Functions of ATP

In all of the reactions in which ATP is utilized, what is observed overall is hydrolysis of ATP to ADP and phosphate. However, as discussed below, although this is the overall reaction, simple hydrolysis of ATP does not achieve any useful result it is the intermediate steps in the reaction of ATP + H O — ADP + phosphate that are important. 

As discussed in section 2.1.1, the equilibrium of an endothermic reaction A + B C + D lies well to the left unless there is an input of energy. The hydrolysis of ATP is 

Such linkage between two apparently unrelated reactions can easily be achieved in enzyme-catalysed reactions there are three possible mechanisms  

Not only is the hydrolysis of ATP to ADP and phosphate an exothermic reaction, but the concentration of ATP in cells is always very much higher than that of ADP (the ratio of ATP to ADP is about 500 1), again ensuring that the reaction will indeed proceed in the direction of ATP hydrolysis. Furthermore, the concentration of ADP 

Compounds that are lipid soluble will diffuse freely across cell membranes, as they can dissolve in the lipid of the membrane - this is passive diffusion. Hydrophilic compounds require a transport protein in order to cross the lipid membrane - this is facilitated or carrier-mediated diffusion. Neither passive nor facilitated diffusion alone can lead to the concentration of the transported material being greater inside the cell than outside. 

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THE MANY FUNCTIONS OF ATP-DEPENDENT CHROMATIN REMODELLING FACTORS... 

Tsukiyama, T. (2002) The in vivo functions of ATP-dependent chromatin-remodelling factors. Nat. Rev. Mol. Cell Biol. 3, 422-429. 

ATP, adenosine triphosphate, provides the currency unit for energy in biochemical reactions (see Section 15.1.1) and is simply a triphosphate variant of a standard RNA nucleotide. It is, of course, the biosynthetic precursor for adenine-based units in RNA (see Section 14.2.5). As we have already seen (see Box 7.25), the functions of ATP can be related to hydrolytic reactions in the triphosphate (anhydride) part of the molecule. 

This phosphorylation step is achieved by reaction of the 6-hydroxyl with the anhydride ATP, during which process ATP is converted into ADP. This process is driven by the energy contained in the anhydride function of ATP, and represents an expenditure of energy to get the metabolic process started, though... 

Additional information <2, 3> (<2> the concentration of glutamate which yields half-maximal activity is 33 mM for y-glutamyl kinase DHPr, and 37mM for y-glutamyl kinase w-l-, no typical Michealis-menten kinetics [2] <3> plots of the enzyme activity as a function of ATP concentration are non-hyperbolic [5]) [2, 5]... 

Ca2+ cycling into and out of the mitochondria leads to NAD depletion and a fall in ATP. The entry of Ca2+ into the mitochondria dissipates the potential difference across the mitochondrial membranes and so inhibits the function of ATP synthase, which relies on the charge difference across the membrane (Fig. 6.13 and 7.60). Export of Ca2+ from the mitochondrial matrix may occur and be stimulated by some chemicals. However, this will lead to repeated cycling, which damages the membrane and further compromises ATP synthesis. The export of Ca2+ also uses up ATP as a result of the Ca2+ ATPases involved. Hence ATP levels fall. 

The appearance of I80 in inorganic phosphate verifies that the function of ATP in the reaction is essentially the dehydration of bicarbonate. 

Because the primary metabolic function of ATP is to drive reactions, biochemists often refer to it as a "high-energy molecule" or an "energy storehouse." These terms don t mean that ATP is somehow different from other compounds they mean only that ATP releases a large amount of energy when its P-O-P (phosphoric anhydride) bonds are broken and a phosphate group is transferred. 

ATP is not a high-energy compound in comparison with many other biological compounds. The functions of ATP depend on its having a AG value for hydrolysis that is intermediate in value compared with AG values for hydrolysis of other phosphate esters. Thus, ATP and ADP can act as a donor-acceptor pair for phosphoryl-group transfer. In many cases the free energy of ATP hydrolysis is used to support reactions that would otherwise be thermodynamically unfavorable. This usually occurs via phosphorylation of one of the reactants in an otherwise unfavorable reaction. 

