Cation effects on the purple membrane of Halobacterium halobium
My more recent research shows that the binding of one divalent cation is directly related to the blue-to-purple transition of bacteriorhodopsin; whereas, four other divalent cation binding sites involving carboxyl groups on the bacteriorhodopsin surface do not directly affect the color change. The intrinsic pK$\sb{\rm a}$ of the binding of the special divalent cation and of the color transition is at about pH 2. The events involve the exchange of one divalent cation for two protons; I suggest that the two groups which release a proton upon binding of the divalent cation are aspartates-85 and -212 in the bacteriorhodopsin active site.
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign