Patch-clamp study on flash-induced secondary electrogenic transport in the thylakoid membrane. Interpretation in terms of a Q-cycle

Photo-electrical signals are studied using a patch-clamp method on single chloroplasts of Peperomia metallica in the so-called 'whole-thylakoid' configuration. The thylakoid transmembrane potential after a single-turnover flash shows multiphasic kinetics which can be described as the sum of two components: (1) Reaction R1/RC associated with fast primary charge separation in the reaction centers of photosystems I and II and (2) Reaction R1/Q, a slow rising component most likely associated with secondary electrogenic steps within the cyt, b(6)f complex. The latter assignment is supported by the inhibition of the slow rising component by 2.5 mu M DBMIB. The dark relaxation time of both R1/RC and R1/Q associated with the field dissipative back flow of ions was independently and unambiguously obtained from the discharge RC-time after current-injection. Reaction R1/Q shows sigmoid rise kinetics which is modelled by two populations of cyt. b(6)f complexes incorporating a 2-step consecutive reaction scheme. The average time constants associated with the two steps are 13 ms and 28 ms and might be related to plastoquinol oxidation at the lumenal interface and plastoquinol formation at the stromal interface, respectively. The ratio of charges displaced by R1/Q and R1/RC (Q/RC-ratio) varied between 0.15 and 0.95 depending, amongst others, on pre-treatment with flashes.