Enzymes Adenosine triphosphatases

Similarly, specific catalysts called enzymes are important factors in determining what reactions occur at an appreciable rate in biological systems. For example, adenosine triphosphate is thermodynamically unstable in aqueous solution with respect to hydrolysis to adenosine diphosphate and inorganic phosphate. Yet this reaction proceeds very slowly in the absence of the specific enzyme adenosine triphosphatase. This combination of thermodynamic control of direction and enzyme control of rate makes possible the finely balanced system that is a hving cell. 

Schistocephalus solidus has its plerocercoid stage in an ectotherm (the fish Gasterosteus aculeatus) and its adult stage in an endoderm (a fish-eating bird) and it serves as an excellent model for the study of temperature adaptation in parasites. Walker Barrett (922,923) have studied the effect of temperature on (a) the activities of the mitochondrial enzyme adenosine triphosphatase (ATPase) and (b) the physical state of mitochondrial membranes in adult and larval S. solidus. 

Adenylate cyclase is considered as a second messenger that catalyzes the formation of cAMP (cyclic adenosine monophosphate) from ATP this results in alterations in intracellular cAMP levels that change the activity of certain enzymes—that is, enzymes that ultimately mediate many of the changes caused by the neurotransmitter. For example, there are protein kinases in the brain whose activity is dependent upon these cyclic nucleotides the presence or absence of cAMP alters the rate at which these kinases phosphorylate other proteins (using ATP as substrate). The phosphorylated products of these protein kinases are enzymes whose activity to effect certain reactions is thereby altered. One example of a reaction that is altered is the transport of cations (e.g., Na+, K+) by the enzyme adenosine triphosphatase (ATPase). 

The enzyme adenosine triphosphatase removes phosphate residues from adenosine-5 -phosphate ATP... 

On hydrolysis, particularly under the action of the enzyme adenosine triphosphatase (ATPase), ATP is converted into adenosine diphosphate (ADP) and phosphate. This process is of great importance in metabolism (see p, 246). Under ordinary physiological conditions ATP bears four negative charges, as shown above. 

Sodium pump. This expels Na ions from inside the cell by a mechanism that derives energy from ATP and involves the enzyme adenosine triphosphatase (ATPase). The carrier is linked to the transfer of K" " ions into the cell. The cardiac glycoiides (Chapter 18) act by inliibiting the Na /K" ATPase. Na" and/or CL transport processes in the kidney are inhibited by some diuretics (Chapter 14). 

Furthermore, the values used in the above calculations have been obtained from measurements made in vitro on systems of the purified substances. This is not the situation in the cell. Most important is the fact that it contains an enzyme, adenosine triphosphatase, which hydrolyses ATP into ADP and inorganic phosphate, and it is not impossible that in vivo part of the ATP formed is lost through the action of this enzyme, also this enzyme is involved in a specific way in the transformation of ATP bond energy into work. 

Various enzymes that are involved in the respiratory chain have long been demonstrable in tissue sections. It has been shown that mitochondria contain these enzymes, but otherwise these methods have yielded limited information since all cells contain the enzymes. Recently, however, electron microscopy has provided evidence concerning the mode of electrolyte and water transport in secretory epithelia. Energy-produdng enzymes (adenosine triphosphatase. glucose-6>phospha-tase. and others as well as carbonic anhydrase. occur at the cellular interfoces this fact supports the hypothesis that active transport takes place at these sites rather than diffusely through the cytoplasm... 

Contraction of muscle follows an increase of Ca " in the muscle cell as a result of nerve stimulation. This initiates processes which cause the proteins myosin and actin to be drawn together making the cell shorter and thicker. The return of the Ca " to its storage site, the sarcoplasmic reticulum, by an active pump mechanism allows the contracted muscle to relax (27). Calcium ion, also a factor in the release of acetylcholine on stimulation of nerve cells, influences the permeabiUty of cell membranes activates enzymes, such as adenosine triphosphatase (ATPase), Hpase, and some proteolytic enzymes and facihtates intestinal absorption of vitamin B 2 [68-19-9] (28). 

