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The Modified Q-cycle .

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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.

• oxidation of P and reduction of Q_A (200 ps);
• oxidation of cyt c₂ (<2 µs for bound, ~100 µs for unbound);
• electron transfer to Q_B (or Q_B.-) (30µs);
• uptake of 2H⁺ (<150 µs), exit of QH₂ to the pool, and uptake of Q to form Q_B (<1 ms).

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 Q_oH₂, release of 2H⁺, reduction of cyt b_H (700 µs).
4. Formation of Q_i.- by electron transfer from cyt b_H ( < 1 ms).
5. Binding of quinol at Q_o-site (10 µs - 1 ms, depending on redox state of pool).
6. Oxidation of Q_oH₂, release of 2H⁺, reduction of cyt b_H (700 µs).
7. Reduction of semiquinone by cyt b_H Formation of Q_iH₂ by electron transfer from cyt b_H ( < 1 ms).
8. Exit of QH₂ from Q_i-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 Q_oH₂ in reactions 3, 6), so the faster times for other partial reactions determine the transient kinetics of intermediates.

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 b_H, cyt b_L), Q_A and Q_B 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))