Complex III of Electron Transport Chain: Difference between revisions

<scene name='Complex_III_of_Electron_Transport_Chain/Fes/4'>Fe/S center</scene> is in the head  of each <font color='red'>Rieske protein</font>. Each of the Fe/S centers is complexed with <scene name='Complex_III_of_Electron_Transport_Chain/Fes_2_his/1'>two His</scene>. As a result of bending at the <scene name='Complex_III_of_Electron_Transport_Chain/Fes_hinge/4'>hinge region</scene> (colored cyan) the head can be in one of three possible positions.  Here the Fe/S head is in the <scene name='Complex_III_of_Electron_Transport_Chain/Posit_cytob_closeup/2'>cyto b position</scene> in which a His of the Fe/S/His complex is in contact with the ubiquinol (actually <font color='red'>stigmatellin</font> in this model) bound at the Q_P site of cyto b. Wider view of <scene name='Complex_III_of_Electron_Transport_Chain/Posit_cyto_b/2'>cyto b position</scene>. Notice that the His of the <font color='red'>Risieke head</font> is in contact with <font color='red'>stigmatellin</font> in the Q_P site and the stigmatellin is positioned on a straight line between the two hemes in the cyto c1 subunits. The <scene name='Complex_III_of_Electron_Transport_Chain/Posit_intermed_1bgy/2'>Int position</scene> is shown here with a PDB file<ref>S.IWATA, J.W.LEE,K.OKADA,J.K.LEE, M.IWATA, B.RASMUSSEN, T.A.LINK, S.RAMASWAMY, B.K.JAP, COMPLETE STRUCTURE OF THE 11-SUBUNIT OVINE MITOCHONDRIAL CYTOCHROME BC1 COMPLEX, ''SCIENCE'', '''281''', 64, 1998</ref>  that does not have stigmatellin bound at Q_P, so the surface of the binding sites is shown in orange and the black arrow is pointing to the Q_P pocket.  This pocket is on a straight line between the hemes of cyto c1, as in the previous view, but the Fe/S center is removed from the Q_P binding pocket and is in a position intermediate between the other two positions. In the <scene name='Complex_III_of_Electron_Transport_Chain/Fes_posit_c1_arrow/3'>Cyto c1 position</scene>, the third position, the second His of the Fe/S is in contact with the cyto c1 heme through a hydrogen bond to a carboxylate oxygen of the heme. Black arrow indicates the direction of  movement from Int position to the Cyto c1 position, and the orange arrow indicates the direction of movement from the Int position to the Cyto b position.

<applet load='1kyo_modified.pdb' size='400' frame='true' align='right' scene ='Complex_III_of_Electron_Transport_Chain/View_three_peptides/1' caption='1KYO modified' /> The cycle starts with the binding of UQH₂, ubiquinol, to cytochrome b at a Q_P site. In the applet to the right the Q_P site is binding <font color=red>stigmatellin</font>. This binding causes the Rieske protein to flex at the hinge region rotating the Fe/S head so that the His which is bound to the Fe/S also binds <scene name='Complex_III_of_Electron_Transport_Chain/Posit_cytob_closeup/1'>to the ubiquinol</scene> at Q_P.  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