Metal selectivity of artificial dimeric proteins:

Overcoming universal restrictions on metal selectivity

Metalloproteins are closely associated with vital functions of living organisms such as metal homeostasis and enzymatic reactions. Central to the functions of metalloproteins is selective metal coordination: each metalloprotein must pair with its cognate metallocofactor to fulfill its biological role. Cells regulate selective incorporation of biologically essential, first row mid-to-late transition metal ions using metallochaperones and metal sensor proteins. Without the aid of the intracellular regulatory mechanisms, most of the metalloproteins in heterogeneous environment exhibits low fidelity in metal selectivity. Inherently flexible motions of protein backbones and side chains disrupt steric selection of the metal ions whose d-orbital electron configurations determine preferred coordination geometries. Accordingly, a large number of metalloproteins have been reported to generally follow Irving-Williams (IW) series (Mn^II < Fe^II < Co^II < Ni^II < Cu^II > Zn^II) in binding affinities between the metal ions and the proteins, thereby losing their intrinsic structural/catalytic functionalities.

Here, I will discuss a design strategy of artificial dimeric proteins that thermodynamically overcome the IW restrictions in vitro and in bacterial cells, favoring binding of lower-IW transition metals over Cu^II – the most dominant ion in the IW series. The dimeric proteins were designed to be flexible through single disulfide bond. The flexibility of the dimer interface adopts mutually exclusive, metal-dependent conformational states with structural cooperativity between multiple metal binding sites. Consequently, the binding sites of metal ions (Co^II, Ni^II, and Zn^II) thermodynamically disfavor Cu^II ions by enforcing an unfavorable coordination geometry. I will explain metal-selection mechanisms of the dimeric proteins based on X-ray crystallography and multiple analytical approaches.