Evidence for the involvement of more than one metal cation in the Schiff base deprotonation process during the photocycle of bacteriorhodopsin

Journal Article · Mon Jun 01 04:00:00 EDT 1987 · Proc. Natl. Acad. Sci. U.S.A.; (United States)

DOI:https://doi.org/10.1073/pnas.84.12.4094· OSTI ID:5460830

Corcoran, T C; Ismail, K Z; El-Sayed, M A

The removal of metal cations inhibits the deprotonation process of the protonated Schiff base during the photocycle of bacteriorhodopsin. To understand the nature of the involvement of these cations, a spectroscopic and kinetic study was carried out on bacteriorhodopsin samples in which the native Ca/sup 2 +/ and Mg/sup 2 +/ were replaced by Eu/sup 3 +/, a luminescent cation. The decay of Eu/sup 3 +/ emission in bacteriorhodopsin can be fitted to a minimum of three decay components, which are assigned to Eu/sup 3 +/ emission from three different sites. This is supported by the response of the decay components to the presence of /sup 2/H/sub 2/O and to the changes in the Eu/sup 3 +//bR molar ratio. The number of water molecules coordinated to Eu/sup 3 +/ in each site is determined from the change in its emission lifetime when /sup 2/H/sub 2/O replaces H/sub 2/O. Most of the emission originates from two wet sites of low crystal-field symmetry-e.g., surface sites. Protonated Schiff base deprotonation has no discernible effect on the emission decay of protein-bound Eu/sup 3 +/, suggesting an indirect involvement of metal cations in the deprotonation process. Adding Eu/sup 3 +/ to deionized bacteriorhodopsin increases the emission intensity of each Eu/sup 3 +/ site linearly, but the extent of the deprotonation (and color) changes sigmoidally. This suggests that if only the emitting Eu/sup 3 +/ ions cause the deprotonation and bacteriorhodopsin color change, ions in more than one site must be involved-e.g., by inducing protein conformation changes. The latter could allow deprotonation by the interaction between the protonated Schiff base and a positive field of cations either on the surface or within the protein.

• Research Organization: Univ. of California, Los Angeles
• OSTI ID: 5460830
• Journal Information: Proc. Natl. Acad. Sci. U.S.A.; (United States), Journal Name: Proc. Natl. Acad. Sci. U.S.A.; (United States) Vol. 84:12; ISSN PNASA
• Country of Publication: United States
• Language: English

Related Subjects

14 SOLAR ENERGY
140505* -- Solar Energy Conversion-- Photochemical
Photobiological
& Thermochemical Conversion-- (1980-)
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59 BASIC BIOLOGICAL SCIENCES
ALKALINE EARTH METAL COMPOUNDS
BIOCHEMICAL REACTION KINETICS
CALCIUM COMPOUNDS
CATIONS
CHARGED PARTICLES
CHEMICAL REACTIONS
EMISSION SPECTRA
EUROPIUM COMPOUNDS
HEAVY WATER
HYDROGEN COMPOUNDS
IMINES
IONS
KINETICS
MAGNESIUM COMPOUNDS
ORGANIC COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
OXYGEN COMPOUNDS
PHOTOCHEMICAL REACTIONS
PHOTOSYNTHETIC BACTERIA
PIGMENTS
PROTEINS
RARE EARTH COMPOUNDS
REACTION KINETICS
RHODOPSIN
SCHIFF BASES
SPECTRA
WATER