Adenosine diphosphate , enzyme reactions

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. 

In the preceding sections the conversion of purines and purine nucleosides to purine nucleoside monophosphates has been discussed. The monophosphates of adenosine and guanosine must be converted to their di- and triphosphates for polymerization to RNA, for reduction to 2 -deoxyribonucleoside diphosphates, and for the many other reactions in which they take part. Adenosine triphosphate is produced by oxidative phosphorylation and by transfer of phosphate from 1,3-diphosphoglycerate and phosphopyruvate to adenosine diphosphate. A series of transphosphorylations distributes phosphate from adenosine triphosphate to all of the other nucleotides. Two classes of enzymes, termed nucleoside mono-phosphokinases and nucleoside diphosphokinases, catalyse the formation of the nucleoside di- and triphosphates by the transfer of the terminal phosphoryl group from adenosine triphosphate. Muscle adenylate kinase (myokinase)... 

The sirtuins (silent information regulator 2-related proteins class III HDACs) form a specific class of histone deacetylases. First, they do not share any sequence or structural homology with the other HDACs. Second, they do not require zinc for activity, but rather use the oxidized form of nicotinamide adenine dinucleotide (NAD ) as cofactor. The reaction catalyzed by these enzymes is the conversion of histones acetylated at specific lysine residues into deacetylated histones, the other products of the reaction being nicotinamide and the metabolite 2 -0-acetyl-adenosine diphosphate ribose (OAADPR) [51, 52]. As HATs and other HDACs, sirtuins not only use acetylated histones as substrates but can also deacetylate other proteins. Intriguingly, some sirtuins do not display any deacetylase activity but act as ADP-ribosyl transferases. 

This enzyme [EC 2.7.7.28], also known as NDP-hexose pyrophosphorylase, catalyzes the reaction of a nucleoside triphosphate with a hexose 1-phosphate to produce a NDP-hexose and pyrophosphate (or, diphosphate). In the reverse reaction the NDP-hexose can be, in decreasing order of activity, guanosine, inosine, and adenosine diphosphate hexoses in which the sugar is either glucose or mannose. 

This enzyme [EC 2.4.2.30] (also referred to as NAD+ ADP-ribosyltransferase, poly(ADP) polymerase, poly-(adenosine diphosphate ribose) polymerase, and ADP-ribosyltransferase (polymerizing)) catalyzes the reaction of NAD+ with [ADP-D-ribosyl] to produce nicotinamide and [ADP-D-ribosyl]( + i). The ADP-d-ribosyl group of NAD+ is transferred to an acceptor carboxyl group on a histone or on the enzyme itself, and further ADP-ribosyl groups are transferred to the 2 -position of the terminal adenosine moiety, building up a polymer with an average chain length of twenty to thirty units. 

Creatine phosphokinase activity has been reported to be minimally inhibited by hemolysis. Hemoglobin concentrations of 1.25 g/100 ml inhibit 5% and 2.5 g/100 ml, 12% (N5). However, in methods utilizing adenosine diphosphate in the reaction mixture, hemolysates containing 100 mg of hemoglobin per 100 ml may have apparent activities of 5-100 units/liter. The activity is presumably related to adenylate kinase in the erythrocyte (S33). In methods utilizing adenosine diphosphate in a coupled enzyme reaction with hexokinase and glucose-6-phosphatase, the inhibitory effect can be eliminated by adding sufficient adenosine mono-... 

Kinetic studies of the nucleotide analogs, y-phenylpropyl di- and triphosphate, have been undertaken to define the role of the adenosine residue in the chemical and enzymic reactions of adenosine triphosphate. A catalytic function associated with binding of metal ions at the adenine nitrogens has been ascribed to the adenosine moiety in phosphate transfer reactions in which adenosine di- or triphosphates function as the phosphate source109-"2. The pH-rate profile (Fig. 6) for the hydrolysis of -y-phenylpropyl diphosphate... 

As an example of enzyme action, look in Figure 24.11 at the enzyme hexose kinase, which catalyzes the reaction of adenosine triphosphate (ATP) with glucose to yield glucose-6-phosphate and adenosine diphosphate (ADP). The enzyme first binds a molecule of ATP cofactor at a position near the active site, and glucose then bonds to the active site with its C6 hydroxyl group held rigidly in position next to the ATP molecule. Reaction ensues, and the two products are released from the enzyme. 

To demonstrate polymerase activity in a model cell, Chakrabarti et al. [79] encapsulated polynucleotide phosphorylase in vesicles composed of dimyris-toylphosphatidylcholine (DMPC). This enzyme can produce RNA from nucleoside diphosphates such as adenosine diphosphate (ADP) and does not require a template, so it has proven useful for initial studies of encapsulated polymerase activity (Fig. 10a). Furthermore, DMPC liposomes are sufficiently permeable so that 5-10 ADP molecules per second enter each vesicle. Under these conditions, measurable amounts of RNA in the form of polyadenylic acid were synthesized and accumulated in the vesicles after several days incubation. The enzyme-catalyzed reaction could be carried out in the presence of a protease external to the membrane, demonstrating that the vesicle membrane protected the encapsulated enzyme from hydrolytic degradation. Similar behavior has been observed with monocarboxylic acid vesicles [80], and it follows that complex phospholipids are not required for an encapsulated polymerase system to function. 

Figure 29. Z-scheme of the photoinduced electron-transfer and dark enzymatic reactions operating in the photosynthesis of green plants. Mn = Mn-containing enzyme complex catalyzing water oxidation and O2 evolution Chi a and Chi b = photoactivated primary electron acceptors in photosystems I and II, respectively A and I = primary electron donors in photosystems I and II, respectively ADP = adenosine diphosphate ATP = adenosine triphosphate.

Enzymes are highly specific and usually catalyze only one type of reaction. Some enzymes show absolute specificity. For example, pyruvate kinase catalyzes the transfer of a phosphate group only from phosphoenolpyruvate to adenosine diphosphate during glycolysis (Chapter 13). Examples of enzymes that show less specificity are ... 

The reaction is essentially irreversible under physiological conditions and is a major regulatory step of glycolysis. PFK-1 is an inducible, highly regulated, allosteric enzyme. In its active form, muscle PFK-1 is a homotetramer (M.W. 320,000) that requires K+ or NH4, the latter of which lowers Km for both substrates. When adenosine triphosphate (ATP) levels are low during very active muscle contraction, PFK activity is modulated positively despite low concentration of fructose-6-phosphate. Allosteric activators of muscle PFK-1 include adenosine monophosphate (AMP), adenosine diphosphate (ADP), fructose-6-phosphate, and inorganic phosphate (Pi) inactivators are citrate, fatty acids, and ATP. 

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