Biosynthetic approach to modeling and understanding metalloproteins using unnatural amino acids

Metalloproteins have inspired chemists for many years to synthesize artificial catalysts that mimic native enzymes. As a complementary approach to studying native enzymes or making synthetic models, biosynthetic approach using small and stable proteins to model native enzymes has offered advantages of incorporating non-covalent secondary sphere interactions under physiological conditions. However, most biosynthetic models are restricted to natural amino acids. To overcome this limitation, incorporating unnatural amino acids into the biosynthetic models has shown promises. In this review, we summarize first synthetic, semisynthetic and biological methods of incorporates unnatural amino acids (UAAs) into proteins, followed by progress made in incorporating UAAs into both native metalloproteins and their biosynthetic models to fine-tune functional properties beyond native enzymes or their variants containing natural amino acids, such as reduction potentials of azurin, O₂ reduction rates and percentages of product formation of HCO models in Mb, the rate of radical transport in ribonucleotide reductase (RNR) and the proton and electron transfer pathways in photosystem II (PS II). We also discuss how this endeavour has allowed systematic investigations of precise roles of conserved residues in metalloproteins, such as Met121 in azurin, Tyr244 that is cross-linked to one of the three His ligands to CuB in HCO, Tyr122, 356, 730 and 731 in RNR and TyrZ in PS II. These examples have demonstrated that incorporating UAAs has provided a new dimension in our efforts to mimic native enzymes and in providing deeper insights into structural features responsible high enzymatic activity and reaction mechanisms, making it possible to design highly efficient artificial catalysts with similar or even higher activity than native enzymes.