2Faculty of Bioengineering and Bioinformatics, Lomonosov Moscow State University, 119991 Moscow, Russia
* To whom correspondence should be addressed.
Received December 18, 2009; Revision received January 15, 2010
Transfer of electrons between artificial electron donors diphenylcarbazide (DPC) and hydroxylamine (NH2OH) and reaction center of manganese-depleted photosystem 2 (PS2) complexes was studied using the direct electrometrical method. For the first time it was shown that reduction of redox-active amino acid tyrosine YZ· by DPC is coupled with generation of transmembrane electric potential difference (ΔΨ). The amplitude of this phase comprised ~17% of that of the ΔΨ phase due to electron transfer between YZ and the primary quinone acceptor QA. This phase is associated with vectorial intraprotein electron transfer between the DPC binding site on the protein–water interface and the tyrosine YZ·. The slowing of ΔΨ decay in the presence of NH2OH indicates effective electron transfer between the artificial electron donor and reaction center of PS2. It is suggested that NH2OH is able to diffuse through channels with diameter of 2.0-3.0 Å visible in PS2 structure and leading from the protein–water interface to the Mn4Ca cluster binding site with the concomitant electron donation to YZ·. Because the dielectrically-weighted distance between the NH2OH binding site and YZ· is not determined, the transfer of electrons from NH2OH to YZ· could be either electrically silent or contribute negligibly to the observed electrogenicity in comparison with hydrophobic donors.
KEY WORDS: reaction center, photosystem 2, proteoliposomes, photopotential, diphenylcarbazide, hydroxylamine, channels