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 FX and is depleted of iron sulfur centers FA/FB. 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 FX − 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 FX in the heliobacterial RC might be interpreted by the unique orientation of A1-menaquinone.