H+-transporting ATP synthase

ATP synthase, H+ transporting, mitochondrial FI complex, beta polypeptide [Homo sapiens]... 

If noncyclic photosynthetic electron transport leads to the translocation of 3 H /e and cyclic photosynthetic electron transport leads to the translocation of 2 H /A, what is the relative photosynthetic efficiency of ATP synthesis (expressed as the number of photons absorbed per ATP synthesized) for noncyclic versus cyclic photophosphorylation (Assume that the CFiCEq ATP synthase yields 1 ATP/3 H. )... 

Because photosystem 11 and the cytochrome b/f complex release protons from reduced plastoquinone into the lumen (via a Q. cycle), photosynthetic electron transport establishes an electrochemical gradient across the thylakoid membrane (see p. 126), which is used for ATP synthesis by an ATP synthase. ATP and NADPH+H", which are both needed for the dark reactions, are formed in the stroma. 

The respiratory chain is one of the pathways involved in oxidative phosphorylation (see p. 122). It catalyzes the steps by which electrons are transported from NADH+H or reduced ubiquinone (QH2) to molecular oxygen. Due to the wide difference between the redox potentials of the donor (NADH+H or QH2) and the acceptor (O2), this reaction is strongly exergonic (see p. 18). Most of the energy released is used to establish a proton gradient across the inner mitochondrial membrane (see p. 126), which is then ultimately used to synthesize ATP with the help of ATP synthase. 

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). 

The ATP synthase (EC3.6.1.34, complex V) that transports H"" is a complex molecular machine. The enzyme consists of two parts—a proton channel (Fq, for oligomycin-sensitive ) that is integrated into the membrane and a catalytic unit (Fi) that protrudes into the matrix. The Fo part consists of 12 membrane-spanning c-peptides and one a-subunit. The head of the Fi part is composed of three a and three p subunits, between which there are three active centers. The stem between Fo and Fi consists of one y and one e subunit. Two more polypeptides, b and 8, form a kind of stator, fixing the a and p subunits relative to the Fo part. 

The inner membrane itself plays an important part in oxidative phosphorylation. As it is impermeable to protons, the respiratory chain—which pumps protons from the matrix into the intermembrane space via complexes 1, 111, and IV—establishes a proton gradient across the inner membrane, in which the chemical energy released during NADH oxidation is conserved (see p. 126). ATP synthase then uses the energy stored in the gradient to form ATP from ADP and inorganic phosphate. Several of the transport systems are also dependent on the H"" gradient. 

A second membrane transport system essential to oxidative phosphorylation is the phosphate translocase, which promotes symport of one H2PO4 and one H+ into the matrix. This transport process, too, is favored by the transmembrane proton gradient (Fig. 19-26). Notice that the process requires movement of one proton from the P to the N side of the inner membrane, consuming some of the energy of electron transfer. A complex of the ATP synthase and both translocases, the ATP synthasome, can be isolated from... 

Figure 18-5 A current concept of the electron transport chain of mitochondria. Complexes I, III, and IV pass electrons from NADH or NADPH to 02, one NADH or two electrons reducing one O to HzO. This electron transport is coupled to the transfer of about 12 H+ from the mitochondrial matrix to the intermembrane space. These protons flow back into the matrix through ATP synthase (V), four H+ driving the synthesis of one ATP. Succinate, fatty acyl-CoA molecules, and other substrates are oxidized via complex II and similar complexes that reduce ubiquinone Q, the reduced form QH2 carrying electrons to complex III. In some tissues of some organisms, glycerol phosphate is dehydrogenated by a complex that is accessible from the intermembrane space.

An interesting mutation is replacement of alanine 62 of the c subunit with serine. This mutant will support ATP synthase using Li+ instead of H+.237 Certain alkylophilic bacteria, such as Propionigenium modestum, have an ATP synthase that utilizes the membrane potential and a flow of Na+ ions rather than protons through the c subunits.238 240c The sodium transport requires glutamate 65, which fulfills the same role as D61 in E. coli, and also Q32 and S66. Study of mutants revealed that the polar side chains of all three of these residues bind Na+, that E65 and S66 are needed to bind Li+, and that only E65 is needed for function with H+. 

Fillingame, R. H. (1997). Coupling H+ transport and ATP synthesis in FjFo-ATP synthases Glimpses of interacting parts in a dynamic molecular machine. J. Exp. Biol. 200, 217-224. 

Fillingame, R. H., and Dmitriev, O. Y. (2002). Structural model of the Uansmembrane F0 rotary sector of H+-transporting ATP synthase derived by solution NMR and intersubunit cross-linking in situ. Biochim. Biophys. Acta 1565, 232-245. 

The conflicting results are obtained at solving the question if H+ ions are mixed with aqueous medium or, vice versa, are transported by the membrane-water interface (or inside the membrane) to the nearest A/ZH consumer, what the accurate value of proton potential decrease on H+-ATP-synthase molecule is, and if unmixed layers are present at the interface, and what the membrane profile complexity is [22],... 

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). 

Oxidative phosphorylation is ATP synthesis linked to the oxidation of NADH and FADH2 by electron transport through the respiratory chain. This occurs via a mechanism originally proposed as the chemiosmotic hypothesis. Energy liberated by electron transport is used to pump H+ ions out of the mitochondrion to create an electrochemical proton (H+) gradient. The protons flow back into the mitochondrion through the ATP synthase located in the inner mitochondrial membrane, and this drives ATP synthesis. Approximately three ATP molecules are synthesized per NADH oxidized and approximately two ATPs are synthesized per FADH2 oxidized. 

Figure 1. Localization of the major types of ion-motive ATPases in the eukaryotic cell. Na+-K+, H+-K+, and Ca2+-ATPases of P-type transport the respective cations across the plasma membrane or into sarcoplasmic (SR) or endoplasmic (ER) reticulum. H+-ATPase of V-type acidifies different types of vacuoles and vesicles allowing their secondary uptake of amino acids and amines (AA+). H+-ATPase (working as ATP synthase) of F-type (FqF,) generates ATP in the mitochondria. Modified from Pedersen and Carafoli, 1987.

At present, much attention is devoted to enzymes that utilize the energy of ATP hydrolysis for realization of energy-rich mechanics (myosin), transport (Na+,K+-ATPase, Ca2+-ATPase, chemical processes (nitrogenase), polymerases, topoisomerases, GTPases, and for creation of electrochemical gradients in biomembranes (H+-ATPase, ATP synthase ). In this section we focus on the latter process. The coupling mechanism in the nitrogenase reaction is discussed in Section 3.1. 

Inner membrane. The inner membrane is folded into numerous cristae, greatly increasing its total surface area. It contains proteins with three types of functions (1) those that carry out the oxidation reactions of the electron transport chain, (2) the ATP synthase that makes ATP in the matrix, and (3) transport proteins that allow the passage of metabolites into and out of the matrix. An electrochemical gradient of H+, which drives the ATP synthase, is established across this membrane, so the membrane must be impermeable to ions and most small charged molecules. 

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