Complex III- Q-Cytochrome c Oxidoreductase (Cytochrome c reductase)

Step 1

When QH2 is oxidized, it donates 2 e- into the complex

Step 1a

One e- is sent to an iron-sulfur cluster, then to Cyt c1, and finally to Cytc. 2 protons (from QH 2) are transported out.

Step 1b

The remaining Q (ubiquinone) becomes part of the “Q pool”

Step 1c

One e- is sent to Cyt bH (b 566), then to Cyt bL (b 562), and finally back to Q (from the “Q pool”) to generate Q•-, the semi-quinone radical. 2 protons are pumped out.

Step 2

A second QH 2 enters the complex and is oxidized, delivering 2 e- to the complex.

Step 2a

One e- is sent to an iron-sulfur cluster, then to Cyt c1 , and finally to Cyt c. 2 protons (from QH2) are transported out

Step 2b

The remaining Q (ubiquinone) becomes part of the “Q pool”

Step 2c

One e- is sent to Cyt bH (b 566) and then to Cyt bL (b 562). However, the e- is transported to the Q•- (from the first half of the Q cycle). The Q•- is reduced to QH 2 by accepting 2 protons from the matrix.