Functional role of a conserved tryptophan residue of Chromatium vinosum high potential iron protein

A Trp80Asn point mutation of Chromatium vinosum high potential iron protein has provided insight on the functional role of this conserved aromatic residue. Following mutation to Asn, a minimal change in the midpoint potential of the [Fe4S4]3+/2+ cluster is observed (+33 mV). In contrast to other proteins carrying mutations of Tyr and Phe residues, the reduced state of the Trp80Asn derivative is stable in an aerobic environment, but displays a more pronounced temperature-dependent degradation that appears to be mediated by unfolding of the protein prior to degradation of the cluster. Accordingly, Trp80 serves a distinct role from other aromatic residues found in the hydrophobic cluster binding pocket. The rate of electron transfer exchange with ferricyanide is similar to that observed for native protein (knat=0.7 s−1; kTrp80Asn=1.7 s−1), the minor increase in reactivity reflecting the change in driving force from the increase in cluster midpoint potential.