ATPase reactions

The specific effect of actin on myosin ATPase becomes apparent if the product release steps of the reaction are carefully compared. In the absence of actin, the addition of ATP to myosin produces a rapid release of H, one of the products of the ATPase reaction ... 

An insufficient rate of ATP resynthesis for optimal energy supply for actomyosin crossbridge formation and cycling, or for the additional ATPase reactions, Na" -K pumping and Ca reuptake and/or release by the SR. 

Fig. 2. E]-E2 reaction cycle of H,K-ATPase, accounting for the transport of two H and two K ions per molecule of hydrolysed ATP. The ATPase reaction proceeds from 2H E, ATP through 2H E -P and 2H E2-P to 2K E. Details of the reaction cycle are described in the text.

Like Na,K-ATPase, gastric H,K-ATPase also exhibits a p-nitrophenylphosphatase (/>NPPase) activity. This phosphatase activity is dependent on Mg and K, or one of its congeners with the same order of selectivity as for the ATPase activity, yielding a specific activity of 6D84% of the maximal ATPase activity [4,136,137]. Phosphorylation by pNPP has not been demonstrated and transport is also not catalyzed by this substrate. As in the ATPase reaction the effect of on the... 

Figure 29 shows the bifurcation diagram for different values of the saturation parameter 6 of the ATPase reaction. Qualitatively, the plot shows the same... 

Figure 29 Bifurcation diagram of the minimal model of glycolysis as a function of feedback strength and saturation 6 of the ATPase reaction. Shown are the transitions to instability via a saddle node (SN) and a Hopf (HO) bifurcation (solid lines). In the regions (i) and (iv), the largest real part with in the spectrum of eigenvalues is positive > 0. Within region (ii), the metabolic state is a stable node, within region (iii) a stable focus, corresponding to damped transient oscillations.

Figure 32. Bifurcation diagram of the medium complexity model of glycolysis, analogous to Fig. 29. A The largest real part of the eigenvalues as a function of the feedback strength 0 TP, depicted for increasing saturation of the overall ATPase reaction. B The metabolic state is stable only for an intermediate value of the feedback parameter. For increasing saturation of the ATPase reaction, the stable region decreases.

Figure 6.28 The Ca -ATPase reaction cycle. (Adapted by permission from Macmillan Pnblishers Ltd from Fignre 1 of reference 96, copyright 2004.)...

In the so-called Lymm-Taylor model, the actomyosin ATPase reaction proceeds in a stepwise manner ... 

A combination of the novel flash photolysis technique enabling the rapid release of nucleotides such as ATP from inert, photolabile precursors (Kaplan et al., 1978 McCray et al., 1980 Gurney Lester, 1987) with the high x-ray intensities available from synchrotron sources has introduced the possibility of studying the kinetics of structural events associated with the actomyosin ATPase reactions in muscle fibres. The precursor or caged nucleotides can readily diffuse into skinned muscle fibres, where, in the case of caged-ATP, a pulse of ultraviolet (u.v.) light will photolyse... 

Reactions causing PA — ATP P and other exchanges and ATPase reactions... 

In the absence of F0, the F[ is an ATPase, known as the FrATPase, and not a synthase. The rotation of the y subunit has been demonstrated in the ATPase reaction. The most graphic example is in Figure 10.19.94 The a3f3 yt components were attached to a Ni2+-coated glass surface at the opposite side from the normal attachment to a membrane by adding a Ni2+-binding sequence (His10) at the... 

Figure 19.22 The gatekeeping or filtering activity of the GroEL ATPase. The turnover number for the ATPase reaction (0.05-0.1 s-"1) is far slower than the refolding rate constant of 2-2.5 s-1 for GroEL-bound bamase. Only slowly folding proteins bind long enough to enter the chaperoning cycles of Figure 19.23.

In the absence of added glucose, hexokinase was found to catalyze the very slow hydrolysis of MgATP (59). This has been explained by assuming that water has replaced glucose at the active site of the enzyme. This ATPase activity can be inhibited by compounds that inhibit the hexokinase activity (60) and can be stimulated by compounds such as D-xylose or D-lyxose which lack the terminal -CH2OH of glucose (61). The ATPase reaction has been used to support evidence that hexokinase has a random kinetic mechanism, since it shows that ATP can bind to hexokinase in the absence of glucose (62). 

In this model the unimolecular constants are relative to the turnover number and the bimolecular constants are chosen to yield equilibrium constants in units of millimolar. The model is primarily based on dead-end inhibition by CrATP, the Michaelis constant for ATP in the ATPase reaction, the isotope partitioning experiments of Rose et al. (65), and various binding and kinetic constants found in the literature. The final model was based on a computer simulation study attempting to discover what combination of rate constants would lit the isotope partition data and the observed kinetic and binding constants. 

The absence of an isotope effect on the V for the ATPase reaction, however, means that the hydra eXdoes not bind appreciably in the active site as a competitive inhibitor (when a competitive inhibitor is present in the variable substrate, there is no effect on V/K, but V is decreased). A normal... 

Electron-proton coupling. Mechanism of ATPase reactions in energy-conversion... 

The model also assumes that physiologically reversible reactions operate in either direction. Here the reactions PGI (reaction 2), TRALD (reaction 6), TRKET (reaction 7), PGK (reaction 8), PGM (reaction 9), LACDH (reaction 14), PFLASE (reaction 15), PTACET (reaction 16), ACO (reaction 22), ISODH (reaction 23), SCOASN (reaction 26), FUMARASE (reaction 28), TRANSH2 (reaction 32), and ATPASE (reaction 39) are considered reversible. The fluxes of all other reactions are constrained to operate in the direction defined as positive by the stoichiometric matrix. 

Carrying out this optimization, we find that 18.667 ATP may be synthesized in this system for each glucose molecule consumed. The flux distribution at this optimal solution is illustrated in Figure 9.4. Here, 14.667 ATP/glucose are synthesized by the ATPase reaction of the oxidative phosphorylation system, 2 by glycolysis and 2 by the TCA cycle. 

By using actomyosin ATPase reaction, analysis of untreated rat slow muscle soleus (SOL) revealed predominantly type 1 fibers with a few type 2A and 2B fibers. In contrast, the fast muscle EDL is composed predominantly of type 2 fibers with few type 1 fibers. Total fiber numbers are approximately 1,800 in SOL and 2,500 in EDL (Gupta et al, 1989). 

The same treatments which elicit the ATPase reaction in the membrane-bound ATP synthase induce simultaneously a dark ATP-Pj exchange reaction. It was recently demonstrated that this exchange reaction is due to the simultaneous occurrence of phosphorylation and ATPase activity and therefore the use of the term exchange reaction may be a misnomer [37,38]. It is suggested that, as noted above, the induced ATP hydrolysis produces a transmembrane electrochemical proton gradient which in turn drives the ATP synthetic reaction. 

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