Complex III of Electron Transport Chain: Difference between revisions
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<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<sub>P</sub> site 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'>Reload initial scene</scene>) With the binding of <scene name='Complex_III_of_Electron_Transport_Chain/Posit_intermed_1bgy_cycle/2'>ubiquinol</scene>, UQH<sub>2</sub>, to cytochrome b (black arrow) the Rieske protein flexes at the hinge region rotating the Fe/S head so that the His which is bound to the Fe/S also <scene name='Complex_III_of_Electron_Transport_Chain/Posit_cyto_b_cycle/1'>binds to the ubiquinol</scene> at Q<sub>P</sub> (<font color=red>stigmatellin</font> in this model). Binding of the His to UQH<sub>2</sub> reduces its pK, and the [http://en.wikipedia.org/wiki/Ubiquinol UQH<sub>2</sub>] loses a proton to become UQH<sup><big> -</big></sup>. The position of Q<sub>P</sub> 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 UQH<sub>2</sub> loses the proton and becomes UQH<sup> -</sup>, it passes an electron through the His to the Fe<sup>+3</sup> reducing it to Fe<sup>+2</sup>. With the loss of the electron the UQH<sup><big> -</big></sup> becomes UQH<big><sup> •</sup></big>, a [http://en.wikipedia.org/wiki/Ubiquinone#Chemical_properties semiquinone], which loses a proton and becomes UQ<sup><big> • -</big></sup>, 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 is reduced by the UQH<sup><big> -</big></sup>, the Rieske head rotates & the Fe/S head 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. | <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<sub>P</sub> site 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'>Reload initial scene</scene>) With the binding of <scene name='Complex_III_of_Electron_Transport_Chain/Posit_intermed_1bgy_cycle/2'>ubiquinol</scene>, UQH<sub>2</sub>, to cytochrome b (black arrow) the Rieske protein flexes at the hinge region rotating the Fe/S head so that the His which is bound to the Fe/S also <scene name='Complex_III_of_Electron_Transport_Chain/Posit_cyto_b_cycle/1'>binds to the ubiquinol</scene> at Q<sub>P</sub> (<font color=red>stigmatellin</font> in this model). Binding of the His to UQH<sub>2</sub> reduces its pK, and the [http://en.wikipedia.org/wiki/Ubiquinol UQH<sub>2</sub>] loses a proton to become UQH<sup><big> -</big></sup>. The position of Q<sub>P</sub> 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 UQH<sub>2</sub> loses the proton and becomes UQH<sup> -</sup>, it passes an electron through the His to the Fe<sup>+3</sup> reducing it to Fe<sup>+2</sup>. With the loss of the electron the UQH<sup><big> -</big></sup> becomes UQH<big><sup> •</sup></big>, a [http://en.wikipedia.org/wiki/Ubiquinone#Chemical_properties semiquinone], which loses a proton and becomes UQ<sup><big> • -</big></sup>, 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 is reduced by the UQH<sup><big> -</big></sup>, the Rieske head rotates & the Fe/S head 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. | ||
The UQ<sup><big> • -</big></sup>, the conjugate base of the semiquinone, which was formed at Qp (see above) is oxidized to the full UQ when it <scene name='Complex_III_of_Electron_Transport_Chain/Electron_semi_to_hem_l/2'>passes an electron</scene> to heme b<sub>L</sub>. 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<sub>L</sub> to the Fe of Heme b<sub>H</sub> | The UQ<sup><big> • -</big></sup>, the <scene name='Complex_III_of_Electron_Transport_Chain/Focus_semiuq/1'>conjugate base</scene> of the semiquinone, which was formed at Qp (see above) is oxidized to the full UQ when it <scene name='Complex_III_of_Electron_Transport_Chain/Electron_semi_to_hem_l/2'>passes an electron</scene> to heme b<sub>L</sub>. 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<sub>L</sub> to the Fe of Heme b<sub>H</sub>, and with Heme b<sub>H</sub> being next to UQ bound at the Q<sub>n</sub> site (Binding site is shown <scene name='Complex_III_of_Electron_Transport_Chain/Hem_h_next_to_surface/2'>here</scene> as a surface.), the <scene name='Complex_III_of_Electron_Transport_Chain/Electron_hem_h_to_uq/1'>electron</scene> is passed to UQ. With only one electron being passed in this series of reaction the UQ is reduced to UQ<sup><big> • -</big></sup>, and becomes UQH<big><sup> •</sup></big> when it accepts a proton which comes from the matrix. The end products of the first half of the Q cycle are an ubiquinol oxidized to ubiquinone at the Q<sub>p</sub> site, a reduced cyt c and a ubiquinone reduced to semi-ubiquinone at the Q<sub>n</sub> site. | ||
==Footnotes== | ==Footnotes== | ||
<references/> | <references/> | ||
