Integral role of the I′-helix in the function of the “inactive” C-terminal domain of catalase-peroxidase (KatG)

► The C-terminal domain may use its I′-helix to adjust N-terminal domain structure. ► I′-helix variants of intact KatG had increasingly reduced stability and activity. ► Multiple substitutions gave properties of the stand-alone N-terminal domain (KatGN). ► I′-helix variants of KatGC had diminished capacity to reactivate KatGN. ► KatGN reactivation by KatGC is valuable for exploring KatG structure and function.