Manganese-dependent iron-superoxide dismutase drives Acinetobacter baumannii fitness during oxidative stress

Superoxide dismutase (SOD), a widely distributed metalloenzyme across all domains of life, mitigates the harmful effects of reactive oxygen species generated during oxidative stress. The catalytic activity of SOD depends on specific metal cofactor, which is determined by bioavailability, structural compatibility, and environmental factors. The nosocomial pathogen Acinetobacter baumannii has been able to thrive under oxidative stress with SODs imparting a major role in curbing this distress. However, the functional role of two encoded SODs, namely SodB and SodC in A. baumannii, is poorly understood in mitigating oxidative stress. Furthermore, the metal ion specificities of these SodB and SodC families exemplify a knowledge gap, as the metal ion utilized by individual family members cannot be reliably predicted. Our study unveils the specific metal cofactors utilized by SodB and SodC and their role in quenching host-mediated oxidative stress during infection by A. baumannii 5075 (AB5075), a hypervirulent and multidrug-resistant strain. The study reveals that SodB primarily utilizes Mn²⁺, whereas SodC employs Cu²⁺ to achieve optimal catalytic efficiency in superoxide dismutation. The oxidative stress response in AB5075 favors SodB over SodC, highlighting the critical role of Mn-dependent SodB in counteracting oxidative stress. Furthermore, we demonstrated that mutations in metal-binding residues (SodB^H28A, SodB^D164A-H168A, SodC^H87A-H89A, and SodC^H165A) led to significant impairment of SOD activity, thus highlighting the importance of these residues in catalytic function and preference for metal ion cofactor. The study shows that SodB has identical metal ion-binding residues for both Fe²⁺ and Mn²⁺ but is only active with Mn²⁺ ion.