ATPases generation

The charge inside the cell is negative relative to the outside. The actual membrane potential A W is somewhat smaller than AE because of nonspecific cation leakage across membranes. In bacteria, an ATPase generates a potent proton gradient across plasma membranes, the pH outside being lower than that inside the cell. In some bacterial cultures, pH difference may be as high as 2 (alkaline inside). 

Na extrusion from plant cells is powered by the operation of the plasma membrane H -ATPase generating an electrochemical gradient that allows plasma membrane Na /H antiporters to couple the passive movement of inside the cells, along its electrochemical potential, to the active extrusion of Na [21]. Recently, AtSOSl from Arahidopsis thaliana has been shown to encode a plasma membrane Na /H antiport with significant sequence similarity to plasma membrane Na /H antiporters from bacteria and fungi [32]. The overexpression of SOSl improved the salt tolerance of Aro-hidopsis, demonstrating that improved salt tolerance can be attained by limiting Na accumulation in plant cells [33] (Table 10.1). 

Cox and Henick-Kling (1989, 1990) reported activity similar to that of a proton pump and ATPase in the formation of ATP during MLF. L-malate enters the cell through the action of a specific transport enzyme and is decarboxylated as described previously. To prevent proton accumulation and, eventually, cell death, it is necessary to export them continually. This is accomplished by transport (symport) of L-lactate along with a a single proton. Repeated proton translocation creates a protonmotive force, or delta-pH (A-pH), across the membrane. Reentry of protons through membrane-associated ATPase generates ATP. Theoretically, one ATP is... 

The proton pump is the gastric H,K-ATPase, which secretes hydronium ions, H30+, in exchange forK+ into the secretory canaliculus generating a pH of <1.0 in... 

Vacuolar-type proton translocating ATPase is a heter-eomeric protein complex, which appears to translocate two protons across the vesicle membrane for each ATP molecule that is hydrolyzed, generating chemical (ApH) and electrical (A ) gradients. Although the ATPases present on different classes of intracellular vesicle have... 

Synaptic vesicles isolated from brain exhibit four distinct vesicular neurotransmitter transport activities one for monoamines, a second for acetylcholine, a third for the inhibitory neurotransmitters GABA and glycine, and a fourth for glutamate [1], Unlike Na+-dependent plasma membrane transporters, the vesicular activities couple to a proton electrochemical gradient (A. lh+) across the vesicle membrane generated by the vacuolar H+-ATPase ( vacuolar type proton translocating ATPase). Although all of the vesicular transport systems rely on ApH+, the relative dependence on the chemical and electrical components varies (Fig. 1). The... 

In summary, therefore, solution and fiber biochemistry have provided some idea about how ATP is used by actomyosin to generate force. Currently, it seems most likely that phosphate release, and also an isomerization between two AM.ADP.Pj states, are closely linked to force generation in muscle. ATP binds rapidly to actomyosin (A.M.) and is subsequently rapidly hydrolyzed by myosin/actomyosin. There is also a rapid equilibrium between M. ADP.Pj and A.M.ADP.Pj (this can also be seen in fibers from mechanical measurements at low ionic strength). The rate limiting step in the ATPase cycle is therefore likely to be release of Pj from A.M.ADP.Pj, in fibers as well as in solution, and this supports the idea that phosphate release is associated with force generation in muscle. 

The ATP in fatigued muscle is, however, well above the K , for actomyosin ATPase activity (Naiminga and Mommaerts, 1960). This indicates that the decrease in force generation is not related to a lack of ATP for crossbridge formation but... 

Fig. 3-4 Electron transport process schematic, showing coupled series of oxidation-reduction reactions that terminate with the reduction of molecular oxygen to water. The three molecules of ATP shown are generated by an enzyme called ATPase which is located in the cell membrane and forms ATP from a proton gradient created across the membrane.

Heavy meromyosin (HMM molecular mass about 340 kDa) is a soluble protein that has both a fibrous portion and a globular portion (Figure 49-4). It exhibits ATPase activity and binds to F-actin. Digestion of HMM with papain generates two subfragments, S-1 and S-2. The S-2 fragment is fibrous in character, has no ATPase activity, and does not bind to F-actin. 

The T2 site also became protected from tryptic hydrolysis after phosphorylation of the native or solubilized sarcoplasmic reticulum vesicles with inorganic phosphate in a calcium free medium in the presence of dimethylsulfoxide or glycerol [121,252]. Under these conditions the Ca -ATPase is converted into a covalent E2-P intermediate, that is analogous in conformation to the E2V intermediate formed in the presence of vanadate. In contrast to this, the T2 site in the stable phosphorylated Ca2E P intermediate generated by the reaction of the Ca -ATPase with chro-mium-ATP in the presence of Ca [178,253] was fully exposed to trypsin, just as it was in the nonphosphorylated Ca2Ei form. Therefore the phosphorylated intermediates show the same sensitivity to trypsin at the T2 site as the corresponding nonphosphorylated enzyme forms. 

A first-generation model for the tertiary structure of the -ATPase... 

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