THE RUBREDOXIN FROM CLOSTRIDIUM-PASTEURIANUM - MUTATION OF THE IRON CYSTEINYL LIGANDS TO SERINE - CRYSTAL AND MOLECULAR-STRUCTURES OF OXIDIZED AND DITHIONITE-TREATED FORMS OF THE CYS42SER MUTANT

Isolation is reported of the four mutant proteins of the electron-tran sfer protein rubredoxin from Clostridium pasteurianum in which each of the four cysteine ligands is changed in turn to serine. They fall int o two pairs whose properties depend on whether an interior (C6, C39) o r a surface (C9, C42) cysteine ligand is substituted. A crystal struct ure of the oxidized C42S protein (1.65 Angstrom; R, 18.5%) confirms th e presence of an Fe-III(Sv-Cys)(3)(O-gamma-Ser) center (Fe-O, 1.82(8) Angstrom). Significant structural change is restricted to the region a round the mutation. EXAFS experiments confirm (FeS3O)-S-III (O = O-gam ma-Ser or OHx) centers in each oxidized protein at pH 8. The reduction potentials of the Fe-III/II couple are decreased by about 100 and 200 mV, respectively, in the interior and surface ligand mutants. The pot entials are pH-dependent with respective pK(a)(red) values of about 9 and 7. EXAFS data indicate an increase of 0.2-0.3 Angstrom in the Fe-I I-O distances in passing through these characteristic pK(a)(red) value s. H-1 NMR experiments on Cd-II forms, reveal the presence of Cd-II-(S -Cys)(3)(O(H)-Ser) centers in the surface ligand mutants C9S and C42S by the detection of Cd-113-O-CH2beta coupling and S-OHgamma resonances . The assumption of the presence of Fe-II(S-Cys)(3)(O-Ser) centers in each mutant protein at pH values above the characteristic pK(a)(red) a llows a simple interpretation Of the electrochemical behavior. Protona tion of the Fe-O-gamma-Ser link upon reduction is proposed, followed b y hydrolysis at lower pH values: Fe-III-O-gamma-Ser + H+ + e(-) --> Fe -II-O-gamma(H)-Ser; Fe-II-O-gamma(H)-Ser + H2O --> Fe-II-OH2 + HOgamma -Ser. The differences in reduction potentials, their pH-dependence, an d the onset of irreversible electrochemistry can be attributed to diff erences in the Fe-O bonds of the interior and surface ligands. These d ifferences appear to result from variation in the conformational flexi bility of the protein chelate loops which carry the ligands. An attemp t to generate crystals of the reduced Fe-II-C42S protein by treatment of Fe-III-C42S crystals with dithionite at pH 4 led to loss of iron. A crystal structure (1.6 Angstrom; R, 16.8%) reveals that cysteine resi dues 6 and 9 have trapped the oxidation product SO2, a result confirme d by reactions in solution: Cys-SH + SO2 --> Cys-S-II-(SO2-)-O-IV + H.