Design and Construction of Functional Supramolecular Metalloprotein Assemblies.

Nature puts to use only a small fraction of metal ions in the periodic table. Yet, when incorporated into protein scaffolds, this limited set of metal ions carry out innumerable cellular functions and execute essential biochemical transformations such as photochemical H2O oxidation, O2 or CO2 reduction, and N2 fixation, highlighting the outsized importance of metalloproteins in biology. Not surprisingly, elucidating the intricate interplay between metal ions and protein structures has been the focus of extensive structural and mechanistic scrutiny over the last several decades. As a result of such top-down efforts, we have gained a reasonably detailed understanding of how metal ions shape protein structures and how protein structures in turn influence metal reactivity. It is fair to say that we now have some idea-and in some cases, a good idea-about how most known metalloproteins function and we possess enough insight to quickly assess the modus operandi of newly discovered ones. However, translating this knowledge into an ability to construct functional metalloproteins from scratch represents a challenge at a whole different level: it is one thing to know how an automobile works; it is another to build one. In our quest to build new metalloproteins, we have taken an original approach in which folded, monomeric proteins are used as ligands or synthons for building supramolecular complexes through metal-mediated self-assembly (MDPSA, Metal-Directed Protein Self-Assembly). The interfaces in the resulting protein superstructures are subsequently tailored with covalent, noncovalent, or additional metal-coordination interactions for stabilization and incorporation of new functionalities (MeTIR, Metal Templated Interface Redesign). In an earlier Account, we had described the proof-of-principle studies for MDPSA and MeTIR, using a four-helix bundle, heme protein cytochrome cb562 (cyt cb562), as a model building block. By the end of those studies, we were able to demonstrate that a tetrameric, Zn-directed cyt cb562 complex (Zn4:M14) could be stabilized through computationally prescribed noncovalent interactions inserted into the nascent protein-protein interfaces. In this Account, we first describe the rationale and motivation for our particular metalloprotein engineering strategy and a brief summary of our earlier work. We then describe the next steps in the "evolution" of bioinorganic complexity on the Zn4:M14 scaffold, namely, (a) the generation of a self-standing protein assembly that can stably and selectively bind metal ions, (b) the creation of reactive metal centers within the protein assembly, and (c) the coupling of metal coordination and reactivity to external stimuli through allosteric effects.