Metalloproteins Diversified: The Auracyanins Are a Family of Cupredoxins That Stretch the Spectral and Redox Limits of Blue Copper Proteins

The metal sites of electron transfer proteins are tuned for function. The type 1 copper site is one of the most utilized metal sites in electron transfer reactions. This site can be tuned by the protein environment from +80 mV to +680 mV in typical type 1 sites. Accompanying this huge variation in midpoint potentials are large changes in electronic structure, resulting in proteins that are blue, green, or even red. Here, we report a family of blue copper proteins, the auracyanins, from the filamentous anoxygenic phototroph <i>Chloroflexus aurantiacus</i> that display the entire known spectral and redox variations known in the type 1 copper site. <i>C. aurantiacus</i> encodes four auracyanins, labeled A–D. The midpoint potentials vary from +83 mV (auracyanin D) to +423 mV (auracyanin C). The electronic structures vary from classical blue copper UV–vis absorption spectra (auracyanin B) to highly perturbed spectra (auracyanins C and D). The spectrum of auracyanin C is temperature-dependent. The expansion and divergent nature of the auracyanins is a previously unseen phenomenon