechanism of control remain to be elucidated. The NADPH oxidase of neutrophils is a special case in which proton transport is coupled to the cytochrome >557 electron carrier. This type of proton transport has its precedents in the well-characterized proton pumping through electron carriers in mitochondrial and chloroplast membranes and prokaryotic plasma membranes. 

Sciortino, F., Poole, P. H., Stanley, H. E., Havlin, S. (1990). Phys. Rev. Letters, 64,1686-1689. Solaini, G., Baracea, A., Parenti Castilla, G., Lenarz, G. (1984). Temperature dependence of mitochondrial oligomycin-sensitive proton transport ATPase. J. Bioenerg. Biomembr. 16, 391-406. 

The coupling between electron transport from NADH (or FADH2) to O2 and proton transport across the inner mitochondrial membrane, which generates the proton-motive force, also can be demonstrated experimentally with Isolated mitochondria (Figure 8-14). As soon as O2 is added to a suspension of mitochondria, the medium outside the mitochondria becomes acidic. During electron transport from NADH to O2, protons translocate from the matrix to the Intermembrane space since the outer membrane Is freely permeable to protons, the pH of the outside medium Is lowered briefly. The measured change In pH Indicates that about 10 protons are transported out of the matrix for every electron pair transferred from NADH to O2. 

A EXPERIMENTAL FIGURE 8-14 Electron transfer from NADH or FADH2 to O2 is coupled to proton transport across the mitochondrial membrane. If NADH is added to a suspension of mitochondria depleted of O2, no NADH is oxidized. When a small amount of O2 is added to the system (arrow), the pH of the surrounding medium drops sharply—a change that corresponds to an increase in protons outside the mitochondria. (The presence of a large amount of valinomycin and in the... 

The Inner membrane of brown-fat mitochondria contains thermogenin, a protein that functions as a natural uncoupler of oxidative phosphorylation. Like synthetic uncouplers, thermogenin dissipates the proton-motive force across the Inner mitochondrial membrane, converting energy released by NADH oxidation to heat. Thermogenin is a proton transporter, not a proton channel, and shuttles protons across the membrane at a rate that is a millionfold slower than that of typical Ion channels. Its amino acid sequence is similar to that of the mitochondrial ATP/ADP antiporter, and it functions at a rate that Is characteristic of other transporters (see Figure 7-2). 

In brown fat, the inner mitochondrial membrane contains thermogenln, a proton transporter that converts the proton-motive force into heat. Certain chemicals (e.g., DNP) have the same effect, uncoupling oxidative phosphorylation from electron transport. 

Complex I, III, and IV proton transporters of mitochondrial inner membrane... 

For decades, brown fat metabolism has been studied with tissue explants. While WAT is important for the storage of energy in the form of triacylglycerol, BAT functions to dissipate energy in the form of heat through the action of a specific mitochondrial proton transporter, UCPl. While the 3T3-L1 or 3T3-F442A cells lines provided a convenient... 

C) Blocking proton transport across the inner mitochondrial membrane... 

The reaction uses a series of specialized electron- and proton-transporting proteins that are formed into large complexes. These are embedded in the inner mitochondrial membrane and constitute the... 

Only in brush-border membranes of some specialised cells and in erythrocyte membranes there seems to be an Mg-ATPase, which can be stimulated by anions, but only to a minor degree. Moreover, the properties of the enzyme in these membranes differ considerably from those of the activities in the microsomal and mitochondrial fractions of most other tissues. The anion sensitivity of the ATPase activity in the brush-border membranes of placenta and small intestine is a property of the alkaline phosphatase [37,41] and that in erythrocytes is part of the (Ca -b Mg )-ATPase activity [33,34]. Although this does not definitely exclude a role of the enzyme in anion transport, no valid arguments in favour of a role of this enzyme in anion or proton transport have been advanced. 

FIGURE 9.10. General structure of ubiquinone and related quinones. For ubiquinone (coenzyme Q) of human origin, n= 0. Quinones of this type are found in bacterial and mitochondrial membranes, and these are involved in electron and proton transport. Because of their long hydrophobic tail, these quinones are soluble in phospholipid bilayers. 

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