Thermodynamics of Biochemical Reaction Systems at Three Levels

Thermodynamics provides the means for obtaining a broader overview of equilibria in systems of biochemical reactions. Level 1 calculations are those of the usual chemical thermodynamics in terms of species like ATP4-, ATP3-, and H2ATP 2-, etc. Level 2 calculations at specified pH deal with reactants (sums of species) like ATP. Level 3 calculations at specified pH and specified concentrations of coenzymes like ATP and ADP deal with sums of reactants like the sum of glucose, G6P, F6P, and FBP. At the higher levels, the number of reactants is considerably reduced. For example, at level 3 glycolysis is reduced to a single reaction (C6 = 2C3) and the pyruvate dehydrogenase reaction plus the citric acid cycle is reduced to three reactions. Yet no thermodynamic information is lost in going to the higher levels. These calculations can also be described by use of semigrand partition functions. Semigrand partition functions have the advantage of containing all the thermodynamic information on a series of reactions at specified pH or at specified pH and specified concentrations of coenzymes