Abstract Alternative Complex III (ACIII) is an example of the robustness and flexibility of prokaryotic respiratory chains. It performs quinol:cytochrome c oxidoreductase activity, being functionally equivalent to the bc₁ complex but structurally unrelated. In this work we further explored ACIII investigating the role of its monoheme cytochrome c subunit (ActE). We expressed and characterized the individually isolated ActE, which allowed us to suggest that ActE is a lipoprotein and to show its function as a direct electron donor to the caa₃ oxygen reductase.