Equilibrium constants for some steps of the reaction cycle of the sarcoplasmic reticulum calcium pump.

This paper summarizes true equilibrium measurements for some partial reactions of the sarcoplasmic reticulum calcium pump transport cycle. The most important result is the estimation of the equilibrium constant for the interconversion of the two major conformational states of the protein, E (Ca2+ binding sites facing the cytoplasm) and E' (Ca2+ binding sides facing the sarcoplasmic reticulum lumen). The value of K0 = [E']/[E] cannot be evaluated directly by any method available at present, but observed cooperativity in the binding of Mg2+ and Ca2+ to unliganded protein strongly indicates that K0 much greater than 1. The most probable value, valid within an order of magnitude, is K0 congruent to 10(3), i.e., the E' state is more stable than the E state by about 4 kcal/mol.