Complex III of Electron Transport Chain: Difference between revisions

Each cytochrome b contains<scene name='Complex_III_of_Electron_Transport_Chain/Hem_cyto_b/5'> two hemes</scene> (displayed as spacefill and colored cpk). Identify each of the hemes by toggling off the spin and hovering the curser over an atom of the heme.  Hem 501 and Hem 502 are in one cytochrome b, and Hem 521 and Hem 522 are in the other one.  The two hemes in each cytochrome b are in different environments and therefore have different properties, e.g. reduction potential. Hemes 501 & 521 have a lower potential than the other two and are called b_L for low potential, and the other two are called b_H for high potential. Each of the cytochrome b's have two binding sites for substrate. Ubiquinol binds at one of the sites, Q_P, and the inhibitor <font color='red'>stigmatellin</font> also binds at this site in both cytochrome b's (<font color='red'>stigmatellin</font> seen in the applet below)(<scene name='Complex_III_of_Electron_Transport_Chain/Sma_cyto_b1/2' target='second'>return to view of the stigmatellin</scene>), and the site is adjacent to the b_L heme. The other site, Q_N, binds [[Coenzyme_Q10|ubiquinone]], and <scene name='Complex_III_of_Electron_Transport_Chain/Surface_cn/1' target='second'>this surface</scene> outlines this site which is adjacent to the b_H heme.  In this view you are looking into the lit pocket in which the ubiquinone binds. You can rotate the structure and observe the binding pocket in the other subunit.

<applet load='1kyo' size='400' color='black' frame='true' align='right' scene ='Complex_III_of_Electron_Transport_Chain/Sma_525_red/2' name='third'/>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 to the <scene name='Complex_III_of_Electron_Transport_Chain/Posit_cytob_closeup/1'>UQH₂ at Q_P</scene>.  Binding of the His to UQH₂ reduces its pK, and the quinol loses a proton.  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/2'>diffuses to the intermembrane space</scene>.