Importance of Hydrogen Bonding in Fine Tuning the [2Fe-2S] Cluster Redox Potential of HydC from Thermotoga maritima.

Iron-sulfur clusters form one of the largest and most diverse classes of enzyme cofactors in nature. They may serve as structural factors, form electron transfer chains between active sites and external redox partners, or form components of enzyme active sites. Their specific role is a consequence of the cluster type and the surrounding protein environment. The relative effects of these factors are not completely understood, and it is not yet possible to predict the properties of iron-sulfur clusters based on amino acid sequences or rationally tune their properties to generate proteins with new desirable functions. Here, we generate mutations in a [2Fe-2S] cluster protein, the TmHydC subunit of the trimeric [FeFe]-hydrogenase from Thermotoga maritima, to study the factors that affect its redox potential. Saturation mutagenesis of Val131 was used to tune the redox potential over a 135 mV range and revealed that cluster redox potential and electronic properties correlate with amino acid hydrophobicity and the ability to form hydrogen bonds to the cluster. Proline scanning mutagenesis between pairs of ligating cysteines was used to remove backbone amide hydrogen bonds to the cluster and decrease the redox potential by up to 132 mV, without large structural changes in most cases. However, substitution of Gly83 with proline caused a change of HydC to a [4Fe-4S] cluster protein with a redox potential of -526 mV. Together, these results confirm the importance of hydrogen bonding in tuning cluster redox potentials and demonstrate the versatility of iron-sulfur cluster protein folds at binding different types of clusters.