ZINC-RUBREDOXINS AND IRON-RUBREDOXINS FROM CLOSTRIDIUM-PASTEURIANUM AT ATOMIC-RESOLUTION - A HIGH-PRECISION MODEL OF A ZNS4 COORDINATION UNIT IN A PROTEIN

The Zn(S-cys)(4) unit is present in numerous proteins, where it assume s structural, regulatory, or catalytic roles, The same coordination is found naturally around iron in rubredoxins, several structures of whi ch have been refined at resolutions of, or near to, 1 Angstrom. The fo ld of the small protein rubredoxin around its metal ion is an excellen t model for many zinc finger proteins, Zn-substituted rubredoxin and i ts Fe-containing counterpart were both obtained as the products of the expression in Escherichia coli of the rubredoxin-encoding gene from C lostridium pasteurianum. The structures of both proteins have been ref ined with an anisotropic model at atomic resolution (1.1% Angstrom = 8 .3% for Fe-rubredoxin, and 1.2 Angstrom, R = 9.6% for Zn-rubredoxin) a nd are very similar, The most significant differences are increased le ngths pf the M-S bonds in Zn-rubredoxin (average length, 2.345 Angstro m) as compared with Fe-rubredoxin (average length, 2.262 Angstrom), An increase of the CA-CB-SG-M dihedral angles involving Cys-6 and Cys-39 , the first cysteines of each of the Cys-Xaa-Xaa-Cys metal binding mot ifs, has been observed, Another consequence of the replacement of iron by zinc is that the region around residues 36-46 undergoes larger dis placements than the remainder of the polypeptide chain, Despite these changes, the main features of the FeS4 site, namely a local 2-fold sym metry and the characteristic network of N-H...S hydrogen bonds, are co nserved in the ZnS4 site, The Zn-substituted rubredoxin provides the f irst precise structure of a Zn(S-cys)(4) unit in a protein, The nearly identical fold of rubredoxin around iron or zinc suggests that at lea st in some of the sites where the metal has mainly a structural role-e .g., zinc fingers-the choice of the relevant metal may be directed by its cellular availability and mobilization processes rather than by it s chemical nature.