Metals susceptible to hydrolytic precipitation (hydrolysis-prone metals) display diverse chemical and biological roles, but are often avoided because of complications in working with them in aqueous solution. Finding ways to utilize these metals, like titanium and vanadium, in aqueous solution would unlock a wealth of chemistry that often requires less green solvents. Nature works around these difficulties by utilizing ligands that bind and stabilize these metals, keeping them soluble in aqueous systems, and controlling their hydrolysis. Titanium has demonstrated uses ranging from catalysis, to solar energy, to TiO2 materials, which are used as coloring or fillers in a variety of products. Many proteins are known to bind to titanium compounds, but no example of a native titanium-binding protein or enzyme is known. Another hydrolysis-prone metal of interest is vanadium, which is required by some avian and mammalian species, and vanadium enzymes have been found in some bacteria and fungi. Vanadium has a complex speciation with a variety of accessible oxidation states in aqueous solution, giving it broad applications in catalysis. These uses illustrate the diverse potential applications of Ti and V in aqueous solution if properly controlled. They are still underused in comparison to rare precious metal catalysts, largely because of the difficulty of working with them. Many biological molecules have good binding moieties to stabilize these metals against hydrolysis, and have been seen to bind and stabilize these metals. Some such molecules are natural proteins and enzymes, but they have evolved to function with other metals in their active sites. Enzymes which are specifically designed with Ti active sites are currently unknown, and are rare for V. We have modified existing metal binding sites in de novo, due Ferri proteins in order to bind Ti and V. Small de novo proteins are typically less than 50 amino acids, but still have well defined tertiary structures. Initial studies have shown promise for the ability of these proteins to bind to titanium, and further studies utilizing circular dichroism are underway. The modified active sites feature a tyrosine residue, which has been shown to be a good ligand for both Ti and V in proteins like transferrin. These metal-bound proteins can mimic enzyme active sites, providing a tunable environment to afford unique substrate specificity.

Towards new titanium and vanadium enzymes

255th National Meeting (2018)

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