Acid-Base Equilibrium of the Schiff Base in Bacteriorhodopsin

Aqueous suspensions of dark-adapted bacteriorhodopsin (bR₅₆₀) in the purple membrane of Halobacterium halobium are exposed to rapid jumps to high pH. Optical and resonance Raman measurements are carried out by using flow and stationary methods. Above pH~11.5 bR₅₆₀ starts to be reversibly converted to a species absorbing at 460 nm (bR₄₆₀) characterized by an unprotonated Schiff base chromophore. Above pH~13.0 bleaching takes place, first reversibly and subsequently irreversibly, to a species absorbing around 365 nm (bR₃₆₅). This process competes with the formation of bR₄₆₀. The pK_a corresponding to the equilibrium is determined as 13.3 ± 0.3. The value of the corresponding association rate constant determined from the reverse jumps (from pH 12.67 to pH 10 and 9.2) is k_a = (3.5 ± 0.5) X 10¹¹ M^-1 s^-1. Thus, starting with bR at pH 12.67 the reprotonation process is diffusion controlled as observed for homogeneous acid-base equilibria. The observed rate of dissociation when jumping from pH 6.5 to 12-13 is slower than that predicted by including the equilibrium The results imply that the Schiff base is titratable in the dark, but its accessibility to external OH^- ions is limited. The limitations in the significance of the “apparent” value of pXa = 13.3 observed for the Schiff base titration are discussed in light of possible alterations in the structure of bR resulting from the parallel titration of other protein groups. It is suggested that a light-induced pK_a change of at least nine units takes place during the photocycle of light-adapted bR.