Regulation and thermodynamics

Biochemical reactions exist to bring about the net formation of a compound which may be required either of itself or as the starting material of a further process. A metabolic pathway could involve the conversion of a large, or constantly maintained, concentration of substrate S into an equally well maintained pool of product P (Eqn. 10). 

The sequence may involve the formation of several intermediates, B, C, D and require the conversion of coenzymes w and y into x and z, respectively. The chemical flux out of S and P may be controlled by (i) changing the concentrations of S or P, 

At thermodynamic equilibrium there can be no net flow. Enzymes do not alter the position of equilibrium between unbound substrates and products. If B, C or D are removed rapidly their concentration will not correspond to their equilibrium value but rather to their steady state concentrations. The total chemical flux out of C is given by the sum of the fluxes out of C into B and D. The total chemical flux into C is given by the sum of the flux of B into C and that of D into C. The rate of appearance of C is then given by the difference of these two sums (Eqn. 12). 

In this case, it is possible for an enzyme to apparently equilibrium between substrate and product. 

Obedience to Michaelis-Menten kinetics yields interesting conclusions about the specificity of enzyme-catalysed reactions. In vitro non-specific substrates are sometimes described as poor because they show a low value of or a high value of K. However, in vivo specificity results from a competition of substrates for the active site of the enzyme. If two substrates, S and S, compete for the same enzyme different conclusions could be reached about their relative specificity if rates of reaction or the K s of the individual substrates are compared instead of their relative values of K. If the enzyme catalyses the reaction of both S and S (Eqn. 15) the relevant equations may be obtained by the usual procedures (Eqns. 16-18). 

---