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 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/2'>ubiquinol</scene>, UQH₂, 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/2'>binds to the ubiquinol</scene> at Q_P (<font color=red>stigmatellin</font> in this model).  Binding of the His to UQH₂ reduces its pK, and the [http://en.wikipedia.org/wiki/Ubiquinol UQH₂] loses a proton to become UQH.  The position of Q_P in the complex is such that the proton which is lost <scene name='Complex_III_of_Electron_Transport_Chain/Proton_releas/3'>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/Ubiquinone#Chemical_properties 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) After Fe is reduced by the UQH^{<big> -</big>}, the Rieske head rotates & the Fe/S head moves to cytochrome c1, <scene name='Complex_III_of_Electron_Transport_Chain/Move_cytob_to_cytoc1/1'>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