ATP synthase in mitochondria

Liu et al. have described the solution structure of a mutant of the homodimer protein transcription factor 1, TFl. The dimeric core, consisting of the N-terminal helices and beta sheets, is more tightly packed than wire type, and this might be responsible for its increased thermal stability. Also, Gordon-Smith et al reported the dimeric structure of a C-terminal fragment of Bovine IFl, which inhibits the hydrol) ic action of the FIFO ATP synthase in mitochondria under anaerobic conditions. Most imusually, the molecule forms an anti-parallel coiled-coil in which three histidine residues occupy key positions at the dimer interface. ... 

The thylakoid membrane enzyme that couples ATP synthesis to the flow of protons down their electrochemical gradient is called the chloroplast ATP synthase (see Fig. 10). This enzyme has remarkable similarities to ATP synthases in mitochondria and certain bacteria. For example, the subunits of the chloroplast ATP synthase have 76% amino acid sequence identity with the subunits of the ATP synthase of the bacterium E. coli. 

The flow of electrons occurs in a similar manner from the excited pigment to cytochromes, quinones, pheophytins, ferridoxins, etc. The ATP synthase in the mitochondria of a bacterial system resembles that of the chloroplast—chloroplast proton translocating ATP synthase [37]. 

The reaction catalyzed by F-type ATPases is reversible, so a proton gradient can supply the energy to drive the reverse reaction, ATP synthesis (Fig. 11-40). When functioning in this direction, the F-type ATPases are more appropriately named ATP synthases. ATP synthases are central to ATP production in mitochondria during oxidative phosphorylation and in chloroplasts during photophosphorylation, as well as in eubacteria and archaebacteria. The proton gradient needed to drive ATP synthesis is produced by other types of proton pumps powered by substrate oxidation or sunlight. As noted above, we return to a detailed description of these processes in Chapter 19. 

Chemiosmotic theory readily explains the dependence of electron transfer on ATP synthesis in mitochondria. When the flow of protons into the matrix through the proton channel of ATP synthase is blocked (with oligomycin, for example), no path exists for the return of protons to the matrix, and the continued extrusion of protons driven by the activity of the respiratory chain generates a large proton gradient. The proton-motive force builds up until the cost (free energy) of pumping... 

ATP Synthase. The motor enzyme, FqFi-ATP synthase, of mitochondria uses the proton-motive force across the mitochondrial membranes to make ATP from ADP and Pi (H2POJ) [29-32]. It does so under cellular conditions that favor the hydrolysis reaction by a factor of 2 x 10 . As a result of the activity of the FqFi-ATP synthase, the concentration ratio (ATP ADP/Pi) is close to unity in mitochondria [33]. This remarkable property is based on the essential difference between an ordinary enz3Tne, which increases the rate of reaction without shifting the equilibrium, and a catal3dic motor like FqFi-ATP... 

TBTC has been shown to decrease ATP levels in rodent cells and to inhibit ATP synthesis in rat liver mitochondria. It has been shown to bind to the Fo portion of F-ATP synthase in bacteria and to inhibit the ATP synthase of bovine heart sub-mitochondrial particles. Thus, TBTC may be decreasing ATP levels in NK cells and the lowered ATP levels may be, at least in part, responsible for the loss of lytic activity. ATP levels were measured in TBTC-exposed NK cells and the percentage decrease in ATP levels, as compared to control cells, after various TBTC-exposures is given in Table 4.5.1. ... 

The mechanism of ATP in chloroplasts closely resembles the process that takes place in mitochondria. The structure of the ATP synthase in chloroplasts is similar to that in mitochondria. 

Reflect and Apply What are the evolutionary implications of the similarity in structure and function of ATP synthase in chloroplasts and mitochondria ... 

Finally, Ernster and coworkers [187] report that Mn(II) prevents Ca(II)-inhibition of ATP synthase in brain mitochondria, adding evidence that a disproportionate number of brain enzymes may be manganese rather than magnesium-specific. 

