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 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, ''Science'', '''281''', 64, 1998</ref><ref name=1BGYmodified>1BGY.pdb was modified to contain only the six active subunits (chains c, d, e, o, p, q) and their cofactors.  1BGY.pdb contains no cofactors bound at Q_P or Q_N so these binding sites are shown as one large open surface which outlines all four binding sites as pockets.</ref>  that does not have stigmatellin bound at Q_P, 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 the Q_P site was positioned in the previous view of the 'cyto b position', but the Fe/S center is not in contact with the Q_P binding pocket and is in a position intermediate between the cyto b and cyto c1 positions. In the 'Cyto c1 position', 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, but since it can not be shown directly, <scene name='Complex_III_of_Electron_Transport_Chain/Posit_c1_1bgy/1'>here</scene> a black arrow indicates the direction of  movement from 'Int position' to the 'Cyto c1' position, and an orange arrow indicates the direction of movement from the 'Int position' to the 'Cyto b position'.

<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, at Q_P site of 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_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^{<big> -</big>}.  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/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 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.