The Modified Q-cycle

A slice through the photosynthetic chain


A Working Model of the Modified Q-cycle (some assembly required).
Click HERE for a working model in animated GIF format (Netscape >2.0 required), or click here for the model in AVI movie format.
Click here for a 3-D movie of the Qo-site mechanism, based on the structure.

Click here for An animated Q-cycle model with the bc1 complex in Chime.

A "portable" version of the simple model can be set up using the Fig. above (click with the right mouse button to download image), and a pocketfull of change.

The Model

In the model, the enzymes of the photosynthetic electron transfer chain of Rhodobacter sphaeroides are shown as present in a stoichiometry of 2 reaction centers, 1 cytochrome c2 and 1 bc1-complex. The quinone pool (20-40 Q/ bc1-complex) is nominally represented. The redox centers are indicated by circles. For a "portable" model, download the image above by clicking on it with the right mouse-button, save the gif file, and print it. In order to use the model, you will need an assortment of coins of different sizes, for example six quarters (or two if you just represent the pool quinones), six nickels, six (plus) pennies. The quarters will represent quinone (Q), nickels are protons (H+), pennies are electrons (e-).

Initial conditions.

In the example we assume an initial redox poise (Eh ~ 100 mV) with the high potential components reduced (P, cyt c2, cyt c1, and FeS), the cyt b chain and the quinones of the reaction center oxidized, and the quinone pool partly reduced.

Place a penny on each of the high potential components, a quarter on each Q circle, and reduce one pool Q with a pair of nickels and pennies (the reaction center quinones don't need quarters if you don't have enough!). The (#) symbol shows when to move the coins appropriately.

Mechanism of quinol (QH2) oxidation.

The system is activated by light which excites an electron from P to QA (#) in each reaction center. The electrons on the two QA acceptor quinones are transferred to Q through the two-electron gate of the QB-site, producing QH2, with uptake of 2H+ from the N-phase (#).

The two electron-holes on P+ are filled by electron transfer from cyt c2, which shuttles back to the bc1-complex, transferring two electrons from FeS and cyt c1 (#).

QH2 binds at the Qo-site (#). Oxidation involves transfer of one electron to FeS (shared with cyt c1) and the other to cyt bL, and release of 2H+ into the P-phase (#). The electron on cyt bL is passed rapidly to cyt bH (#). The Q exits the Qo-site into the pool (#).

Q from the pool binds at the Qi-site, and is reduced to a semiquinone, with uptake of a H+ from the N-phase (#).

A second QH2 binds at the Qo-ste, and is oxidized as before (#). This completes the re-reduction of the high potential chain, and passes a second electron into the b-cytochrome chain.

The second electron in the b-cytochrome chain reduces the semiquinone with uptake of H+ from the N-phase.

Next turn-over

You have now completed one turn-over of the cycle. You should now be back in the starting state, but with 4H+ transferred from N-phase to P-phase.

You can explore the effects of changing redox poise by varying the initial conditions (see below).

You can explore the effects of inhibitors by placing an immovable foreign coin at the appropriate site: antimycin, funiculosin, HQNO, or diuron (in yeast) at the Qi-site; myxothiazol, mucidin, stigmatellin, or UHDBT at the Qo-site; o-phenanthroline, atrazine, or terbutryne at the QB-site.

Reactions of the Q-cycle

Initial conditions.

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 at 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) is oxidized, and the quinone pool is about 30% reduced.

Over the Eh range below 300 mV, the system behaves as if 1 reaction center has Q and the other QH.- at the QB-site. As a result, following the photochemical activation, one reaction center reduces QH.- to QH2, which is then exchanged with Q in the pool; the other reaction center reduces Q to QH.- which remains bound at the site.

In the model, the redox poise of the system can be set by populating the circle representing the redox centers by either 1 e-, 1e- + 1H+, or 2e- + 2H+, as appropriate.

(Z = -2.303.n.RT/z.F ~ -60 x n/z (mV per pH unit increase at 30°C))