Homeostasis and trafficking of hydrolysis-prone metals in cells, proteins, and small molecules

Abstract

Nature uses inorganic elements for biological processes based on the useful chemistry, abundance, and availability of each metal. Transition metals are critical in the biogeochemical cycling of essential elements and the bioinorganic chemistry of organisms. Hydrolysis-prone metals such as iron and titanium are abundant on Earth but are mostly insoluble in oxic aqueous environments. Nearly every organism requires iron for survival, therefore Nature evolved to stabilize iron from hydrolysis and hydrolytic precipitation through protein and small molecule mechanisms. Like iron, titanium primarily exists as insoluble mineral oxides and is second only to iron as the most abundant transition metal in the Earth’s crust. Despite the reputation as an inert and insoluble metal, titanium can be solubilized and made bioavailable through by chemical and biological weathering. Currently there is no known native role for titanium, however it is quite bioactive. As a stronger Lewis acid, titanium can compete with iron in binding to biomolecules and proteins. It is of interest to investigate the interactions between hydrolysis-prone metals and biological systems, from whole cell organisms to proteins and small molecules. The non-pathogenic bacterium Rhodococcus ruber GIN-1 was isolated for its ability to strongly adhere to titanium dioxide (TiO2) over other metal oxides, providing an opportunity to study the interactions between whole bacterial cells and metal oxides. The GIN-1 strain incorporates Ti(IV) ions into its biomass after adherence to anatase, rutile, and a mixture of the two morphologies. Six metals were quantitated in TiO2-exposed and control (unexposed) cells by inductively coupled plasma optical emission spectroscopy. The exposure to TiO2 caused a significant uptake of titanium with concomitant loss of iron, zinc, and possibly manganese. A collaborative project with the Strongin laboratory at Temple University works to develop stable, biomaterial photocatalysts for environment remediation of toxic inorganic contaminants. Ferritins are a class of proteins that mineralizes and stores iron as a non- toxic ferrihydrite nanoparticle. These proteins can be photoactivated with ultraviolet light to release iron from its core to remediate environmental contaminants. Ferritin can be sensitized with plasmonic gold nanoparticles to extend the photoactivity of the catalyst to the visible spectrum. Work in this thesis highlights the contribution to this collaboration from the Valentine laboratory, included the expression and purification of proteins in E. coli (human H-chain ferritin, human L-chain ferritin, and bacterial DNA protection from starved cells protein), mutation of proteins to improve sensitization of catalyst, and biomineralization with iron and titanium. The trafficking of hydrolysis prone metals is vital for the survival of nearly every organism. Iron transport proteins such as transferrins are studied to understand how nature utilizes a difficult essential metal across the domains of life. Most transferrins have two homologous lobes and are believed to have evolved from a gene duplication of a monolobal transferrin. The ascidian Ciona intestinalis has genes for both a bilobal and monolobal transferrin. Nicatransferrin (nicaTf), the monolobal transferrin from C. intestinalis, is a primitive protein that may provide insight on the evolution of transferrins in higher organisms. It is advantageous to use E. coli expression systems to produce recombinant proteins, however protein misfolding and aggregation can be a concern. To improve expression of nicaTf in E. coli, codon optimization and disulfide bonded protein expression were used. Finally, siderophores are small, high affinity iron-chelating molecules secreted from lower organisms that scavenge iron in iron-limiting conditions. R. ruber GIN-1 and R. ruber DSM 43338 strains both secrete siderophores in artificial seawater media. There are several siderophores identified from Rhodococcus species, however none have been reported from any R. ruber strain. A new siderophore was isolated and preliminary work has been done to purify and characterize the molecule. Understanding the siderophore- metal ion interactions may help elucidate the mechanism of how R. ruber cells obtain titanium from the metal-oxide particles.