Interactions between the donor and acceptor sides in bacterial reaction centers

Area of Science:

• Photosynthesis research
• Biochemistry
• Electron transport chains

Background:

• The reaction center of Rhodobacter capsulatus facilitates crucial electron transfer processes.
• Understanding quinone acceptor interactions is key to photosynthesis research.

Purpose of the Study:

• To measure the apparent equilibrium constant K'(2) for electron transfer between primary (Q(A)) and secondary (Q(B)) quinone acceptors.
• To investigate the influence of the primary donor's redox state (P(+) or P) on this equilibrium.
• To explore the impact of ubiquinone and inhibitor binding on donor-acceptor interactions.

Main Methods:

• Measurement of K'(2) in chromatophores of Rhodobacter capsulatus.
• Utilizing recombination rates from P(+)Q(A-) and P(+)Q(B-) states.
• Analyzing semiquinone oscillations during single turnover flashes.
• Measuring midpoint potential shifts of P in the presence of Q(B-) or Q(A-)S (stigmatellin).

Main Results:

• K'(2)(P(+)) was approximately 100, while K'(2)(P) was approximately 20, indicating a 5-fold difference.
• Stabilization of the oxidized primary donor P(+) by 10 mV (with Q(B-)) and 30 mV (with Q(A-)S).
• Observed modifications in P(+)/P and Q(A)/Q(A)(-) interactions based on Q(B) pocket occupancy.

Conclusions:

• The occupancy of the Q(B) pocket by ubiquinone or stigmatellin significantly alters interactions between the primary donor and quinone acceptors.
• A proposed conformational destabilization of the P(+)Q(A-) state contributes to the high K'(2)(P(+)) value, which is relieved by stigmatellin binding.