Complex III:
Coenzyme C/Cytochrome C Reductase

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I. Introduction

Complex III accepts 4 electrons from 2reduced ubiquione, UQH2, and donates them to oxidized cytochrome C.  In the process it pumps 4H+s to the inner membrane space.  A variety of electron carriers in the complex, including hemes and an FeS Rieske cluster, mediate electron flow. 


For more information see Biochemistry Online: Chapter 8C-6 - Complex III

II. General Structure

Electron flow occurs from mobile but lipophilic reduced UQH2 to mobile but soluble cytochrome C in single electron steps.  At the same time, the two protons from the UQH2 are moved to the inner membrane space of the mitochondria. 

These process are repeated twice, using 2 UQH2s, in a process called the Q cycle.



Cartoon of Complex III without electron carriers

Now let's sequentially add the bound electron carriers which receive electrons from the mobile lipophilic UQH2 donor and deliver it to the mobile soluble receiver, cytochrome C

add the Rieske Fe2S2 Centers
add hemes from cytochrome C1
add low potential hemes from cytochrome b562 (cyto bL)
add high potential hemes from cytochrome b566 (cyto bH)
add cytochrome C - the soluble mobile electron carrier with its own heme

Note the proximity of cytochrome c1 to cytochrome C implicates the c1 heme as the electron donor to the heme in cytochrome C.

add stigmatellin A (green spacefill), an inhibitor of UQH2 transfer of electrons. It presumably shows the binding site for UQH2. Note its proximal location to the Rieske FeS center and to the low potential heme of cytochrome b562 .

This supports the Q cycle in that ubiquinone is in position to interact with both teh FeS Rieske centers and the low potential heme of cytochrome b562.