Evolution of quinol oxidation within the heme‑copper oxidoreductase superfamily.

The heme‑copper oxidoreductase (HCO) superfamily is a large superfamily of terminal respiratory enzymes that are widely distributed across the three domains of life. The superfamily includes biochemically diverse oxygen reductases and nitric oxide reductases that are pivotal in the pathways of aerobic respiration and denitrification. The adaptation of HCOs to use quinol as the electron donor instead of cytochrome c has significant implication for the respiratory flexibility and energetic efficiency of the respiratory chains that include them. In this work, we explore the adaptation of this scaffold to two different electron donors, cytochromes c and quinols, with extensive sequence analysis of these enzymes from publicly available datasets. Our work shows that quinol oxidation evolved independently within the HCO superfamily at least seven times. Enzymes from only two of these independently evolved clades have been biochemically well-characterized. Combining structural modeling with sequence analysis, we identify putative quinol binding sites in each of the novel quinol oxidases. Our analysis of experimental and modeling data suggests that the quinol binding site appears to have evolved at the same structural position within the scaffold more than once.

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