Understanding copper-thiolate containing electron transfer centers by incorporation of unnatural amino acids and the Cu _A center into the type 1 copper protein azurin

Highly covalent copper-thiolate bonds are salient features of ubiquitous type 1 (T1) blue copper and purple Cu _A electron transfer (ET) centers in proteins. These centers are found in a wide variety of proteins, each having its own electron transfer partners, requiring the centers to possess a broad range of reduction potentials to match those of their redox partners and to perform ET functions under various driving forces, all while maintaining high ET efficiency. Unraveling the secrets of the success realized by these ET centers has relied upon the expertise of many scientific disciplines and sub-disciplines, including inorganic chemistry, microbiology, biochemistry, and biophysical chemistry. Here, we review the contribution of protein engineering approaches-namely, the incorporation of unnatural amino acids and a biosynthetic Cu _A cofactor into the T1 copper protein azurin-to advancing the current understanding of how the unique structures of T1 copper and Cu _A centers confer their proteins with efficient and tailored ET properties.