Kinetics of electron transfer from Q(A) to Q(B) in photosystem II

The oxidation kinetics of the reduced photosystem II electron acceptor Q(A) (-) was investigated by measurement of the chlorophyll fluorescence yield t ransients on illumination of dark-adapted spinach chloroplasts by a series of saturating flashes. Q(A)(-) oxidation depends on the occupancy of the "Q (B) binding site", where this reaction reduces plastoquinone to plastoquino l in two successive photoreactions. The intermediate, one-electron-reduced plastosemiquinone anion Q(B)(-) remains tightly bound, and its reduction by Q(A)(-) may proceed with simple first-order kinetics. The next photoreacti on, in contrast, may find the Q(B) binding site occupied by a plastoquinone , a plastoquinol, or neither of the two, resulting in heterogeneous Q(A)(-) oxidation kinetics. The assumption of monophasic Q(B)(-) reduction kinetic s is shown to allow unambiguous decomposition of the observed multiphasic Q (A)(-) oxidation. At pH 6.5 the time constant for Q(A)(-) oxidation was fou nd to be 0.2-0.4 ms with Q(B) in the site, 0.6-0.8 ms with Q(B)(-) in the s ite, 2-3 ms when the site is empty and Q(B) has to bind first, and of the o rder of 0.1 s if the site is temporarily blocked by the presence Of Q(B)H(2 ) or other low-affinity inhibitors such as carbonyl cyanide m-chlorophenylh ydrazone (CCCP). Effects of pH and H2O/D2O exchange were found to be remark ably nonspecific, No influence of the S-states could be demonstrated.