Non-equilibrium steady states and cycle kinetics

As discussed in Chapter 3, living cells exist away from thermodynamic equilibrium. When a reaction such as S = P is maintained in a steady state, we refer to this state 

For the example of the reversible Michaelis-Menten enzyme catalyzing S P, in the steady state S and P are transported into and out of the system at the constant rate J. The positive and negative fluxes of the catalytic cycle are given by 

When an enzyme-catalyzed biochemical reaction operating in an isothermal system is in a non-equilibrium steady state, energy is continuously dissipated in the form of heat. The quantity J AG is the rate of heat dissipation per unit time. The inequality of Equation (4.13) means that the enzyme can extract energy from the system and dissipate heat and that an enzyme cannot convert heat into chemical energy. This fact is a statement of the second law of thermodynamics, articulated by William Thompson (who was later given the honorific title Lord Kelvin), which states that with only a single temperature bath T, one may convert chemical work to heat, but not vice versa. 

In Chapter 9 we will see that the second-law inequality of Equation (4.13) will form a cornerstone of the constraint-based approach to analyzing biochemical networks. 

---