Modeling binding kinetics at the Q(A) site in bacterial reaction centers

Area of Science:

• Biochemistry
• Photosynthesis research
• Electron transfer mechanisms

Background:

• Bacterial reaction centers (RCs) are crucial for photosynthesis, catalyzing electron transfer.
• The Q(A) site within RCs binds quinones, reducing them to semiquinones.

Purpose of the Study:

• To compare the binding kinetics of neutral and anionic quinones to the bacterial RC Q(A) site.
• To investigate the factors influencing quinone association and dissociation rates.

Main Methods:

• Measured dissociation constants (K(d)) and association rates (k(on)) for 13 different quinones.
• Utilized varying quinone concentrations and solvent viscosities.
• Determined rate constants at specific pH values for hydroxyl naphthoquinones.

Main Results:

• Neutral quinones associated and dissociated rapidly (milliseconds) with second-order kinetics.
• Anionic hydroxyl naphthoquinones exhibited significantly slower binding and dissociation (minutes) via first-order kinetics.
• Slow dissociation rates for anionic quinones were independent of ionic strength, viscosity, and concentration.

Conclusions:

• The bacterial RC Q(A) site imposes kinetic barriers for anionic quinone binding and release.
• These kinetic barriers, rather than thermodynamic stabilization, likely explain the slow dissociation of anionic semiquinones.