Cyanoba
cteria are shown to be unique in
containing membrane-bound manganese superoxide dismutases (MnSOD). They are homodimeri
c type 2 membrane proteins that prote
ct this phototrophi
c organism against oxidative stress. We have determined, for the first time, the 2.0Å resolution stru
cture of the
catalyti
c portion of the MnSOD from the filamentous
cyanoba
cterium
Anabaena PCC 7120. Within ea
ch subunit, both the N-terminal heli
cal hairpin (His94 and His145) and the C-terminal α/β domain (His232 and Asp228)
contribute ligands to the
catalyti
c manganese site. Together with a water or hydroxide ion (OH
x) a five-
coordinated trigonal bipyramidal geometry is formed, with OH
x and His90 forming the axial ligands and manganese shifted out of the equatorial plane in the dire
ction of OH
x. The ligands in
cluding OH
x 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 symmetri
c dimeri
c stru
cture of MnSODs in
contributing elements to both the a
ctive site and the substrate funnel. The Mn
![]()
center border=0 SRC=/images/glyphs/BN9.GIF>Mn distan
ce (18.4Å) is bridged by the hydrogen-bonded His232 of one monomer with Glu231 of the other monomer. A detailed dis
cussion of the stru
cture, a
comparison with known stru
ctures of soluble MnSODs as well as a model of the
cyanoba
cterial membrane-bound MnSOD is presented.