Activation Free Energies in Partial Steps of the Sarcoplasmic Reticulum Ca-ATPase Cycle during the Transient States of the Reaction

The step-by-step free energy (ΔG) profile of the sarcoplasmic reticulum CaATPase cycle during the transient state of simulated reactions was analyzed previously by Alonso and Hecht (1990). ΔG was negative at all the steps. Here we calculated the step-by-step activation energies (ΔG ) in the same model according the transition state theory. Transient species were introduced as chemical intermediates at each step; they react with enzyme species accordingly with the mass law, decaying at 6.56x10 12 s -1 . Other rate constants are as in the referenced paper. The transient evolution of the model was followed with a computer program. The evolution of the ligands concentrations agree with experimental results. Enzymatic species concentrations lie within 10 -10 –10 -6 M, transient species lie within 10 -19 -10 -17 M, at 0.1 s, when the reaction is in a cuasi-steady-state. These differences preclude the use of the standard (ΔG o ) or basic (ΔGbasic) free energies, to describe ΔG fall along the cycle. Forward and backward ΔG are shown for 10 and 100 ms of simulated reaction; both are negative in forward cycles, at any reaction time, and their sum equals the partial ΔG. The results show the absence of any activation energy hill to be surmounted upon the reaction advance. Similar results were obtained for the successive binding of 2 Ca ions to the enzyme through (+)cooperative reactions.