Adenylate kinase reaction

The problem can be solved using the equilibrium expression for the adenylate kinase reaction ... 

Because many cells maintain ATP, ADP, and AMP concentrations at or near the mass action ratio of the adenylate kinase reaction, the cellular content of this enzyme is often quite high. A consequence of such abundance is that, even after extensive purification, many proteins and enzymes contain traces of adenylate kinase activity. The presence of this kinase can confound the quantitative analysis of processes that either require ADP or are carried out in the presence of both ATP and AMP. Furthermore, the equilibrium of any reaction producing ADP may be altered if adenylate kinase activity is present. To minimize the effect of adenylate kinase, one can utilize the bisubstrate geometrical analogues Ap4A and ApsA to occupy simultaneously both substrate binding pockets of this kinase . Typical inhibitory concentrations are 0.4 and 0.2 mM, respectively. Of course, as is the case for the use of any inhibitor, one must always determine whether Ap4A or ApsA has a direct effect on a particular reaction under examination. For example. Powers et al studied the effect of a series of o ,co-di-(adenosine 5 )-polyphosphates (e.g., ApnA, where n =... 

In order to obtain a more intuitive insight into the mechanism of thermodynamic buffering we calculated the effects of thermodynamic buffering on the entropy production of the system. The entropy production of oxidative phosphorylation with an attached load is given in equation (8). A convenient way to introduce the contribution of the adenylate kinase reaction to this system is to consider L/ as an overall load conductance embracing the effects of the adenylate kinase reaction as well as the effects of the true extrinsic load conductance of the irreversible ATP utilizing... 

Fig. 7. Reaction scheme for oxidative phosphorylation with load and adenylate kinase. LP, conductance of phosphorylation Lt, load conductance and Z.Ak conductance of adenylate kinase reaction. For details see text.

For this simulation we took again the model as was used for the previous calculations of the steady state adenine nucleotides, namely, oxidative phosphorylation with an attached load plus the adenylate kinase reaction. The only modification introduced into this scheme now was to consider a fluctuating rather than a constant load conductance. In order to arrive at a realistic description, a stationary process with a Lorentzian... 

Relative concentrations of ATP, ADP, and AMP as a function of the adenylate energy charge. The adenylate kinase reaction was assumed to be at equilibrium, and a value of 1.2 was used for its effective equilibrium constant in the direction shown in the equation in the text. 

Savina, 1992). Moreover, as these authors and Silkina (1990) note, the white muscle of sluggish fish often exceeds that of highly active species in the characteristics described above. This thesis is supported by the data of Emeretli (1990) on LDH and ATPase activity measured in the muscles of horse-mackerel and scorpion fish (Figure 17). The creatine- and adenylate kinase reactions in the white muscle of sluggish fish appear to proceed at a greater rate than in more... 

During glycolysis, glyceraldehyde 3-phosphate is converted to 1,3-bisphosphoglycerate and the equilibrium of the adenylate kinase reaction lies in favor of 3-phosphoglycerate, so the metabolites are drawn through the pathway of reactions. 

Following its formation, the AMP can have at least two fates. These are illustrated in Figure 10.11, where the reactions are shown. Reaction 2 of this figure is the deamination of AMP to IMP discussed above, a reaction catalyzed by AMP deaminase. Reaction 4 is the adenylate kinase reaction in which ATP is involved and two ADP molecules are formed. This reaction was also discussed above. 

M. soehngenii lacks acetate kinase, phosphotransacetylase, and PPi AMP- or PPi ADP-phosphotransferase[241], but possesses high levels of adenylate kinase (Reaction 37) and pyrophosphatase (Reaction 38) activities ... 

The adenylate pool in a culture of lymphosarcoma cells was found to consist of 10 M ATP, 3 X M ADP, and 10 M AMP. (a) Calculate the energy charge of the cells, (b) Assuming that the adenine nucleotides are at equilibrium for the adenylate kinase reaction, calculate the of the reaction. 

During muscle contraction AMP deaminase activity increases. Nucleoside triphosphates are negative modulators, whereas nucleoside di- and monophosphates are positive modulators of the enzyme. The increased AMP deaminase activity prevents accumulation of AMP so that the adenylate kinase reaction favors the formation of ATP ADP - -ADP ATP -k AMP. 

