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Moderator: Wolfgang Lubitz
Max Planck Institute for Chemical Energy Conversion, Germany

News From Structural Biology – Terminal Oxidases, by Hartmut Michel

Our main interest lies in understanding the mechanism of action of terminal oxidases and their biosynthesis. Terminal oxidases are the final enzymes of the respiratory chains reducing molecular oxygen (dioxygen) to water. They are integrated into membranes. Due to the usage of protons from one side of the membrane and of electron providing substrates from the opposite side they create electric voltages (“membrane potentials“) and pH gradients across the membrane which are e.g. used by ATP synthases to produce ATP. There are two major classes: the heme-copper containing terminal oxidases (“HCOs“) and the bd oxidases. The HCOs, in contrast to the bd oxidases, are able to pump protons across the membrane thus doubling the energy yield.

We have used single particle electron cryomicroscopy to determine the structures of all intermediates of the catalytic cycle of the cytochrome c oxidase from the soil bacterium Paracoccus denitrificans. Our results are not compatible with the general expectations. The oxidized form of the enzyme appears to contain a peroxide bridging the copper CuB and the heme a3 iron, whereas the so-called two-electron reduced P-state appears to contain a dioxygen molecule and the three-electron reduced F-state, a superoxide radical.

We have determined the structures of a number of bd oxidases among them the one from the tuberculosis causing enzyme of Mycobacterium tuberculosis. We aim to develop inhibitors against this enzyme which might be used to cure tuberculosis. In addition, the group has determined the structure of the ABC transporter CydDC which appears to catalyze the incorporation of hemes of the B-type into the maturing bd oxidase. Attempts to predict the structures of CydDC with and without hemes using Alphafold3 were unsatisfactory. The predictions yield one structure which is intermediate between the experimentally determined structures with and without heme which are significantly different. The presence or absence of the substrate heme has no influence on the predicted structures of CydDC.

Hartmut Michel, Schara Safarian, Di Wu, Daniel Hatlem, Haoran Fu
Max Planck Institute of Biophysics, Frankurt am Main, Germany