C(13)-substituted bacteriorhodopsin analogs.

Area of Science:

• Biochemistry
• Photochemistry
• Structural Biology

Background:

• Bacteriorhodopsin (bR) is a light-driven proton pump crucial for cellular energy generation.
• The retinal chromophore within bR undergoes photoisomerization, initiating the proton pumping cycle.
• Understanding the role of specific retinal modifications is key to elucidating bR's mechanism.

Purpose of the Study:

• To synthesize novel bacteriorhodopsin analogs by modifying the C(13) position of the retinal chromophore.
• To investigate the functional consequences of introducing bulky hydrocarbon substituents at the C(13) position.
• To explore the structural and dynamic effects of these modifications on the bR photocycle.

Main Methods:

• Synthesis of various retinals with different C(13) substituents (ethyl, isopropyl, tert-butyl, phenyl, alpha-naphthyl, demethyl).
• Reconstitution of synthesized retinals with bacterioopsin (bO) to form bR analogs.
• Spectroscopic and electrophysiological measurements to study light-induced conversions and photoresponses.

Main Results:

• Successful synthesis and reconstitution of bR analogs with diverse C(13) substituents.
• Demonstrated photocycling capability for 13-tert-butyl- and 13-alpha-naphthyl-bRs, indicating a cavity around C(13).
• Larger C(13) substituents significantly diminished electric photoresponse amplitudes.
• Bulky C(13) groups or lack of substitution decelerated M-intermediate relaxation and disrupted cis-trans isomerization equilibrium.

Conclusions:

• The cavity around the C(13) position of the retinal chromophore in bacteriorhodopsin can accommodate bulky substituents.
• Steric hindrance at C(13) negatively impacts the efficiency of light-induced proton pumping and alters photocycle kinetics.
• These findings provide insights into the structure-function relationship of the retinal chromophore in bacteriorhodopsin.