Effect of Iron-Sulfur Cluster Environment in Modulating the Thermodynamic Properties and Biological Function of Ferredoxin from Pyrococcus furiosus

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Abstract
The ferredoxin (7.5 kDa) of the hyperthermophilic archaeon, Pyrococcus furiosus, contains a single [4Fe-4S]1+,2+ cluster that is coordinated by three Cys and one Asp residue rather than the expected four Cys. The role of this Asp residue was investigated using a series of mutants, D14X, where X = C, S, H, N, V, and Y, prepared by heterologous gene expression in Escherichia coli. While the recombinant form of the wild-type and the D14S and D14C mutants contained a [4Fe-4S]1+,2+ cluster, the D14V, D14H, D14Y, and D14N proteins contained a [3Fe-4S]0,+ center, as determined by visible spectroscopy and electrochemistry. The redox potentials (at pH 7.0, 23 °C) of the D14C and D14S mutants were decreased by 58 and 133 mV, respectively, compared to those of the wild-type 4Fe-ferredoxin (Em −368 mV), while those of the 3Fe-protein mutants (including the 3Fe-form of the D14S, generated by chemical oxidation) were between 15 and 118 mV more positive than that of wild-type 3Fe-form (obtained by chemical oxidation, Em −203 mV). The reduction potentials of all of the 3Fe-forms, except the D14S mutant, showed a pH response over the range 3.0−10.0 with a pK of 3.3−4.7, and this was assigned to cluster protonation. The D14H mutant and the wild-type 3Fe-proteins showed an additional pK (both at 5.9) assumed to arise from protonation of the amino acid side chain. With the 4Fe-proteins, there was no dramatic change in the potentials of the wild-type or D14C form, while the pH response of the D14S mutant (pK 4.75) was ascribed to protonation of the serinate. While the ferredoxin variants exhibited a range of thermal stabilities (measured at 80 °C, pH 2.5), none of them showed any temperature-dependent transitions (0−80 °C) in their reduction potentials, and there was no correlation between the calculated ΔS°‘ values and the absorbance maximum, reduction potential, or hydrophobicity of residue 14. In contrast, there was a linear correlation between the ΔH°‘ value and reduction potential. Kinetic analyses were carried out at 80 °C using the ferredoxin as either an electron acceptor to pyruvate oxidoreductase (POR) or as an electron donor to ferredoxin:NADP oxidoreductase (FNOR, both from P. furiosus). The data showed that the reduction potential of the ferredoxin, rather than cluster type or the nature of the residue at position 14, appears to be the predominant factor in determining efficiency of electron transfer in both systems. However, compared to all the variants, the reduction potential of WT Fd makes it the most appropriate protein to both accept electrons from POR and donate them to FNOR.

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