Redox chemistry of the Schizosaccharomyces pombe ferredoxin electron-transfer domain and influence of Cys to Ser substitutions

Abstract

Schizosaccharomyces pombe ( Sp) ferredoxin contains a C-terminal electron transfer protein ferredoxin domain (etp Fd) that is homologous to adrenodoxin. The ferredoxin has been characterized by spectroelectrochemical methods, and Mössbauer, UV–Vis and circular dichroism spectroscopies. The Mössbauer spectrum is consistent with a standard diferric [2Fe–2S] 2+ cluster. While showing sequence homology to vertebrate ferredoxins, the E°' and the reduction thermodynamics for etp Fd (− 0.392 V) are similar to plant-type ferredoxins. Relatively stable Cys to Ser derivatives were made for each of the four bound Cys residues and variations in the visible spectrum in the 380–450 nm range were observed that are characteristic of oxygen ligated clusters, including members of the [2Fe–2S] cluster IscU/ISU scaffold proteins. Circular dichroism spectra were similar and consistent with no significant structural change accompanying these mutations. All derivatives were active in an NADPH-Fd reductase cytochrome c assay. The binding affinity of Fd to the reductase was similar, however, V max reflecting rate limiting electron transfer was found to decrease ~ 13-fold. The data are consistent with relatively minor perturbations of both the electronic properties of the cluster following substitution of the Fe-bond S atom with O, and the electronic coupling of the cluster to the protein.