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The tertiary structural changes in bacteriorhodopsin occur between M states: X-ray diffraction and Fourier transform
H J Sass1, I W Schachowa, G Rapp
1Forschungszentrum Jülich, IBI-2: Structural Biology, Germany. Jurgen@ibistr.dnet.kfa-juelich.de
The EMBO Journal
|April 1, 1997
Summary
Tertiary structural changes in bacteriorhodopsin (BR) during its photocycle are linked to M states. These changes, crucial for reprotonation, are hydration-dependent and observed in Asp96Asn mutants.
Area of Science:
- Structural biology
- Biophysics
- Spectroscopy
Background:
- Bacteriorhodopsin (BR) is a light-driven proton pump essential for cellular energy generation.
- The photocycle of BR involves distinct intermediate states (M1, M2) characterized by tertiary structural changes.
- Understanding these conformational changes is key to elucidating BR's mechanism and related signaling pathways.
Purpose of the Study:
- To investigate the tertiary structural dynamics of bacteriorhodopsin during its photocycle.
- To determine the role of hydration levels on BR conformational changes.
- To assign specific structural transitions to the M1 and M2 states of the photocycle.
Main Methods:
- X-ray diffraction and Fourier transform infrared (FTIR) spectroscopy were employed in parallel.
- Studies were conducted on purple membranes (PM) of the Asp96Asn BR mutant.
- Samples were analyzed at varying relative humidity levels (15%, 57%, 75%, 100% r.h.).
Main Results:
- Light-dependent conformational changes in BR-Asp96Asn were observed only at high hydration (75% and 100% r.h.).
- FTIR spectra indicated the M intermediate, with significant tertiary structural changes occurring between the M2 and M(G) states.
- X-ray diffraction data showed M2 state changes comparable to wild-type M(G) state.
Conclusions:
- Tertiary structural changes between M1 and M2 states are critical for opening the cytoplasmic half-channel in BR.
- This channel opening facilitates reprotonation, completing the catalytic cycle.
- These hydration-dependent structural shifts, possibly triggered by charge redistribution, may be a conserved mechanism in retinal proteins for signal transduction.