Cooper J B, McIntyre K, Badasso M O, Wood S P, Zhang Y, Garbe T R, Young D
Department of Crystallography, Birbeck College, University of London, U.K.
J Mol Biol. 1995 Mar 3;246(4):531-44. doi: 10.1006/jmbi.1994.0105.
The X-ray structure of the tetrameric iron-dependent superoxide dismutase from Mycobacterium tuberculosis has been refined to an R-factor of 0.167 and a correlation coefficient of 0.954 at 2.0 A resolution. The crystals are monoclinic P2(1) and have four subunits related by strong non-crystallographic 222 (or D2) symmetry in the asymmetric unit. 198 of the 207 amino acids of each subunit are defined by the electron 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 followed by two projecting antiparallel alpha-helices, and the C-terminal domain containing four more helical segments with a three-stranded antiparallel beta-sheet inserted sequentially between the fourth and fifth helices. The catalytic iron is co-ordinated by five ligands: three histidines (residues 28, 76 and 164), one aspartate (160) and a solvent molecule. The inferred positions of protons at the active site are consistent with the solvent ligand being a hydroxide ion. This ligand interacts with His145 in the Mycobacterium tuberculosis SOD. In the highly homologous Mycobacterium leprae Mn-SOD, the histidine is replaced by glutamine, this being the only significant residue difference within 10 A 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 Fe- or Mn-SODs. However, the dimer-dimer interactions within the tetramer are novel, being dominated by dimerisation of the 144 to 152 loop regions which connect the outer two beta-strands of the three-membered beta-sheet. This contrasts strongly with the other tetrameric Fe- or Mn-SODs where the dimer-dimer association is dominated by the projecting alpha alpha-turn in the N-terminal domain.
结核分枝杆菌四聚体铁依赖性超氧化物歧化酶的X射线结构已精修至2.0埃分辨率，R因子为0.167，相关系数为0.954。晶体为单斜P2(1)，不对称单元中有四个通过强非晶体学222（或D2）对称性相关的亚基。每个亚基207个氨基酸中的198个由电子密度确定，这表明它们采用了其他铁或锰依赖性超氧化物歧化酶的保守折叠。该结构可分为两个结构域，N端结构域包含一个延伸区域，其后是两个突出的反平行α螺旋，C端结构域包含另外四个螺旋段，在第四和第五个螺旋之间依次插入一个三链反平行β折叠。催化铁由五个配体配位：三个组氨酸（残基28、76和164）、一个天冬氨酸（160）和一个溶剂分子。活性位点质子的推断位置与溶剂配体为氢氧根离子一致。该配体与结核分枝杆菌超氧化物歧化酶中的His145相互作用。在高度同源的麻风分枝杆菌锰超氧化物歧化酶中，组氨酸被谷氨酰胺取代，这是Fe3+周围10埃范围内唯一显著的残基差异。该位置氨基酸的性质可能影响这些酶的金属离子特异性。结核分枝杆菌超氧化物歧化酶的亚基通过极性接触缔合形成二聚体，这与其他二聚体或四聚体铁或锰超氧化物歧化酶的二聚体非常相似。然而，四聚体内的二聚体-二聚体相互作用是新颖的，主要由连接三元β折叠外部两条β链的144至152环区域的二聚化主导。这与其他四聚体铁或锰超氧化物歧化酶形成强烈对比，在其他四聚体中，二聚体-二聚体缔合主要由N端结构域中突出的αα转角主导。
