Cytochrome bc1 (also known as complex III) is an enzyme of electron transport chain and is found in both bacteria and mitochondria. It has an important role in cellular respiration as it contributes to the generation of the proton motive force which is later used for ATP synthesis. It does this by coupling oxidation and reduction of quinones (commonly called Q-cycle) to proton translocation across inner mitochondrial or bacterial plasma membrane. Stable semiquinone is formed in one of the catalytic sites of cytochrome bc1 (Qi site) during its enzymatic activity. We analysed mutants of cytochrome bc1 that were thought to have even more stable semiquinone than wild type due to steric hinderance introduced at the Qi site. Experiments performed on Rhodobacter capsulatus mutants indicate that during reverse reaction hemes bH get reduced more than in the same reaction in wild type. However, despite this no semiquinone signal could be detected at Qi site in those mutants.

Citokrom bc1 (također poznat i kao kompleks III) je enzim transportnog lanca elektrona te ga se može naći u bakterijama i mitohondrijima. Ima važnu ulogu u staničnom disanju, budući da doprinosi stvaranju elektrokemijskog gradijenta protona koji se kasnije koristi za sintezu ATP-a. To čini tako da spaja oksidaciju i redukciju kinona (zajednički nazvane Q ciklus) sa premještanjem protona preko unutarnje membrane mitohondrija ili plazma membrane bakterija. Tijekom ove enzimatske aktivnosti, stabilni semikinon se stvara u jednom od katalitičkih mjesta citokroma bc1 (Qi mjesto). Analizirali smo mutante citokroma bc1 za koje se smatralo da bi mogli imati još stabilniji semikinon nego divlji tip zbog prostornih smetnji induciranih u Qi mjestu. Pokusi rađeni na Rhodobacter capsulatus mutantima ukazuju da su tijekom povratne reakcije hemovi bH reducirani više nego tijekom iste reakcije u divljem tipu. Ipak, unatoč tome nije bilo moguće opaziti nikakav signal semikinona u Qi mjestu u tim mutantima.