F. 22.13. Changes in ATP, ADP, and AMP concentrations in skeletal muscle during exercise. The concentration of ATP decreases by only approximately 20% during exercise, and the concentration of ADP rises. The concentration of AMP, produced by the adenylate kinase reaction, increases manyfold and serves as a sensitive indicator of decreasing ATP levels. 

AMP is a much more sensitive indicator of low energy levels because of the adenylate kinase reaction. The [AMP] to [ATP] ratio is proportional to the square of the [ADP] to [ATP] ratio, so a fivefold change in ADP levels corresponds to a 25-fold change in AMP levels. 

Fig. 47.5. Regulation of fatly acyl CoA entry into muscle mitochondria. 1. Acetyl CoA carboxylase-2 (ACC-2) converts acetyl CoA to malonyl CoA, which inhibits carnitine pahnitoyl transferase I (CPT-I), thereby blocking fatty acyl CoA entry into the mitochondria. 2. However, as energy levels drop, AMP levels rise because of the activity of the adenylate kinase reaction. 3. The increase in AMP levels activates the AMP-activated protein kinase (AMP-PK), which phosphorylates and inactivates ACC-2, and also phosphorylates and activates malonyl CoA decarboxylase (MCoADC). The decarboxylase converts malonyl CoA to acetyl CoA, thereby relieving the inhibition of CPT-1, and allowing fatty acyl CoA entry into the mitochondria. This allows the muscle to generate ATP via the oxidation of fatty acids.

F. 47.9. Activation of muscle glycogenolysis and glycolysis by AMP. As muscle contracts, ATP is converted to ADP and Pj. In the adenylate kinase reaction, two ADP react to form ATP and AMP. The ATP is used for contraction. As AMP accumulates, it activates glycogenolysis and glycolysis. 

At very low values of EC, when AMP is elevated it is deaminated via AMP deaminase to inosine monophosphate (IMP). This further displaces the adenylate kinase reaction in the direction of ATP synthesis. The IMP is dephosphorylated by nucleotide phosphatase, and the inosine is phosphorylyzed via purine nucleotide phosphorylase, releasing hypoxanthine and ribose 1-phosphate. The latter is metabolized via the pentose phosphate pathway, and most of the carbon atoms enter glycolysis. Because this course of events depletes the overall adenine nucleotide pool, and hence the scope for ATP production in the longer term, it represents a metabolic last ditch stand by the cell to extract energy even from the energy currency itself ... 

Quantitatively, the most efficient method for producing ATP is by aerobic metabolism by oxidative phosphorylation (Chapters 16 and 33). However, ATP can also be produced albeit less efficiently under anaerobic conditions by substrate-level phosphorylation, from phosphocreatine, and by the adenylate kinase reaction. Although less efficient, the ability to produce ATP without oxygen can be of life-saving importance. 

Figure 10.5 Formation of ATP from two molecules of ADP by the adenylate kinase reaction.

When ATP has been hydrolysed to provide energy for muscle contraction, ADP accumulates. Remember that ADP stiU has a source of imtapped energy in the a-phosphoanhydride bond (Fig. 10.1). With ingenious hiochemical resourcefulness, this energy is salvaged when two molecules of ADP form ATP under anaerobic conditions using the adenylate kinase reaction (previously known as myokinase) (Fig. 10.5). 

Biosynthesis of ATP. ATP is the irrunediate product of all cellular processes leading to the chemical storage of energy. It is biosynthesized by phosphorylation of ADP in the course of Substrate phosphorylation (see). Oxidative phosphorylation (see) and non-cyclic Photophosphorylation (see) in plants. Energy in the form of a third phosphate may also be transferred to ADP from other high-energy phosphates, such as creatine phosphate (see Creatine) or other nucleoside triphosphates, or in the adenylate kinase reaction. 

Because the majority of the known nucleoside monophosphate kinase reactions require an adenosine phosphate as one of the substrates, they may be classified as those which (1) are specific for adenylate as a phosphoryl acceptor, and (2) as those which require ATP as a phosphoryl donor. (The adenylate kinase reaction obviously may be placed in either category.) There may exist an additional class of nucleoside monophosphate kinase reactions in which adenosine phosphates do not participate however, such enzyme activity has not been unequivocally demonstrated. 

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