Rubredoxin redox stability

The redox stability of the various model complexes was examined in aqueous Triton X-100 solutions. Only [Fe(Z-cys1-Pro-Leu-cys-OMe)2]2 exhibits a quasireversible redox couple at — 0.37 V versus SCE, which is considered to simulate closely the value of native rubredoxin, although a small difference (0.1 V) still remains (7). Other model peptide complexes are rapidly decomposed by hydrolysis in an aqueous micellar solution and do not exhibit even an oxidation peak. The simple alkythiolate model [Fe(S2-o-xyl)2]2 exhibits a quasireversible redox couple at —1.0 V versus SCE in an aqueous micellar solution. Therefore, macro-ring peptide chelation with some hydrophobic side chains, is required in order to induce redox stability (reversibility of cyclic voltammogram measurements) of the Fe(III/II) couple. 

The difference in redox potential and in thermal stability between native rubredoxin and the simple model complexes has been suggested to be brought about by the different protein environments (18). This is as yet unproved, however. The amino acid sequences of many rubredoxins isolated from various sources have been determined, as shown in Fig. 5 (19). A sequence around the Fe active site, Cys-X-Y-Cys, is an invariant fragment and primarily determines the chemical and physical properties. For example, C. pasteurianum rubredoxin has such sequences, Cys6-Thr-Val-Cys9 and Cys39-Pro-Leu-Cys42. 

The chelation effect also brings about a stabilization of the — 1 state of the peptide model complexes as indicated by the thermal stability and redox behavior. Only [Fe(Z-cys-Pro-Leu-cys-OMe)2] exhibits a relatively reversible redox couple in the cyclic voltammogram measurement, but the others do not (20). The bulkiness of side chains of the X and Y residues in Cys-X-Y-Cys probably restricts the adoption of the inherent by preferable conformation (ift = 0°), resulting in a more restricted orientation of Fe-S-C. In fact, the X-ray analysis of native rubredoxin shows that two of the Fe-S torsion angles are restricted and the other two are normal, i.e., conformationally more stable. 

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