The 2.0 Å Resolution Structure of the Catalytic Portion of a Cyanobacterial Membrane-bound Manganese Superoxide Dismutase

Cyanobacteria are shown to be unique in containing membrane-bound manganese superoxide dismutases (MnSOD). They are homodimeric type 2 membrane proteins that protect this phototrophic organism against oxidative stress. We have determined, for the first time, the 2.0 Å resolution structure of the catalytic portion of the MnSOD from the filamentous cyanobacterium Anabaena PCC 7120. Within each subunit, both the N-terminal helical hairpin (His94 and His145) and the C-terminal α/β domain (His232 and Asp228) contribute ligands to the catalytic manganese site. Together with a water or hydroxide ion (OHx) a five-coordinated trigonal bipyramidal geometry is formed, with OHx and His90 forming the axial ligands and manganese shifted out of the equatorial plane in the direction of OHx. The ligands including OHx are tightly constrained by hydrogen bonding with surrounding residues either from the same monomer (Tyr98, Asn144, Trp194, Gln213, Val229, Trp230) or from the neighbouring subunit (Glu231, Tyr235). This underlines the important role of the symmetric dimeric structure of MnSODs in contributing elements to both the active site and the substrate funnel. The Mn⋯Mn distance (18.4 Å) is bridged by the hydrogen-bonded His232 of one monomer with Glu231 of the other monomer. A detailed discussion of the structure, a comparison with known structures of soluble MnSODs as well as a model of the cyanobacterial membrane-bound MnSOD is presented.