Kinetic isotope effects on the reduction of the Yz radical in oxygen evolving and tris-washed Photosystem II membranes by time-resolved EPR

Deuterium kinetic isotope effects on the electron transfer reactions for the reduction of the tyrosyl radical Y-Z(.) were studied with time resolved electron paramagnetic resonance in oxygen evolving N and tris-washed photosystem II membranes. For the electron transfers from the (Mn)(4)/H2O complex to Y-Z(.), S-state dependent kinetic isotope effects were observed with k(H)/k(D) values of 2.9, 1.3 and 1.6 for the transitions S-1 --> S-2, S-2 --> S-3 and S-3 --> [S-4] --> S-0, respectively. In tris-washed samples, oxygen evolution is abolished and the reduction of Y-Z(.) proceeds at the same rate in (H2O)-H-1 and in (H2O)-H-2. Taken together, these results show that proton-electron coupling during the reduction of Y(Z)(.)is in effect when water is the electron donor to PSII, but not when oxygen evolution is inhibited. This conclusion is consistent with a mechanism for oxygen evolution in which Y-Z(.) abstracts a hydrogen atom from the (Mn)(4)/H2O complex on each S-state transition. By demonstrating proton/electron coupling, our data suggest that these hydrogen atom abstraction reactions proceed along a pathway that has a concerted character. The atom transfer character in the transition state serves to minimize charge development during the SnYZ. --> Sn+1YZ reactions and is consistent with the relatively low activation energies observed in the S-state transitions in PSII. (C) 1997 Elsevier Science B.V.