Detection of Quinone Function in the Homodimeric Type-I Reaction Center of Heliobacterium modesticaldum

Abstract

Heliobacteria have the type I homodimeric reaction center (RC) complex that is symmetrical in contrast to the heterodimeric RCs of PS I and type II. The function of quinone in the heliobacterial RCs has been unclear. We purified the RC core complex of Heliobacterium modesticaldum, which shows turnover of F_X and is depleted of iron sulfur centers F_A/F_B. We studied the flashinduced ESR signal and detected the E/A pattern spin polarized ESR signal that decays with a time constant of 4 ms and assigned it to originate from the P⁺ ₈₀₀ F_X ⁻ radical pair as reported in the membranes. The RC also showed another new signal of A/E/A pattern that decays with a time constant of 70 μs. We assigned this signal to be originated from the P⁺ ₈₀₀ A₁ ⁻ radical pair in comparison with the P⁺ ₈₀₀ A₁ ⁻ state signal in PS I RCs. The result suggests the function of quinone as A₁ in this RC. The A/E/A ESR spectral pattern was ascribed to the menaquinone which has an orientation different from that of phylloquinone in PS I. The distances and the electron transfer rates between A₀, A₁, and F_X in the heliobacterial RC might be interpreted by the unique orientation of A₁-menaquinone.