The carbodiimide 2, in contrast to 1, displays biological effects in vitro that may be responsible for the activity in vivo. In particular, 2 is a potent inhibitor of mitochondrial ATP synthesis at the ATP synthase level in vitro [10,18] and in vivo [22-24]. Radiolabeling experiments with [ " Cj-2 confirm that it covalently binds to the 8-kDa proteolipid of Fo of the mitochondrial ATP synthase in isolated mitochondria from insect flight muscle and rat liver. Because binding is competitively blocked by DCCD and partly inhibited by venturicidin it has been concluded that 2 and the classical and well-studied inhibitor DCCD [25] share the same binding site on the Fo proteolipid [26]. 

Figure 8.28. Stereo view of ATP synthase from mitochondria of yeast, neutral residues in light gray, aromatics in black, other hydrophobics in gray, and charged residues in white. (A) Rotating wheel of Fo-motor, side view. (B) Rotating wheel of Fo-motor, top view (cytoplasmic side). (C) Rotating wheel of Fo-motor, bottom view (matrix side). (Prepared using the crystallographic results of Stock et al. as obtained from the Protein Data Bank, Structure File IQOl.)...

Abbreviations used Fi Fq, H+-translocating ATP synthases from mitochondria, E. coli plasma membranes, plasma membranes of the thermophilic bacterium PS3 and chloroplast membranes CFi, chloroplast coupling factor one ATPase, CFiE. C. 3.6.1.3 SDS-PAGE, polyacrylamide gel electrophoresis in the presence of sodium dodecyl... 

The mitochondrial complex that carries out ATP synthesis is called ATP synthase or sometimes FjFo-ATPase (for the reverse reaction it catalyzes). ATP synthase was observed in early electron micrographs of submitochondrial particles (prepared by sonication of inner membrane preparations) as round, 8.5-nm-diameter projections or particles on the inner membrane (Figure 21.23). In micrographs of native mitochondria, the projections appear on the matrixfacing surface of the inner membrane. Mild agitation removes the particles from isolated membrane preparations, and the isolated spherical particles catalyze ATP hydrolysis, the reverse reaction of the ATP synthase. Stripped of these particles, the membranes can still carry out electron transfer but cannot synthesize ATP. In one of the first reconstitution experiments with membrane proteins, Efraim Racker showed that adding the particles back to stripped membranes restored electron transfer-dependent ATP synthesis. 

F-ATPases (including the H+- or Na+-translocating subfamilies F-type, V-type and A-type ATPase) are found in eukaryotic mitochondria and chloroplasts, in bacteria and in Archaea. As multi-subunit complexes with three to 13 dissimilar subunits, they are embedded in the membrane and involved in primary energy conversion. Although extensively studied at the molecular level, the F-ATPases will not be discussed here in detail, since their main function is not the uptake of nutrients but the synthesis of ATP ( ATP synthase ) [127-130]. For example, synthesis of ATP is mediated by bacterial F-type ATPases when protons flow through the complex down the proton electrochemical gradient. Operating in the opposite direction, the ATPases pump 3 4 H+ and/or 3Na+ out of the cell per ATP hydrolysed. 

Transport of protons from the intermembrane space into the matrix across the inner membrane of the mitochondria occurs via the ATP synthase complex (Fo-Ei), which generates ATP. Somewhat surprisingly, it was discovered that proteins that transport protons back into the matrix, but without generating ATP, do, in fact, exist and are... 

The activity of the ATP synthase is low in comparison with that in mitochondria from other tissues i.e. mitochondria in brown adipose tissue can generate very little ATP. 

The synthesis of ATP is catalyzed by the enzyme ATP synthase (or FiFq-ATP synthase) the Fj portion of this enzyme was first isolated by Racker and coworkers in 1960 [4]. ATP synthase is present in abundance in the membranes of animal mitochondria, plant chloroplasts, bacteria and other organisms. ATP synthesized by our ATP synthase is transported out of mitochondria and used for the function of muscle, brain, nerve, liver and other tissues, for active transport, and for synthesizing myriad compounds needed by the cell. Since the pool of adenosine phosphates in the body is limited, the use of ATP must be continually compensated by its synthesis, and an active person synthesizes his own body weight of ATP every day. The synthesis of ATP is the most prevalent chemical reaction in the body [5]. This is indeed a very important reaction. How exactly does it occur ... 

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