X-RAY STRUCTURE-ANALYSIS OF THE IRON-DEPENDENT SUPEROXIDE-DISMUTASE FROM MYCOBACTERIUM-TUBERCULOSIS AT 2.0-ANGSTROMS RESOLUTION REVEALS NOVEL DIMER-DIMER INTERACTIONS

The X-ray structure of the tetrameric iron-dependent superoxide dismut ase from Mycobacterium tuberculosis has been refined to an R-factor of 0.167 and a correlation coefficient of 0.954 at 2.0 Angstrom resoluti on. The crystals are monoclinic P2(1) and have four subunits related b y strong non-crystallographie 222 (or D-2) symmetry in the asymmetric unit. 198 of the 207 amino acids of each subunit are defined by the el ectron density which shows that they adopt the conserved fold of other iron- or manganese-dependent SODs. The structure can be divided into two domains, the N-terminal domain involving an extended region follow ed by two projecting antiparallel a-helices, and the C-terminal domain containing four more helical segments with a three-stranded antiparal lel beta-sheet inserted sequentially between the fourth and fifth heli ces. The catalytic iron is co-ordinated by five ligands: three histidi nes (residues 28, 76 and 164), one aspartate (160) and a solvent molec ule. The inferred positions of protons at the active site are consiste nt with the solvent ligand being a hydroxide ion. This ligand interact s with His145 in the Mycobacterium tuberculosis SOD. In the highly hom ologous Mycobacterium leprae Mn-SOD, the histidine is replaced by glut amine, this being the only significant residue difference within 10 An gstrom of the Fe3+. The nature of the amino acid at this position may influence the metal ion specificity of these enzymes. The subunits of the Mycobacterium tuberculosis SOD associate by polar contacts to form dimers, which closely resemble those of other dimeric or tetrameric F e- or Mn-SODs. However, the dimer-dimer interactions within the tetram er are novel, being dominated by dimerisation of the 144 to 152 loop r egions which connect the outer two beta-strands of the three-membered beta-sheet. This contrasts strongly with the other tetrameric Fe- or M n-SODs where the dimer-dimer association is dominated by the projectin g alpha alpha-turn in the N-terminal domain.