Expression, purification and metal utilization of recombinant SodA from Borrelia burgdorferi

Borrelia are microaerophilic spirochetes capable of causing multisystemic diseases such as Lyme disease and Relapsing Fever. The ubiquitous Fe/Mn-dependent superoxide dismutase (SOD) provides essential protection from oxidative damage by the superoxide anion. Borrelia possess a single SOD enzyme - SodA that is essential for virulence, providing protection against host-derived reactive oxygen species (ROS). Here we present a method for recombinant expression and purification of Borrelia burgdorferi SodA in E. coli. Metal exchange or insertion into the Fe/Mn-SOD is inhibited in the folded state. We therefore present a method whereby the recombinant Borrelia SodA binds to Mn under denaturing conditions and is subsequently refolded by a reduction in denaturant. SodA purified by metal affinity chromatography and size exclusion chromatography reveals a single band on SDS-PAGE. Protein folding is confirmed by circular dichroism. A coupled enzyme assay demonstrates SOD activity in the presence of Mn, but not Fe. The apparent molecular weight determined by size exclusion corresponds to a dimer of SodA; a homology model of dimeric SodA is presented revealing a surface Cys distal to the dimer interface. The method presented of acquiring a target metal under denaturing conditions may be applicable to the refolding of other metal-binding proteins.