Thermodynamics biochemical reaction coupling

To explore the consequences of coupling ATP hydrolysis under physiological conditions to a thermodynamically unfavorable biochemical reaction, consider the hypothetical transformation for which AG ° = 20 kJ/mol. 

The concepts involved in this approach are simple, but the equations become rather complicated. Biochemical reactions are written in terms of reactants like ATP that are made up of sums of species, and they are referred to as biochemical reactions to differentiate them from the underlying chemical reactions that are written in terms of species. The thermodynamics of biochemical reactions is independent of the properties of the enzymes that catalyze them. However, the fact that enzymes may couple reactions that might otherwise occur separately increases the number of constraints that have to be considered in thermodynamics. 

Many biochemical reactions are nonspontaneous (that is, they have a positive AG° valne), yet they are essential to the maintenance of life. In living systems these reactions are conpled to an energetically favorable process, one that has a negative AG° valne. The principle of coupled reactions is based on a simple concept we can nse a thermodynamically favorable reaction to drive an nnfavorable one. Consider an in-dnstrial process. Snppose we wish to extract zinc from the ore sphalerite (ZnS). The following reaction will not work since it is highly nonspontaneons ... 

The compulsory energetic coupling of chemical (biochemical) reactions and of diffusive mass-transfers was phenomenologically identified and also introduced in the well-known case for example of the so-called active transport by biologists it was explicited theoretically in several treatments using mostly calculations based on thermodynamics of non-equilibrium processes [7-11]. 

Reactions that are thermodynamically unfavorable can be coupled to favorable reactions. The coupling of the reactions may be direct or sequential, as in a biochemical pathway. 

Hydrolase reactions do not involve coupling, but they do involve a problem with the treatment of water in biochemical thermodynamics. Therefore, it is of interest to consider their representation by matrices. As a prototypical hydrolase reaction, consider the hydrolysis of glucose 5-phosphate. 

Chapters 15 through 24 explore intermediary metabolism. Chapter 15 opens the topic with chemical principles that provide some unifying themes. Thermodynamic concepts learned earlier in general chemistry and in Chapter 1 are applied specifically to biochemical topics such as coupled reactions. In addition, this chapter explic-idy makes the connection between metabolism and electron transfer (oxidation-reduction) reactions. 

We saw in Section 4.2 that in biochemical work it is common to adopt the biological standard state (pH = 7, corresponding to neutral solution), rather than the thermodynamic standard state (pH = 0). To convert standard potentials to biological standard potentials, , we must first consider the variation of potential with pH. The two potentials differ when hydrogen ions are involved in the half-reaction, as in the fumaric acid/succinic acid couple fum/suc with fum = HOOCCH=CHCOOH and sue = HOOCCH2CH2COOH, which plays a role in the citric acid cycle (Case study 4.3) ... 

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