The long Q-loop of Escherichia coli cytochrome bd oxidase is required for assembly and structural integrity.

Cytochrome bd-I oxidase is a terminal reductase of bacterial respiratory chains produced under low oxygen concentrations, oxidative stress, and during pathogenicity. While the bulk of the protein forms transmembrane helices, a periplasmic domain, the Q-loop, is expected to be involved in binding and oxidation of (ubi)quinol. According to the length of the Q-loop, bd oxidases are classified into the S (short)- and the L (long)-subfamilies. Here, we show that either shortening the Q-loop of the Escherichia coli oxidase from the L-subfamily or replacing it by one from the S-subfamily leads to the production of labile and inactive variants, indicating a role for the extended Q-loop in the stability of the enzyme.