An in-depth study of DNA repair systems (Aravind et al., 1999a) has concluded that few, if any, repair proteins occur with identical collinear domain arrangements in all three kingdoms of life. Approximately 10 enzyme families of adenosine triphosphatases (ATPases), photolyases, helicases, and nucleases were identified that are all likely to have been present in the cenancestor. These enzymatic domains are accompanied in DNA repair proteins by numerous regulatory domains. This indicates that the domain architectures of these proteins are labile, with incremental addition and/or subtraction of domains to conserved cores to be a common phenomenon except in the most closely related species. 

The catecholamines can play an important role in the short-term regulation of plasma potassium levels. Stimulation of hepatic a-adrenoceptors will result in the release of potassium from the liver. In contrast, stimulation of (32-adrenoceptors, particularly in skeletal muscle, will lead to the uptake of potassium into this tissue. The (32-adrenoceptors are linked to the enzyme Na"", K+ adenosine triphosphatase (ATPase). Excessive stimulation of these (32-adrenoceptors may produce hypokalemia, which in turn can be a cause of cardiac arrhythmias. 

Mechanism of Action A proton pump inhibitor that selectively Inhibits the parietal cell membrane enzyme system (hydrogen-potassium adenosine triphosphatase) or proton pump. Therapeutic Effect Suppresses gastric acid secretion. Pharmacokinetics ... 

Mecfianism of Action A benzimidazole that is converted to active metabolites that irreversibly bind to and inhibit hydrogen-potassium adenosine triphosphatase, an enzyme on the surface of gastric parietal cells. Inhibits hydrogen ion transport into gastric lumen. Therapeutic Effect Increases gastric pH, reduces gastric acid production. Pharmacokinetics ... 

MEDICINAL CHEMISTRY 8.2.3 Enzyme Targets Adenosine Triphosphatase... 

Amzel, L. M., and Pedersen, P. L. (1978). Adenosine triphosphatase from rat liver mitochondria Crystallization and x-ray diffraction studies of the F -component of the enzyme. / Biol. Chem. 253, 2067-2069. 

One in vitro study on rat renal tissue homogenate showed barium weakly inhibited the sodium-potassium-adenosine triphosphatase enzyme system (Kramer et al. 1986). A second study on mouse kidney tubules showed barium chloride could depolarize the membrane and inhibit potassium transport (Volkl et al. 1987). A similar defect in cell membrane transport in humans could be responsible for the renal involvement observed in some cases of acute barium poisoning. 

Three membrane-bound adenosine triphosphatase enzymes have been characterized using Mn(II) and Gd(III) electron paramagnetic resonance (EPR) and a variety of NMR techniques. Mn(II) EPR studies of both native and partially delipidated (Na+ + K+)-ATPase from sheep kidney indicate that the enzyme binds Mn2+ at a single, catalytic site with Kq = 0.21 x 10- M. The X-band EPR spectrum of the binary Mn(II)-ATPase complex exhibits a powder line shape consisting of a broad transition with partial resolution of the 55 n nuclear hyperfine structure, as well as a broad component to the low field side of the spectrum. ATP, ADP, AMP-PNP and Pj all broaden the spectrum, whereas AMP induces a substantial narrowing of the hyperfine lines of the spectrum. 

Alkaline phosphatase, acid phosphatase, 5 -nucleotidase, monoacyl hydrolase, ribonuclease, type 1 phosphodiesterase, adenosine triphosphatase, adenyl cyclase, glycosyl transferase, esterases and disaccharidase have been biochemically or cytochemically demonstrated in the tegument of various cestodes (152, 210, 250, 374, 491, 620, 624-626, 651, 718, 763, 776, 898). Several of these enzymes - phosphatases, 5 -nucleotidase and phosphodiesterase - probably have a digestive and/or absorptive function but the role of the others is uncertain. 

Some of the enzymes of the MFGM, such as 5 -nucleotidase, adenosine triphosphatase and phosphodiesterase I, are known to be enriched in plasma membranes. However, other enzymes found in the MFGM are known constituents of intracellular membranes or are cytosolic. Why some of these are present in the MFGM remains to be explained some may possibly originate from material entrained in cytoplasmic crescents and therefore are not true MFGM constituents. Perhaps some enzymes become... 

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