Menaquinone as the Secondary Electron Acceptor in the Type I Homodimeric Photosynthetic Reaction Center of Heliobacterium modesticaldum.

The type 1 photosynthetic reaction center (RC) of heliobacteria (hRC) is a homodimer containing cofactors almost analogous to those in the plant photosystem I (PS I). However, its three-dimensional structure is not yet clear. PS I uses phylloquinone (PhyQ) as a secondary electron acceptor (A(1)), while the available evidence has suggested that menaquinone (MQ) in hRC has no function as A(1). The present study identified a new transient electron spin-polarized electron paramagnetic resonance (ESP-EPR). signal, arising from the radical pair of the oxidized electron donor and the reduced electron acceptor (P800(+)MQ(-)), in the hRC core complex and membranes from Heliobacterium modesticaldum. The ESP signal could be detected at 5-20 K upon flash excitation only after prereduction of the iron sulfur center, F-x, and was selectively lost by extraction of MQ with diethyl ether. MQ was suggested to be located closer to F-x than PhyQ in PS I based on the simulation of the unique A/E (A, absorption; E, emission) ESP pattern, the reduction/oxidation rates of MQ, and the power saturation property of the static MQ signal. The result revealed the quinone usage as the secondary electron acceptor in hRC, as in the case of PS I.