PARENT SESSION
Posters P4Ab Type II reaction centres: Acceptor side. Abstracts (272-288)

Reconstitution of Q_B function with 1,4-naphthoquinone in bacteria reaction centers with different low potential Q_A's. Xinyu Zhang^*,1, M. Gunner², ¹ The Graduate Center at City University of New York., New York, NY, USA² Physics Department, City College of New York., New York, NY, USA

ABSTRACT- In the purple reaction centers from Rb. sphaeroides, ubiquinone is used at both primary (Q_A) and secondary (Q_B) sites, but they perform different functions. Many different quinones can functional at Q_A site, but no other quinone (XQ) will support electron transfer from XQ_A to XQ_B. In addition, ubi-Q_A has not been shown to easily reduce other quinones at Q_B. Previous work of Giangiacomo and Dutton (Proc. Natl. Acad. Sci. USA (1989)86, 2658-2662) showed that increasing the driving force between Q_A and Q_B site could allow reduction of non-native Q_Bs. A low potential 2-methyl-3-Dimentylamino-1,4-Naphthoquinone (DMA-NQ) was synthesized. The binding affinity of DMA-NQ, Anthraquinone (AQ) and 1,4-Naphthoquinone (NQ) at Q_A are 0.06uM, 0.2uM and 8uM respectively. The quantum yield of the different quinones at Q_A site is 0.5, 0.9, and 1.0. The Em, relative to UQ_A is -423meV, -340meV, and 30meV. With an appropriate concentration of AQ or DMA-NQ and NQ, the low potential quinone is predominant occupant of Q_A while NQ is in the Q_B site. The K_D of NQ for the Q_B site is around 20uM. The reaction kinetics slow as expected if there is electron transfer from Q_A– to Q_B. The back reaction rate with NQ at Q_B is 3.5s if DMA-NQ at Q_A, 2.1 s if AQ at Q_A. The reaction -G_AB^o (DMA-NQ at Q_A and NQ at Q_B) = 248 meV; the reaction of -G_AB^o(AQ at Q_A and NQ at Q_B) = 154 meV. Reduction of 1,4-naphthoquinone at Q_B is 240meV less favorable than with UQ at Q_B site. The Em of NQ is and 180mV lower than Ubiquinone at Q_A explaining why there is no electron transferred from UQ_A to NQ at Q_B. Supported by CREES 2003-02123.

KEY WORDS: electron transfer, bacteria reaction center, reaction rates, redox potential