Biochemical systems, free energy change

Warshel is to utilize a formula identical to (11.22) in this chapter to compute the free energy change. They employed an empirical valence bond (EVB, below) approach to approximately model electronic effects, and the calculations included the full experimental structure of carbonic anhydrase. An H/D isotope effect of 3.9 1.0 was obtained in the calculation, which compared favorably with the experimental value of 3.8. This benchmark calculation gives optimism that quantum effects on free energies can be realistically modeled for complex biochemical systems. 

If the reactions are independent, each term in the sum must be positive. However, if the reactions are coupled, most simply by having reactants and products in common, only the total entropy generation must be positive. Thus, as is well known in biochemical systems, it is possible for reactions with negative affinity (positive free-energy change) to be driven to products by reactions with positive affinity. 

The free energy change in biochemical systems becomes... 

Free energy changes for systems described in terms of biochemical standard states are symbolized with a prime, such as AG°, AH°, AS°. ... 

The ability to rapidly and accurately calculate free energy changes in complex biochemical systems would make possible the computational design of new... 

ATP " + H2O ADP + HP04 + H+ AC° = -30.5 kJ (For biochemical systems, the standard-state concentration of is 10 M, not the usual 1 M, and the standard free energy change has the symbol AG . ) In the metabolic breakdown of glucose, fra- example, the initial step, which is nonspontaneous, is the addition of a phosphate group to a glucose molecule ... 

The energy relations associated with the redox processes in wastewater follow the general rules of thermodynamics (Castellan, 1975 Atkins, 1978). The Gibbs free energy, G, of the system is the major thermodynamic function defining the state — and the change in state — of the biochemical redox processes. At constant temperature and under constant pressure, AG is equal to the maximum work, which can be produced by the redox process ... 

However, using entropy as a criterion of whether a biochemical process can occur spontaneously is difficult, as the entropy changes of chemical reactions are not readily measured, and the entropy change of both the system and its surroundings must be known. These difficulties are overcome by using a different thermodynamic function, free energy (G), proposed by Josiah Willard Gibbs which combines the first and second laws of thermodynamics ... 

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