Kinetic mechanism of calcium binding to whiting parvalbumin

Calcium binding to whiting parvalbumin induces large changes in the fluorescence, absorption, and circular dichroism spectra of the protein. The fluorescence emission maximum of the single tryptophan shifts from 325 to 348 nm upon the removal of calcium and decreases in intensity by 50%. All of the spectral changes are linear between 0 and 2 mol of calcium bound/mol of protein, which suggests that the only protein species present in significant concentration are PA0 and Pa-Ca2. The kinetics of calcium binding measured by stopped-flow fluorescence are accurately single exponential from 2 X 10(-7) to 2 X 10(-4) M free calcium. The kinetics of calcium dissociation show a pronounced lag and are best fit by two rate constants of 1.2 and 3.0 s-1. The minimal kinetic mechanism that adequately describes the rate and equilibrium data is a branched pathway mechanism in which the rate and equilibrium constants are markedly different for each pathway: (formula; see text) At [Ca] less than 2 microM the upper kinetic pathway of calcium binding predominates whereas at [Ca] greater than 2 microM calcium binding occurs predominantly by the lower kinetic pathway. Calcium dissociates primarily by the upper kinetic pathway.