ON THE ROLE OF TYROSINE IN THE PHOTOCYCLE OF BACTERIORHODOPSIN

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Abstract
‐The rate of formation of the M intermediate (kM) in the photocycles of bacteriorhodopsin (bR570) and of nitrated bacteriorhodopsin (bR532n), is measured over the range between pH 6.5 and 11.5. In the case of bR570, kM is markedly pH dependent, exhibiting a titration‐like curve with pK? 10.3. The pH dependency is completely eliminated by nitration. On the basis of previous work by Lemke and Oesterhelt (1981), the effect is attributed to the specific modification of the Tyr 26 residue. The data are rationalized by a mechanism in which deprotonation of Tyr 26 at the stage of the L intermediate constitutes a prerequisite for deprotonation of the retinal‐lysine SchifT base. Both reactions are intimately associated with the photo‐induced proton pump mechanism.