Figure 2.5 Upper panel shows change in pH as a function of ATP, ADP, and PI concentrations predicted by Equation (2.20). The lower panel illustrates Ar G ° and ArG as function of reactant concentration. The solid line represents predictions for the closed system, where pH varies with reactant concentrations dashed line represents a system with pH clamped.

Figure4.11 Fits to kinetic data from [63] on the forward operation of liver enzyme. Measured flux in arbitrary units was obtained from Figures 1,2, 5, 6,13, and 14 of [63], For all cases the product (CIT and COASH) concentrations are zero and total substrate and inhibitor concentrations are indicated in the figure. Data obtained with no inhibitors present are plotted in A and B. In C the relative activity (normalized to its maximum) of the enzyme is plotted as functions of [ATP], [ADP], and [AMP] measured at [ACCOA] = 11 TM and [OAA] = 1.9 uM. D. The measured flux is plotted as a function of [ACCOA] at [OAA] = 34 qM with ATP, ADP, and AMP present as indicated in the figure. In E the relative activity of the enzyme is plotted as functions of [ATP] at [Mg2+] = 0 mM (shaded circles), 0.5 mM (shaded triangles), 1.0 mM (shaded squares), 2.0 mM (open circles), and 4.0 mM (diamonds). In F relative activity is plotted as a function of pH. Substrate concentrations are [ACCOA] = 21 qM and [OAA] = 8.6 qM. All data were obtained at 25 °C. pH is fixed a 7.4 for A. Model fits are plotted as solid lines.

Figure 7.14 Model prediction of ADP concentration and inorganic phosphate concentration in cytoplasm as a function of ATP hydrolysis rate in the healthy subjects. Data are from [106] solid lines are prediction of model of Wu et al. [213],...

Histone chaperones bind histones and facilitate their proper deposition onto DNA by preventing nonspecific histone-DNA interactions (17). Two major histone chaperones are CAF-1 and NAP-1. CAF-1 localizes to the replication fork by binding PCNA and facilitates the deposition of histones H3 and H4 onto the newly synthesized DNA strands (18,19). Subsequently, NAP-1 facilitates the deposition of histones H2A and H2B to complete the nucleosome (20). Using in vitro nucleosome assembly and nuclease digestion mapping assays, it was shown that the periodic spacing of nucleosomes requires the function of ATP-dependent chromatin remodeling factors, such as the ACF/ISWI complex (16, 21). 

The rate of ATP hydrolysis by myosin has been examined as a function of ATP concentration, as shown in graph A. 

The heteroexchanges ATP o for ADP,j j, and ADP m) for ATP,j ), were also studied, but under conditions where both substrates were present in equimolar concentration on both sides of the membrane. The rate of entry of radiolabelled ADP under these conditions was studied at saturating substrate levels at variable Aif/ from -Hl20 to —180 mV with respect to the external side of the membrane. The ratio of the rate of ADP entry to ATP entry was plotted as a function of Atp and the ratio varied from 5 to 0.2 as a linear function of Aip from -1-120 to —90 mV. 

From the step movement trace of kinesin, which was obtained by laser trap measurements, the interval time between steps and directionality of the step were calculated as a function of ATP concentration, load, and temperature [19, 20]. The data were analyzed statistically and interpreted thermodynamically since the mechanochemical processes underlying the step movement of kinesin occur in accordance with thermodynamic rules. At no load limit, the probability of a backward step was estimated as 1 of 2,000, which corresponded to the difference in activation free energy ( -. 6 k T) in the energy landscape of forward and backward step movement (Fig. 12.2c). From structural studies, it has... 

Fig. 13.5. Mean burst size and dwell and burst duration as a function of [ATP], (a) The spatial periodicity observed in the average PWD for the low force data in Fig. 13.4b as a function of [ATP]. Error bars represent the standard deviation in a linear fit to the position of each of the peaks in the average PWD. (b) Mean dwell and burst durations for the low force data in Fig. 13.4b as a function of [ATP]. The mean dwell time before the 10-bp bnrsts circles) is well described by a Michaelis-Menten [ATP] dependence solid line). The average bnrst dnration squares) shows no apparent [ATP] dependence, with a mean of 10ms solid line). Modified from [74]...

