Metal specificities and catalytic activities of the two superoxide dismutases of Staphylococcus aureus

Abstract

The superoxide dismutase (SOD) metalloenzymes play a major role in the cellular oxidative stress defence systems of microorganisms including that of an important pathogenic bacterium Staphylococcus aureus. Staphylococcus aureus, unlike other staphylococci, possesses two isozymes of the Fe/Mn-dependent SOD superfamily, designated SodA and SodM, both of which are predicted to utilise manganese as their essential metal cofactor. The two SODs are critical for the resistance to reactive oxygen species (ROS) and contribute to the pathogenicity of S. aureus. The immune system can utilise a Mn-restriction as one of its defence mechanism against pathogens, suggesting a possibility that one of the S. aureus SODs can use another metal in order to overcome this host-imposed Mn-starvation. To clarify the metal requirements of the two S. aureus SODs, the recombinant Fe- and Mnmetalated isoforms of S. aureus SodA and SodM were produced. All four forms were characterised as catalytically active, regardless of utilised metal. The relative activity analysis showed that SodA exhibits a strong metal preference of Mn over Fe, whereas SodM presented highly cambialistic properties, i.e. it was equally active with either Mn or Fe. Crystal structures of all four forms of the S. aureus SOD proteins were solved and showed a high level of identity. Structure-based mutagenesis led to the successful swapping of catalytic properties between the two proteins, yielding a Mn-specific SodM and a cambialistic SodA, with no significant change to an overall enzyme architecture. HF-EPR analysis gave insight into the mechanism of metal-specific catalysis of the two enzymes. Phylogenetic analyses suggested the cambialistic SodM originated from a gene duplication of a single, likely Mn-specific SOD, in the common ancestor of all analysed S. aureus isolates. The evolution of SodM to work with both metals could have provided an important adaptation for resisting manganese-starvation during infection. Characterisation of SodM purified directly from S. aureus, as well as studies in an animal model of infection, provided evidence consistent with the hypothesis that the cambialistic SodM contributes to resisting host-imposed metal starvation during S. aureus infection.