The Dawn of Metal-Oxo Clusters as Artificial Proteases: From Discovery to the Present and Beyond

The selective cleavage of peptide bonds in proteins is of paramount importance in many areas of the biological and medical sciences, playing a key role in protein structure/function/folding analysis, protein engineering, and targeted proteolytic drug design. Current applications that depend on selective protein hydrolysis largely rely on costly proteases such as trypsin, which are sensitive to the pH, ionic strength, and temperature conditions. Moreover, >95% of peptides deposited in databases are generated from trypsin digests, restricting the information within the analyzed proteomes. On the other hand, harsh and toxic chemical reagents such as BrCN are very active but cause permanent modifications of certain amino acid residues. Consequently, transition-metal complexes have emerged as smooth and selective artificial proteases owing to their ability to provide larger fragments and complementary structural information. In the past decade, our group has discovered the unique protease activity of diverse metal-oxo clusters (MOC) and pioneered a distinctive approach to the development of selective artificial proteases. In contrast to classical coordination complexes which often depend on amino acid side chains to control the regioselectivity, the selectivity profile of MOCs is determined by a complex combination of structural factors, such as the protein surface charge, metal coordination to specific side chains, and hydrogen bonding between the protein surface and the MOC scaffold.In this Account, we present a critical overview of our detailed kinetic, spectroscopic, and crystallographic studies in MOC-assisted peptide bond hydrolysis, from its origins to the current rational and detailed mechanistic understanding. To this end, reactivity trends related to the structure and properties of MOCs based on the hydrolysis of small model peptides and key structural aspects governing the selectivity of protein hydrolysis are presented. Finally, our endeavors in seeking the next generation of heterogeneous MOC-based proteases are briefly discussed by embedding MOCs in metal-organic frameworks or using them as discrete nanoclusters in the development of artificial protease-like materials (i.e., nanozymes). The deep and comprehensive understanding sought experimentally and theoretically over the years in aqueous systems with intrinsic polar and charged substrates provides a unique view of the reactivity between inorganic moieties and biomolecules, thereby broadly impacting several different fields (e.g., catalysis in biochemistry, inorganic chemistry, and organic chemistry).