Complex III of Electron Transport Chain: Difference between revisions

<applet load='1bgy_modified.pdb' size='400' frame='true' align='right' scene ='Complex_III_of_Electron_Transport_Chain/Posit_intermed_1bgy_arrowless/1'  /> At the start of the cycle the Q_P site of cytochrome b is empty and the Fe/S center of the Rieske protein is in the 'Int position'. (<scene name='Complex_III_of_Electron_Transport_Chain/Posit_intermed_1bgy_arrowless/1'>Reset initial scene.</scene>) With the binding of <scene name='Complex_III_of_Electron_Transport_Chain/Posit_intermed_1bgy_cycle/2'>ubiquinol</scene>, UQH₂, to the Q_P site (black arrow) of cytochrome b the Rieske protein moves to the 'cyto b position' by flexing at the hinge region and rotating the Fe/S head so that the His which is bound to the Fe/S also binds to <scene name='Complex_III_of_Electron_Transport_Chain/Posit_cyto_b_cycle/1'>ubiquinol</scene> (<font color=red>stigmatellin</font> in this model)<ref name=1KYOmodified/>.  Binding of the His to UQH₂ reduces its pK, and the UQH₂ loses a proton to become UQH^{<big> -</big>}<ref>External link to structure of [http://en.wikipedia.org/wiki/Ubiquinol UQH^{<big> -</big>}]</ref>.  The position of Q_P in the complex is such that the proton which is lost <scene name='Complex_III_of_Electron_Transport_Chain/Posit_cyto_b_cycle_arrow/1'>diffuses to the intermembrane space</scene>. After UQH₂ loses the proton and becomes UQH^-, it passes an electron through the His to the Fe⁺³ reducing it to Fe⁺².  With the loss of the electron the UQH^{<big> -</big>} becomes UQH<big>^•</big>, a [http://en.wikipedia.org/wiki/Ubiquinol semiquinone], which loses a proton and becomes UQ^{<big> • -</big>}, the conjugate base of the semiquinone.  The proton diffuses to the intermembrane space, as the first one did.  (The fate of the semiquinone can be traced starting with the next paragraph.) After Fe in the Fe/S center is reduced by the UQH^{<big> -</big>}, the Rieske head rotates & the Fe/S moves to cytochrome c1, <scene name='Complex_III_of_Electron_Transport_Chain/Posit_cyto_b_cycle_move_c1/1'>the "c1" position</scene>, so that the second His bound to Fe/S binds to the heme of cytochrome c1. When the His contacts the heme of cytochrome c1 an electron is rapidly passed from the Fe/S through the His to the Fe of the cytochrome c1 heme, and since it is now in the oxidized form, the Rieske protein returns to the "Int" position. The cytochrome c1 heme is now reduced, and when <scene name='Complex_III_of_Electron_Transport_Chain/Cyto_c_2_cycle/1'>cytochrome c binds</scene> to it the electron is passed from the c1 heme to the c heme (black arrow). The cytochrome c then releases from the membrane and diffuses through the intermembrane space to Complex IV.  

Returning to UQ^{<big> • -</big>}, the conjugate base of the [http://en.wikipedia.org/wiki/Ubiquinol semiquinone], which was formed at Qp as described above and is shown <scene name='Complex_III_of_Electron_Transport_Chain/Focus_semiuq/1'>here</scene> as <font color=red>stigmatellin</font>, the UQ^{<big> • -</big>} is oxidized to the full UQ when it <scene name='Complex_III_of_Electron_Transport_Chain/Electron_semi_to_hem_l/3'>passes an electron</scene> to heme b_L. The <scene name='Complex_III_of_Electron_Transport_Chain/Electron_hem_l_to_hem_h/1'>electron</scene> is then passed from the Fe of heme b_L to the Fe of Heme b_H, and with Heme b_H being next to UQ bound at the Q_n site (Binding site is located on the <scene name='Complex_III_of_Electron_Transport_Chain/Hem_h_next_to_surface/3'> surface</scene> with a black arrow.), the <scene name='Complex_III_of_Electron_Transport_Chain/Electron_hem_h_to_uq/3'>electron</scene> is passed to UQ. With only one electron being passed in this series of reaction, the UQ is reduced to UQ^{<big> • -</big>}, and when it accepts a <scene name='Complex_III_of_Electron_Transport_Chain/Proton_matrix_in/1'>proton</scene> which comes from the matrix it becomes UQH<big>^•</big>. The end products of the first half of the Q cycle are UQ at the Q_p site, reduced cyt c and semi-ubiquinone at the Q_n site.