Involved in the reductive branch of L-leucine fermentation. Required for the activation of (R)-2-hydroxyisocaproyl-CoA dehydratase. The reduced activator transfers one electron to the dehydratase concomitant with hydrolysis of ATP. This protein is extremely sensitive towards oxygen. Binds 1 [4Fe-4S] cluster per dimer. Homodimer. Only in the reduced state, HadI exhibits significant ATPase activity, which appears to be essential for unidirectional electron transfer. Belongs to the HadI activator family.