Scientists from the Faculty of Biochemistry, Biophysics and Biotechnology (WBBiB) and the Małopolska Centre of Biotechology (MCB) of the Jagiellonian University Krakow teamed up to disclose intriguing features of the energy conserving machinery in bacterial cells at the atomic level.
Using cryo-electron microscopy (cryo-EM), the UJ scientists have captured the high-resolution structure of the entire cytochrome bc1 complex together with native molecules of ubiquinone, also known as coenzyme Q, bound at its catalytic sites. The structures revealed two distinct modes of coenzyme Q binding, reflecting the different architecture and functional properties of the two catalytic sites. These findings provide new insights into the molecular mechanisms that enable efficient catalysis by cytochrome bc1. The seminal study was published recently in Proceedings of the National Academy of Sciences USA.
Coenzyme Q – Head and Tail
Cytochrome bc1 plays a central role in cellular respiration and bacterial photosynthesis, where it catalyzes reactions involving coenzyme Q. The coenzyme Q molecule consists of a head group and a long hydrophobic tail. The head group contains the chemical groups required for catalytic reaction, while the tail allows the molecule to remain mobile within the lipid membrane in which cytochrome bc1 is embedded. In cells the reactions involving coenzyme Q are essential to conserve energy and subsequently use it to drive different molecular processes. Cytochrome bc1 contains two distinct catalytic sites for coenzyme Q, which operate in fundamentally different ways. One of these sites undergoes transition between active and inactive state due to the movement of a protein domain containing an iron–sulfur cluster. This movement positions the cluster close or distant from the catalytic site, which renders the site active or inactive, respectively.
The structures obtained in this study reveal how coenzyme Q is handled by this dynamic catalytic site. When the site is active the coenzyme Q molecule extends its tail into a cleft running through the complex, while its head group enters a pocket where it becomes firmly positioned close to the iron–sulfur cluster.
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Image: Structure of cytochrome bc1 with highlighted position of the part of coenzyme Q (orange sticks) and its head group in the active state of the catalytic site.