Thermodynamic buffering

After these general comments let us further test the idea of thermodynamic buffering on an experimental basis by repeating the above experiment but this time in the presence of an inhibitor of adenylate kinase, namely, diadenosine pentaphosphate. As is depicted in Fig. 6b the buffering effect of the adenylate kinase is abolished by inhibiting this enzyme and it becomes now possible to drive the system beyond the state of optimal efficiency by increasing the hexokinase concentration in the medium. Note that it was not possible to measure points closer to level flow than the ones shown in the figure. This is due to technical reasons. At the lowest phosphate potentials the ATP/ADP ratios where of the order... 

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... 

Finally, we might wish to observe thermodynamic buffering at work in an intact cell in a fluctuating environment. Unfortunately there is no experimental technique available at present which would permit a continuous measurement of the phosphate potential in the cytosol of a living cell. Therefore, we are left with the only possibility to give such a demonstration by a numerical computer simulation. 

Fig. 8. Effect of thermodynamic buffering on entropy production, (a) Plot of Su versus x [equations (8) and (15) with LAK = 0]. The plots were normalized with Lp = L = X2 = I. The other values in the plot As = 8.5 kcal/mole, AGak = 0.15 kcal/mole, P, = 0.008 M, and M = 0.005. LJ was varied between 0.0 and 0.9 in steps of 0.1. Thin line, rop, for q%c broken tine, loci of steady states.

It is interesting to note that the subcellular distribution of the thermodynamic buffer enzymes is also in accord with their physiological... 

This chapter starts with a simplified analysis of biological processes using the basic tools of physics, chemistry, and thermodynamics. It provides a brief description of mitochondria and energy transduction in the mitochondrion. The study of proper pathways and multi-inflection points in bioenergetics are summarized. We also summarize the concept of thermodynamic buffering caused by soluble enzymes and some important processes of bioenergetics using the linear nonequilibrium thermodynamics formulation. 

Gain ratio 17 r can be calculated at a reference force ratio, such as xopt, which is a natural steady-state force ratio of oxidative phosphorylation. This is seen as a result of the adaptation of oxidative phosphorylation to various metabolic conditions and also as a result of the thermodynamic buffering of reactions catalyzed by enzymes. The experimentally observed linearity of several energy converters operating far from equilibrium may be due to enzymatic feedback regulations with an evolutionary drive towards higher efficiency. 

Dividing Eq. (11.179) withEq. (11.167) shows the effect of thermodynamic buffering on the exergy loss... 

Within the framework of the theory of dissipative structures, thermodynamic buffering represents a new bioenergetics regulatory principle for the maintenance of a nonequilibrium conditions. Due to the ATP production in oxidative phosphorylation, the phosphate potential is shifted far from equilibrium. Since hydrolysis of ATP drives many processes in the cell, the shift inXp to far from equilibrium results in a shift of all the other potentials into the far from equilibrium regime. 

Matching the supply of chemical energy WcM to the chemical energy WcA2 for active transport requires complex and organized thermodynamic buffering systems, which may include uncoupling, slips, and leaks. 

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