Modified Q-cycle mechanism for photosynthetic chain of Rb. sphaeroides.
Partial reactions of the bc-complex in the cycle
Reactions are as follows:
Photochemical and rapid secondary reactions introduce the substrates for the bc-complex in ~150 µs.
The times shown are reaction half-times, either derived from direct measurement of rate constants (in bold), or by estimation from the kinetic characteristics. For the bc-complex, the rate of turn-over is determined by the rate limiting step (oxidation of QoH2 in reactions 3, 6), so the faster times for other partial reactions determine the transient kinetics of intermediates. The Table below summarizes other properties
Component Em,7 (mV) z n pH dependence Notes
cyt c1 270 1 0 none in physiological range
FeS300 1 1 -60 above pK on oxidized form at 7.6
cyt bH 50 1 1 -60 below pK on reduced form at 7.8
cyt bL -90 1 ~0.5 -30 over pH range 6 to 8
Q/QH2 90 2 2 -60 over physiological pH range
P+/P 450 1 0 none in physiological range
QA 0 1 1 -60 below pK on reduced form at 8.9
cyt c2 340 1 0 none in physiological range
At Eh ~ 100 mV and pH 7.0, the components of the high potential chain (P, cytc2, cyt c1, FeS) are all reduced, the cytochrome b chain (cyt bH, cyt bL), QA and QB are oxidized, and the quinone pool is about 30% reduced.
(Z = -2.303.n.RT/z.F ~ -60 x n/z (mV per pH unit increase at 30°C))