Figure 5.

Putative superoxide-dependent proton shuttling mechanism. Black dashed lines indicate the proton transfer relay during catalysis. Red dashed line indicates the proton transfer from WAT1 to Asp159 that results in the inactive complex whereas the backwards transfer restores activity of the enzyme (model based on PDB entry 5T30). Green dashed lines indicate stabilizing hydrogen-bond interactions for molecules of the proton relay. Of note, superoxide is thought to bind Mn³⁺ directly but not Mn²⁺, where it is instead bound to the active site solely through hydrogen bonding to members of the hydrogen-bond network. Single letter amino acid code is used.