Complex III of Electron Transport Chain: Difference between revisions

Complex III of the electron transport chain contains as many as 11 subunits per monomer.  The structure shown to the right has 9. (The 'default scene' green link available in the Jmol applet shows the dimer structure along with Heavy Chain (Vh) Of Fv-Fragment, Light Chain (Vl) Of Fv-Fragment and Cytochrome C, Iso-1 all of which are a part of 1KYO.PDB. The link to OCA in the green box below contains additional information on the complete complex and the individual peptide components.) <scene name='Complex_III_of_Electron_Transport_Chain/Labels_applied/3'>Show orientation</scene> of the complex within the inner mitochondrial membrane with labels. <scene name='Complex_III_of_Electron_Transport_Chain/View_one_subunit/3'>Coloring one monomeric unit grey</scene> reveals that one of the <font color='red'>peptides</font> of each subunit invades the space of the other subunit. <scene name='Complex_III_of_Electron_Transport_Chain/View_3_active_subunits/4'>Three of the subunits</scene> of each monomeric unit have a direct role in the passage of electrons in the respiratory chain. The subunits that are colored are active in the electron transport chain. The grey peptides have other catalytic activities and functions, and the interior spaces which are created by the positions of the other subunits have a role in the movement of the substrates from one active site to another active site within the complex. The two subunits of cytochrome b (colored green) for the most part are buried in the complex and have minimal exposure to the intermembrane space and matrix.  <font color='#0000CD'>Cytochrome c1 subunits</font> are positioned on top of cytochrome b and their outer surfaces are exposed to the intermembrane space.  They are held in place by helical tails that extend deep into the complex and membrane. The <font color=red>Rieske subunits</font> are Fe/S proteins with three domains: membrane domain (long helical segment that extends into the membrane), head domain which contains the Fe/S center and hinge domain (short segment between the other two).

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. [http://en.wikipedia.org/wiki/Ubiquinol Ubiquinol] binds at one of the sites, Q_P, and the inhibitor stigmatellin also binds at this site in both cytochrome b's (<font color='red'>Stigmatellin</font> is shown in the applet below.<ref>The structure shown in the second applet was produced by modifying 1KYO.pdb. The Jmol command 'write file' was used to make a pdb file that contained only the 6 active subunits and cytochrome c (c,d,e,n,o,p,w)and the cofactors of those peptides.</ref>)(<scene name='Complex_III_of_Electron_Transport_Chain/Stigmatellin/1' 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_antimycin/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_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

==Footnotes==