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

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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 b_L)
add high potential hemes from cytochrome b₅₆₆ (cyto b_H)
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.