Fig. 6: Reaction scheme showing the Mn4CaOx cluster as well as the D61, Y161 (YZ), H190 and N298 residues during the S3→S0 transition.
a Oxygen atom O6 is oxidized by transferring an electron to YZ; the proton bound to O6 is concomitantly transferred to D61 in a Grotthus-type mechanism. Subsequently, O6 and O5 form an O=O bond while reducing Mn4, Mn3, and Mn1 from +IV to +III. This step takes place within about 5 ms in wild-type PSII as well as in PSII containing the N298A mutation, but is severely slowed down in the D61A mutant. Here a specific mode of O-O bond formation is indicated10,43,88,89; alternatives have been discussed1,90. b A new water molecule is inserted into the Mn4CaOx cluster, and a proton is simultaneously released to the bulk, leading to the formation of the S0 state (shown in c). This step is faster than O2 release and thus “invisible” in wild-type PSII as well as in PSII containing the D61A mutation. We propose that in the N298A mutant, this step is slowed down roughly ten-fold and thus becomes visible for spectroscopic methods. The given time constants are approximate values at 10 °C. The arrangements of the non-hydrogen atoms schematically shown in (a, c) correspond to crystallographically resolved structures16,46. The structures in panel b, the location of H-atoms, and all the particle movements indicated by arrows have been deduced by structure-based computational chemistry previously10 and thus are more hypothetical than the arrangement of non-hydrogen atoms shown in (a, c).
