The EF loop in green proteorhodopsin affects conformation and photocycle dynamics
Michaela Mehler1, Frank Scholz, Sandra J Ullrich
1Institute of Biophysical Chemistry and Centre for Biomolecular Magnetic Resonance, Goethe-University Frankfurt, Germany.
Biophysical Journal
|July 23, 2013
Summary
A mutation in the EF loop of green proteorhodopsin significantly alters its photocycle kinetics and optical properties by affecting the chromophore binding pocket. This study reveals a conserved communication pathway between the EF loop and retinal binding pocket.
Area of Science:
- Biochemistry
- Spectroscopy
- Structural Biology
Background:
- Proteorhodopsins are marine bacterial retinal proteins with environment-tuned optical properties.
- A specific EF loop mutation (A178R) in green proteorhodopsin causes a large redshift despite its distance from the chromophore.
Purpose of the Study:
- To investigate the structural and functional effects of the A178R EF loop mutation in green proteorhodopsin.
- To elucidate the molecular mechanisms behind the observed spectral and kinetic alterations.
Main Methods:
- Time-resolved optical spectroscopy
- Solid-state Nuclear Magnetic Resonance (NMR), including multidimensional MAS-NMR and DNP-enhanced (13)C-double quantum MAS-NMR
Main Results:
- The mutation alters the primary photoreaction, making it pH-independent and slower, while accelerating K-intermediate decay.
- The photocycle is elongated due to longer lifetimes of late photo intermediates.
- NMR data show mutation-induced chemical shift changes propagating to the chromophore pocket and reveal a modified chromophore-Schiff base-counterion interaction network.
Conclusions:
- The EF loop mutation distorts helix F reorientation during reprotonation, leading to a slower photocycle.
- The EF loop is crucial for proton uptake, and a communication pathway exists between the EF loop and the retinal binding pocket.
- This pathway may be evolutionarily conserved in retinal proteins.
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