The Modified Q-cycle .


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.

  1. Oxidation of the high potential chain of the bc-complex by cyt c2(150 - 200 µs).
  2. Binding of QH2 to the bc-complex (10 µs - 2 ms, depending on concentration of QH2 in pool).
  3. Oxidation of QoH2, release of 2H+, reduction of cyt bH (700 µs).
  4. Formation of Qi.- by electron transfer from cyt bH ( < 1 ms).
  5. Binding of quinol at Qo-site (10 µs - 1 ms, depending on redox state of pool).
  6. Oxidation of QoH2, release of 2H+, reduction of cyt bH (700 µs).
  7. Reduction of semiquinone by cyt bH Formation of QiH2 by electron transfer from cyt bH ( < 1 ms).
  8. Exit of QH2 from Qi-site to the pool, and uptake of Q (10 - 500 µs, depending on redox state of pool).

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

Reaction # in the Table corresponds to the reaction identified by letter in the Scheme below, which shows the simplified Q-cycle as it operates in the presence of antimycin. The two-electron gate at the Qi-site is blocked by antimycin, which displaces all quinone species by occupying the binding site.

The redox centers have the following properties.

 
Component      Em,7 (mV)      z       n       pH dependence  Notes
                                              (Z)
bc1-complex
cyt c1         270            1       0       none in physiological range
FeS            300            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
 
quinone pool
Q/QH2            90            2       2       -60     over physiological pH range
 
reaction center
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

Notes:

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))