Geoffrey B. Jameson,a Julian J. Adams,a Paul D. Hempstead,a Irene Morgenstern-Badarau,b James W. Whittaker,c Edward N. Bakerd and Bryan F. Andersona
aCentre for Structural Biology, Institutes of
Fundamental Sciences and Molecular BioSciences, Massey University, Palmerston
North, New Zealand; bInstitut de Chimie Moleculaire dOrsay, Bat. 420 Universit Paris-Sud
XI, 91405 Orsay, France; cDepartment of Biochemistry and Molecular Biology, Oregon Graduate Institute School of Science and Engineering,
Beaverton, Oregon 97006, USA; dSchool of Biological Sciences, University of
Auckland, Auckland, New Zealand (G.B.Jameson@massey.ac.nz)
The structures of
the apo, Fe and Mn forms of the manganese-preferring superoxide dismutase from
the hyperthermophilic microaerobic archaeon Pyrobaculum aerophilum (Pa-SOD) have been determined at 2.0, 2.2 and 1.9 , respectively, and the
structure of the Fe-specific superoxide dismutase from the thermophilic
methanogenic anerobic archaeon Methanobacterium thermoautotrophicum (Mt-FeSOD) has been determined at 2.6 . Corresponding values for R (Rfree) are, presently, 0.16 (0.20), 0.19 (0.24), 0.17 (0.20),
and 0.22 (0.24); the asymmetric unit is a dimer for Pa-SODs and a tetramer and half-tetramer for Mt-FeSOD.
The extreme
thermal stability of Pa-SOD, which survives the autoclaving necessary to insert
metal ions into the E. coli-expressed
apo-protein, appears to arise from a combination of previously identified
structural determinants for thermal stability. In particular, both the Pa-SODs and the Mt-FeSOD form extremely compact tetramers.
In common with
all other structurally characterised Mn- and FeSODs, the central metal ions of Pa-FeSOD, Pa-MnSOD and Mt-FeSOD
have an identical primary coordination sphere. Three histidines, a monodentate
aspartate and a solvent-derived ligand (OH- in oxidised MIII
forms) coordinate in a trigonal bipyramidal manner, in which the solvent-derived
ligand and a histidine ligand are trans to each other. No differences exist in the second coordination sphere
for both species a histidine C-H moiety hydrogen bonds to the solvent-derived
ligand, although in many other SODs this aprotic C-H moiety is substituted by a
glutamine NH2 moiety. It is only in the third coordination sphere,
involving the hydrogen-bonding network to the amine moiety of the coordinated
axial histidine that significant differences can be found not only for these
SODs but also for other SODs that have been well-characterised both
structurally and functionally. A methionine thioether moiety appears to direct
a different hydrogen-bonding network to the axial histidine for Mn-specific
SODs, in contrast to Fe-specific SODs where an aliphatic residue is found at
this location, position n
on the helix supporting the axial histidine at position n+3.