Complexes proton transport

Traditionally, the electron and proton transport pathways of photosynthetic membranes (33) have been represented as a "Z" rotated 90° to the left with noncycHc electron flow from left to right and PSII on the left-most and PSI on the right-most vertical in that orientation (25,34). Other orientations and more complex graphical representations have been used to depict electron transport (29) or the sequence and redox midpoint potentials of the electron carriers. As elucidation of photosynthetic membrane architecture and electron pathways has progressed, PSI has come to be placed on the left as the "Z" convention is being abandoned. Figure 1 describes the orientation in the thylakoid membrane of the components of PSI and PSII with noncycHc electron flow from right to left. 

Complex I Transports Protons from the Matrix to the Cytosol... 

FIGURE 21.8 A probable scheme for electron flow in Complex II. Oxidation of succinate occurs with rednction of [FAD]. Electrons are then passed to Fe-S centers and then to coenzyme Q (UQ). Proton transport does not occur in this complex. 

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. 

Mitchell s chemiosmotic hypothesis. The ratio of protons transported per pair of electrons passed through the chain—the so-called HV2 e ratio—has been an object of great interest for many years. Nevertheless, the ratio has remained extremely difficult to determine. The consensus estimate for the electron transport pathway from succinate to Og is 6 H /2 e. The ratio for Complex I by itself remains uncertain, but recent best estimates place it as high as 4 H /2 e. On the basis of this value, the stoichiometry of transport for the pathway from NADH to O2 is 10 H /2 e. Although this is the value assumed in Figure 21.21, it is important to realize that this represents a consensus drawn from many experiments. 

The total electro-osmotic coefficient = Whydr + mo includes a contribution of hydrodynamic coupling (Whydr) and a molecular contribution related to the diffusion of mobile protonated complexes—namely, H3O. The relative importance, n ydr and depends on the prevailing mode of proton transport in pores. If structural diffusion of protons prevails (see Section 6.7.1), is expected to be small and Whydr- If/ ori the other hand, proton mobility is mainly due to the diffusion of protonated water clusters via the so-called "vehicle mechanism," a significant molecular contribution to n can be expected. The value of is thus closely tied to the relative contributions to proton mobility of structural diffusion and vehicle mechanism. ... 

Proton transport via complexes I, III, and IV takes place vectorially from the matrix into the intermembrane space. When electrons are being transported through the respiratory chain, the concentration in this space increases—i. e., the pH value there is reduced by about one pH unit. For each H2O molecule formed, around 10 H ions are pumped into the intermembrane space. If the inner membrane is intact, then generally only ATP synthase (see p. 142) can allow protons to flow back into the matrix. This is the basis for the coupling of electron transport to ATP synthesis, which is important for regulation purposes (see p. 144). 

Transporting ATP synthase [EC 3.6.1.34] in plants, also referred to as chloroplast ATPase and CFiCFo-ATPase, catalyzes the hydrolysis of ATP to produce ADP and orthophosphate. When coupled with proton transport the reverse reaction results in the synthesis of ATP by this multisubunit complex. CFi, isolated from the rest of the membrane-bound complex, retains the ATPase activity but not the proton-translocating activity. 

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 Fe-S Reaction Center (Type I Reaction Center) Photosynthesis in green sulfur bacteria involves the same three modules as in purple bacteria, but the process differs in several respects and involves additional enzymatic reactions (Fig. 19-47b). Excitation causes an electron to move from the reaction center to the cytochrome bei complex via a quinone carrier. Electron transfer through this complex powers proton transport and creates the proton-motive force used for ATP synthesis, just as in purple bacteria and in mitochondria. 

The performance of calixarenes as cation carriers through H20-organic solvent H20 liquid membranes has also been studied.137 In basic metal hydroxide solutions, the monodeprotonated phenolate anions complex and transport the cations, while [18]crown-6 does not, under the same conditions. Low water solubility, neutral complex formation and potential coupling of cation transport to reverse proton flux have been cited as desirable transport features inherent in these molecules.137... 

Hydrogen transport. The approach that they have used to predict proton transport through complex membranes such as Nation is to use ab initio methods to determine the barriers for migration of hydrogen as a function of the donor-acceptor separation and then to employ a statistical method that is based on the ab initio results. This method allows a proton jump among water clusters when the configuration around the proton is appropriate. 

Because of the complexity of hydrated PEMs, a full atomistic modeling of proton transport is impractical. The generic problem is a disparity of time and space scales. While elementary molecular dynamics events occur on a femtosecond time scale, the time interval between consecutive transfer events is usually 3 orders of magnitude greater. The smallest pore may be a few tenth of nanometer while the largest may be a few tens of nanometers. The molecular dynamics events that protons transfer between the water filled pores may have a timescale of 100-1000 ns. This combination of time and spatial scales are far out of the domain for AIMD but in the domain of MD and KMC as shown in Fig. 2. Because of this difficulty, in the models the complexity of the systems is restricted. In fact in many models the dynamics of excess protons in liquid water is considered as an approximation for proton conduction in a hydrated Nation membrane. The conformations and energetics of proton dissociation in acid/water clusters were also evaluated as approximations for those in a Nation membrane.16,19 20 22 24 25... 

It consists of two so-called sub-complexes responsible for proton transport (F0) and ATP synthesis or hydrolysis (Fj). 

Complex I mediates electron transfer from NADH + H+ to coenzyme Q. The consequent reduction of coenzyme Q is coupled to vectorial proton transport the H+/e stoichiometry (ratio) has been variously estimated to be 1.0, 1.5, and 2.0. 

ATP synthase activity can be restored by adding back the F] complex to the depleted membranes. The F[ complexes bind to membrane channels known as the F complex, which are also composed of multiple subunits. The polypeptides of the F0 component are very hydrophobic and form a proton transport channel through the membrane, which links the proton gradient to ATP synthesis. This channel appears to be lined with hydrophilic residues such as seryl, threonyl and carboxyl groups. The stalk that connects the F, to the F complex comprises one copy each of the polypeptide known as the oligomycin-sensitivity-conferring protein (OSCP) and another protein known as F6. 

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