Ferredoxin and ferredoxin-heme maquettes.

A 16-amino acid residue peptide derived from a consensus motif of natural ferredoxins incorporates a tetranuclear iron sulfur cluster under physiological conditions. Successful assembly of the [4Fe-4S]2+/1+ cluster within a monomeric peptide was demonstrated using size exclusion chromatography, UV-visible, visible CD, and cryogenic EPR spectroscopies. The robustness of [4Fe-4S]2+/1+ formation was tested using peptides with either the ligating cysteine exchanged for alanine or with the intervening amino acids replaced by glycine. The small size of the peptide allows for modular incorporation into more complex protein structures. In one larger structure, we describe a tetra-alpha-helix bundle that self-assembles both iron-sulfur clusters and hemes, thereby demonstrating feasibility for the general synthesis of maquettes containing multiple, juxtaposed redox cofactors. This is a motif common to the catalytic sites of native oxidoreductases.