Subunit CcoQ is involved in the assembly of the Cbb-type cytochrome c oxidases from Pseudomonas stutzeri ZoBell but not required for their activity.

The Cbb-type cytochrome c oxidases (Cbb-CcOs), the second most abundant CcOs, catalyze the reduction of molecular oxygen to water, even at micromolar oxygen concentrations. In Pseudomonas stutzeri ZoBell, two tandemly organized cbb-operons encode the isoforms Cbb-1 and Cbb-2 both possessing subunits CcoN, CcoO and CcoP. However, only the cbb-2 operon contains an additional ccoQ gene. CcoQ consists of 62 amino acids and is predicted to possess one transmembrane spanning helix. The physiological role of CcoQ was investigated based on a CcoQ-deletion mutant and wild-type Cbb-2 crystals not containing subunit CcoQ. Cbb-2 isolated from the deletion mutant is inactive and appears as a dispersed band on blue native-PAGE gels. Surprisingly, in the absence of ccoQ, Cbb-1 also shows a strongly reduced activity. Our data suggest that CcoQ primarily functions as an assembly factor for Cbb-2 but is also required for correct assembly of Cbb-1. In contrast, once correctly assembled, Cbb-1 and Cbb-2 possess a full enzymatic activity even in the absence of CcoQ.