Fig. 13.7. Fluctuations in the dwell time before the 10-bp bursts, (a) Probability distributions for the dweii times before the 10-bp bursts in Fig. 13.4b as a function of [ATP]. Peaked distributions are indicative of multiple rate-limiting kinetic transitions. [ATP] decreases from 250 to 5 xM left to right, (b) The effective number of rate-limiting transitions, Umin, as a function of [ATP]. The values at low and high [ATP] indicate the minimum number of ATP binding events and of nonbinding events that must occur during each dwell, 2 and 4 respectively. Modified from [74]...

In addition, our data place strict limits on the possible mechanisms by which these ATPs load to the ring. Interestingly, these restrictions arise out of efforts to resolve an apparent contradiction in our data. In our measurements of the mean dwell time as a function of [ATP] (Fig. 13.5b), we found that the [ATP] dependence of the mean dwell time was well described by a Hill coefficient of 1, i.e., this data displayed a simple Michaelis-Menten-like [ATP] dependence. However, the requirement that multiple ATPs bind before... 

It is believed that the function of ATP is to phosphorylate mevalonic acid pyrophosphate at the 3-position. 

DBTC, like TBTC, appears to interfere with the function of ATP synthase. Table 4.5.3 indicates the decreases in ATP levels that occurred when NK cells were exposed to DBTC. These decreases did not... 

The function of ATP is to store energy until it is needed by cells. Under appropriate... 

Function of ATP in the ATP-dependent luminescence reaction catalyzed by the arm-organ extract. The amounts of coelenteramide disulfate, AMP and ADP produced in luminescence reaction under the conditions of standard assay were measured by HPLC. In this experiment, the luminescence reaction mixture had initially contained 4 nmol of coelenterazine disulfate and 250 nmol of ATP, and after 10 min of luminescence reaction, 0.58 nmol of coelenteramide disulfate, 0.1 nmol or less of AMP, and 15 nmol of ADP were found. Thus the molar amount of AMP produced is much less than that of coelenteramide disulfate, indicating that the production of AMP is unrelated to the luminescence reaction. Therefore, the mechanism of ATP-dependent luminescence reaction suggested by Tsuji that involved the formation of adenyl coelenterazine disulfate as a key intermediate, must be incorrect. 

The formation of A -carboxybiotin from bicarbonate and biotin on an enzyme involves the stoichiometric hydrolysis of ATP to produce ADP and inorganic phosphate (Scheme 17). This is particularly significant because the hydrolysis of ATP is coupled to formation of the carbon-carbon bond (45, 46) but is not explicitly involved in the apparent stoichiometry of the biosynthetic process. Therefore, the function of ATP is cryptic, as it is in most processes in which ATP hydrolysis accompanies a biosynthetic process. Although biotin-dependent reactions are not a general model for other ATP-dependent processes, the patterns that emerge from the study of such a mechanism guides one in thinking about the other processes. In the biotin-dependent case, the reaction with ATP... 

Figure 4.6 Apparent inhibition constant as a function of ATP-concentration relative to Km for competitive inhibition (solid line), pure noncompetitive inhibition (dashed line) and uncompetitive inhibition (dotted line). The inhibition constant is 5 nM for each mechanism.

Preparations are listed in Table 47. Main interest lies in determining how metal ions catalyze the hydrolysis of ADP and ATP. ATP plays a crucial role in the energy metabolism of all living cells and divalent metal ions (Mg +, Mn " and Ca ) play an important role in these phosphoryl transfer processes. Divalent metal ions such as Mg +, Ca, Zn, Cu and Mn " provide only modest in vitro catalysis "" and stronger, more specific coordination to phosphate units appears to be required by the enzyme. Co " (and Cr" ) complexes of ADP and ATP have been shown to mimic many of the biological functions of the Mg" enzyme, and since the cobalt(III)-phosphatc coordination remains intact, the specificity of alternative coordination sites, and the stereochemical requirements at phosphorus, have been elucidated in some cases. Often the Co "-enzyme species is biologically active and several enzymic functions of ATP have been examined in this manner. 

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