Redox Potential of Quinones in Both Electron Transfer Branches of Photosystem I

Hiroshi Ishikita,Ernst-Walter Knapp

doi:10.1074/jbc.m306434200

Abstract

The redox potentials of the two electron transfer (ET) active quinones in the central part of photosystem I (PSI) were determined by evaluating the electrostatic energies from the solution of the Poisson-Boltzmann equation based on the crystal structure. The calculated redox potentials are -531 mV for A1A and -686 mV for A1B. From these results we conclude the following. (i) Both branches are active with a much faster ET in the B-branch than in the A-branch. (ii) The measured lifetime of 200-290 ns of reduced quinones agrees with the estimate for the A-branch and corroborates with an uphill ET from this quinone to the iron-sulfur cluster as observed in recent kinetic measurements. (iii) The electron paramagnetic resonance spectroscopic data refer to the A-branch quinone where the corresponding ET is uphill in energy. The negative redox potential of A1 in PSI is primarily because of the influence from the negatively charged FX, in contrast to the positive shift on the quinone redox potential in bacterial reaction center and PSII that is attributed to the positively charged non-heme iron atom. The conserved residue Asp-B575 changes its protonation state after quinone reduction. The difference of 155 mV in the quinone redox potentials of the two branches were attributed to the conformation of the backbone with a large contribution from Ser-A692 and Ser-B672 and to the side chain of Asp-B575, whose protonation state couples differently with the formation of the quinone radicals.

Highlights

The crystal structure of Photosystem I (PSI)1 from Synechococcus elongatus that is available at atomic resolution of 2.5 Å (1) solved the riddle of the microscopic structure of this membrane protein and has promoted the challenge to understand its function
The A1A redox potential obtained from electrostatic energy computations is in good agreement with the estimate between Ϫ577 and Ϫ522 mV computed from the rate expression (Equation 3)
Both branches are probably electron transfer (ET) active, and the redox potential difference by 155 mV between A1A and A1B leads to a biphasic ET process, which is fast in the B-branch and slow in the A-branch

Summary

Introduction

The crystal structure of Photosystem I (PSI) from Synechococcus elongatus that is available at atomic resolution of 2.5 Å (1) solved the riddle of the microscopic structure of this membrane protein and has promoted the challenge to understand its function. Kinetic investigations showed a biphasic forward ET from P700 to A1 (4) and from A1 to FX (5, 6), which can be interpreted as the action from two active branches This view is supported by point mutagenesis studies replacing the tryptophans Trp-A697 and Trp-B677 in the vicinity of each of the quinones (see Fig. 2) by His, Leu, and Phe residues. The complexity of the PSI membrane protein complex, the large number of possible variably charged groups (redox-active and titratable), and the unknown interactions among them make it difficult to attribute experimentally measured signals uniquely to a specific redoxactive cofactor. Our calculated redox potentials are capable of explaining both the kinetic and the EPR studies and resolving the apparent conflict between these different experimental data

Methods

Results

Conclusion