<p>Ubiquinol-cytochrome c reductase (bc1 complex or complex III) is an enzyme complex of bacterial and mitochondrial oxidative phosphorylation systems It catalyses the oxidoreduction of the mobile redox components ubiquinol and cytochrome c, generating an electrochemical potential, which is linked to ATP synthesis [<cite idref="PUB00001341"/>, <cite idref="PUB00001342"/>]. The complex consists of three subunits in most bacteria, and nine in mitochondria: both bacterial and mitochondrial complexes contain cytochrome b and cytochrome c1 subunits, and an iron-sulphur `Rieske' subunit, which contains a high potential 2Fe-2S cluster [<cite idref="PUB00002441"/>].The mitochondrial form also includes six other subunits that do not possess redox centres. Plastoquinone-plastocyanin reductase (b6f complex), cyanobacteria and the chloroplasts of plants, catalyses the oxidoreduction of plastoquinol and cytochrome f. This complex, which is functionally similar to ubiquinol-cytochrome c reductase, comprises cytochrome b6, cytochrome f and Rieske subunits [<cite idref="PUB00004558"/>].</p><p>The Rieske subunit acts by binding either a ubiquinol or plastoquinol anion, transferring an electron to the 2Fe-2S cluster, then releasing the electron to the cytochrome c or cytochrome f haem iron [<cite idref="PUB00001341"/>, <cite idref="PUB00004558"/>]. The 2Fe-2S cluster is bound in the highly conserved C-terminal region of the Rieske subunit.</p>