| The thermodynamics and kinetics of non-linear biochemical reactions existing far from equilibrium are currently being studied because of their importance to the understanding of fundamental biological and chemical processes. Such systems may exist in a stable node (a stable stationary state), a stable focus (a stationary steady state which is approached with an oscillatory component), or a stable limit cycle (stable oscillations).;Energy or power transduction in systems far from equilibrium must take place at a non-zero rate to be effective, and thus entropy must be produced. The rate of entropy production times the temperature is defined to be the dissipation. For an isothermal chemical reaction, the dissipation is the $Delta$G of reaction times the rate. The thermodynamic efficiency of power transduction is defined to be the power output divided by the power input. A reduction in dissipation is equivalent to an increase in efficiency.;As an example of a biochemical reaction, a model of a light driven biochemical pump is studied. The light intensity is subject to a periodic perturbation, and the efficiency of the pump is calculated. When the system is in a stable focus or stable limit cycle, maxima and minima in the efficiency are observed, depending upon the frequency of perturbation. Further calculations show that three factors play a role in changing the efficiency: changes in the average $Delta$G and rates of reaction, phase shifting between the force ($Delta$G) and the fluxes (rates), and the magnitude of the system's response to the perturbation.;Experimental results are presented from the oscillatory oxidation of the coenzyme NADH by oxygen, catalyzed by the enzyme horseradish peroxidase. The oxygen input is subject to a periodic perturbation, and changes in average NADH concentration, average $Delta$G, average rate of reaction, phase difference between $Delta$G and the rate, and the response of the system to the perturbation are observed. The magnitude of these changes depend upon the frequency and magnitude of perturbation. These are the first results which confirm the possibility of an 'alternating chemistry' for an isothermal reaction and control and optimization of thermodynamic efficiency and dissipation by means of external variation of constraints in non-linear biochemical reactions and biological pumps. |
