Adenosine triphosphatase, reaction

Bagshaw, C. R., and Trentham, D. R. (1974). The characterization of myosin-product complexes and of product-release steps during the magnesium ion-dependent adenosine triphosphatase reaction. Biochem.J. 141, 331-349. 

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. 

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). 

Figure 10-14 Ion and fluid movement in the nonpigmented ciliary epithelium. Na+ enters the nonpigmented ciliary epithelium from the stromal side either by diffusion or by NaVH+ exchange. Na+, the main cation involved in aqueous formation, is transported extraceUularly into the lateral intercellular channel by a Na+-K+-adenosine triphosphatase-dependent transport system. HC03 forms from the hydration of CO2, a reaction catalyzed by carbonic anhydrase. HC03", the major anion involved in aqueous formation, balances a portion of the Na+ being transported into the lateral intercellular channel. Cl" enters the intercellular space by a mechanism that is not understood. This movement of ions into the lateral intercellular space creates a hypertonic fluid, and water enters by osmosis. Because of the restriction on the stromal side of the channel, the newly formed fluid moves toward the posterior chamber. A rapid diffusional exchange of CO2 allows for its movement into the posterior chamber. (Adapted from Cole DF. Secretion of aqueous humor. Exp Eye Res 1977 25(suppl) l6l-176.)...

Until fairly recently there was no convincing evidence that inactivation of any one enzyme or group of related enzymes has a causal relationship to the development of vesicant-induced burns. However, most of the earlier work concentrated on enzymes restricted to carbohydrate metabolism and many of the enzymes known today were undiscovered at the time of these early observations, e.g. adenosine triphosphatase (Jprgensen et al., 1971). Even if it is assumed that vesicants act by alkylating or disrupting a single enzyme or group of similar enzymes, since the reaction time, i.e. exposure to full alkylation, is so short, therapy would need to be directed towards prophylaxis rather than treatment. 

The magnitudes of entropies of activation have provided valuable information regarding the details of the interactions between enzymes and substrates. The process of muscular contraction involves an interaction between the muscle enzyme myosin and adenosine triphosphate (ATP). Myosin is an enzyme which catalyzes the hydrolysis of ATP, a process which we have seen (p. 246) to be more exergonic than is the case for many other phosphates, and this hydrolysis contributes energy for contraction. Because of its catalytic action, myosin is also referred to as adenosine triphosphatase (ATP-ase). When the activated complex is formed from ATP ase and its substrate ATP, the entropy of activation is about 41 cal K" mol under approximately normal physiological conditions. We saw on p. 400, on the basis of a very simple electrostatic theory of AS values for ionic reactions in aqueous solution, that there will be a positive contribution of about 10 cal mol for each unit of the product [ [ % ( The long myosin molecules bear a series of positive... 

Later, Meyerhof showed that the addition of adenosine triphosphatase (ATPase, an enzyme that hydrolyzes ATP to yield ADP and Pj) to the reaction mixture stimulates the evolution of CO2. Explain this result. 

Bacterial and Fungal Glycoproteins. — No evidence has been found to substantiate the claim for the presence of covalently-bound carbohydrate in energy-transducing adenosine triphosphatases of Escherichia coli. Incomplete removal of sodium dodecyl sulphate (SDS) after SDS-electrophoresis of the protein results in a positive reaction with the periodate-Schiff staining method. 

Adenosine 5 - phosphate (ADP) is formed either by adding a second phosphate to AMP (see Adenylate kinaseX or by removal of a phosphate from ATP the latter conversion may be cataly by adenosine triphosphatases (EC 3.6.13), or by kinases which trarrsfer the phosphate to another organic molecule. The energy stored in the anhydride bond of ADP is made available by the reaction 2ADP ATP-H AMP, catalysed by adenylate kinase. ADP is the phosphate acceptor (i.e. it is converted into ATP) in Substrate-level phosphorylation (see). Oxidative phosphorylation (see) and Photophosphorylation (see). 

The Pd-catalyzed cascade Heck reaction of 5-methylenecycloheptene precursor 108 was utilized to construct the scopadulan ring system and chiral centers at C9 and C12 of the bicyclo-octane 109 and 110 for the first total synthesis of scopadulcic acid B, which is a powerful inhibitors of H+, K+ -adenosine triphosphatase and have potential for the treatment of peptic ulcers, gastritis, and esophagitis. (Scheme 55) (103,104). 

The circulating and intracellular calcium participate in many vital metabolic reactions muscle contraction, neuromuscular excitability, blood coagulation, adenosine triphosphatase activations, and other reactions. 

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