Jp. Wang et Ma. El-sayed, The effect of metal cation binding on the protein, lipid and retinal isomeric ratio in regenerated bacteriorhodopsin of purple membrane, PHOTOCHEM P, 73(5), 2001, pp. 564-571
The effect of metal cation binding on bacteriorhodopsin (bR) in purple memb
rane has been examined using in situ attenuated total reflection-Fourier tr
ansform infrared difference spectroscopy in aqueous media. It is known that
adding metal cations to deionized bR regenerates the purple state from its
blue state and recovers the proton pump function, During this process, inf
rared spectral changes in the frequency region of 1800-1000 cm(-1) are moni
tored. The results reveal that metal cation binding affects the protein con
formation, the retinal isomeric composition as well as lipid head groups. I
t is also observed that metal cation binding induces conformational changes
in the alpha (1)-helix region of bR, converting the portion of its alpha (
1)-helical domain into beta -turn or disordered coil, In addition, the infl
uence of Ho3+ binding on the protein and lipid is observed to be larger tha
n that of Ca2+, These results suggest that some of the metal cation binding
sites are on the membrane lipid domain, while others could be on the intra
helical domain or interhelical loops where the Asp and Glu are located (bin
ding with their COO- groups). Our results also suggest that the removal of
the C-terminal of bR increase the accessibility of the binding site of meta
l cations, which affects protein conformational structure. All these observ
ations are discussed in terms of the two proposals given in the literature
regarding the metal cation binding sites.