Light emission from the calcium-sensitive bioluminescent protein aequorin was measured at calcium ion concentrations of 10-9 to 10-2 molar. At very low Ca2+ concentrations, light emission is independent of calcium ion concentration. The maximum slope of the log-log plot of light as a function of calcium ion concentration is about 2.5. The complete relation is well described by a two-state model involving three calcium-binding sites.
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