Proteorhodopsin Photocycle Kinetics Between pH 5 and pH 9

Area of Science:

• Biochemistry
• Molecular Biology
• Photophysics

Background:

• Proteorhodopsin is a light-driven proton pump and a homolog of bacteriorhodopsin.
• While proton transport mechanisms are conserved, proteorhodopsin exhibits distinct pH ranges for activity.
• The protonation state of the primary proton acceptor, Asp-97, is crucial for proton transport and is unusually close to physiological pH in proteorhodopsin.

Purpose of the Study:

• To investigate the photoinduced kinetics of proteorhodopsin across a pH range of 5 to 9.
• To understand the correlation between Asp-97 protonation and proteorhodopsin activity.
• To explore potential differences in proton acceptor function compared to bacteriorhodopsin and molecular adaptation.

Main Methods:

• Time-resolved UV/Vis absorption spectroscopy was employed.
• Kinetics were measured across a pH gradient from 5 to 9.
• Data analysis focused on identifying kinetic fractions and their relation to Asp-97 titration.

Main Results:

• Proteorhodopsin kinetics were found to be inhomogeneous between pH 5 and 9.
• The observed kinetics can be described as a superposition of two distinct fractions.
• These kinetic fractions directly correlate with the titration curve of Asp-97.
• Other protonation equilibria also influence kinetics, but the core proton acceptor function remains similar to bacteriorhodopsin.

Conclusions:

• The pK_a of Asp-97 in proteorhodopsin (≈7.5) is close to environmental pH (≈8), potentially rendering a fraction inactive.
• This pH-dependent behavior suggests molecular adaptation of proteorhodopsin to the specific physiological conditions of its host organisms.
• Despite pH-dependent variations, the fundamental mechanism of the primary proton acceptor function is conserved between proteorhodopsin and bacteriorhodopsin.