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  • Title: Water structural changes at the proton uptake site (the Thr46-Asp96 domain) in the L intermediate of bacteriorhodopsin.
    Author: Yamazaki Y, Hatanaka M, Kandori H, Sasaki J, Karstens WF, Raap J, Lugtenburg J, Bizounok M, Herzfeld J, Needleman R.
    Journal: Biochemistry; 1995 May 30; 34(21):7088-93. PubMed ID: 7766618.
    Abstract:
    Fourier transform infrared spectra of the L intermediate of light-adapted bacteriorhodopsin were examined for recombinant proteins with amino acid substitutions at Thr46 and Asp96. Two O-H stretching vibrational bands of water, at 3607 and 3577 cm-1, change into stronger H-bonding states in L of the wild type. Thr46-->Val substitution abolished these bands in spite of the fact that [3-18O]threonine-labeling did not shift them, indicating that they correspond to coordination of the water with Thr46. The two water bands were restored, although with changed frequencies, by an additional Asp96-->Asn substitution in Thr46-->Val/Asp96-->Asn. A single Asp96-->Asn substitution abolished the 3607 cm-1 band. Thus, Asp96 also takes part in structural changes in water. The perturbations of these water molecules in the L intermediate displayed a weak correlation with the ratio of intensity change in the two vibrational bands of the Schiff base mode at 1312 and 1301 cm-1 and the rate for the deprotonation of the Schiff base at the L-to-M reaction of the photocycle. We find, therefore, that the water molecules in the cytoplasmic Asp96-Thr46 domain, which comprises the site of proton uptake after formation of the M intermediate, undergo structural changes in the L intermediate already. These changes are transmitted to the extracellular domain and affect interaction of the Schiff base with Asp85, that is far removed from this region.
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