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INSIGHTS INTO THE METAL HANDLING MECHANISMS OF TITANIUM DIOXIDE-BINDING BACTERIA RHODOCOCCUS RUBER
Cobani, Lori
Cobani, Lori
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2024-08
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Chemistry
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http://dx.doi.org/10.34944/dspace/10581
Abstract
Titanium is an abundant element in the Earth’s crust, making up about 0.6% of itscomposition and predominantly occurring as Ti(IV) in slightly soluble mineral oxides. Titanium
has a wide range of uses due to its excellent properties such as high strength-to-weight ratio,
corrosion resistance, and biocompatibility. Despite its abundance and being readily absorbed by
certain organisms, no essential biological function for titanium has been identified. Nonetheless,
there is evidence that titanium is bioactive, including in its interactions with microbes. Titanium
interacts with specific unicellular organisms and with biomolecules found in them, increasing the
likelihood of the potential presence of titanium in microbial metalloproteomes. Understanding
titanium’s presence and varied roles across different microbial phyla may eventually lead to the
discovery of an essential biological role for this element.
Another hydrolysis-prone metal is iron, which unlike titanium, is an essential nutrient for
almost all living organisms. While higher organisms obtain it through their diet, some lower
organisms have developed sophisticated mechanisms to acquire it. Siderophores are small, metalchelators
secreted by bacteria that scavenge iron and bring it to the organism for its metabolic
needs. The significance of this class of molecules is that they can be used in the pharmaceutical
industry for delivering a drug to its specific target site, including antibiotics. Rhodococcus ruber
is a siderophore-producing marine organism, isolated first due to its high affinity for metal
oxides, particularly titanium dioxide. R. ruber GIN.1 and DSM 43338 strains were grown under
laboratory conditions and a growth media recipe was developed to trigger the secretion of
siderophores by these two strains. Here, we present the isolation, purification and structure
elucidation of the first siderophore molecule secreted by this species. Similar to other
siderophores, it contains two catechol groups and a hydroxamic group. This is supported by
genomic analysis of the GIN.1 strain which revealed a similar cluster to some heterobactin
structures. UV-Vis spectroscopy data shows that this novel molecule has high affinity for iron,
comparable to other siderophores. Understanding the siderophore-iron interaction may help to
study the mechanism of how these bacterial cells obtain titanium from the metal oxide particles.
Future research will focus on the further characterization of this new siderophore molecule and its
potential applications.
Other molecules that were investigated from R. ruber were the surface binding proteins
that exhibit specific affinity for titanium dioxide (TiO2). Knowing the fact that this bacteria binds
strongly and selectively to TiO2 among other metal oxides, the question was raised to identify the
molecules in this organism that mediate the affinity for titanium dioxide. After growing the
bacteria for the purpose of siderophore production, a separate procedure was developed to isolate
the binding proteins. Using mutanolysin treatment, the proteins were released from the bacterial
cell walls and subsequently purified. SDS-PAGE analysis, combined with silver stain technique,
revealed distinct protein bands, which were further characterized using mass spectrometry and
sequencing techniques. The main focus was to identify and elucidate the primary structure of
these TiO2-binding proteins. These proteins share similarities with known metal-binding proteins,
suggesting specific amino acid motifs responsible for TiO2 affinity.
On a different project related to metals in biology, ferritin is a protein that stores iron and
releases it as needed by the organism. It plays a crucial role in maintaining iron homeostasis, and
has several applications in biotechnology, biomedical research, as well as environmental and
industrial uses. The ability to reduce toxic chromate (Cr(VI)) to nontoxic chromium (Cr(III)) is of
interest due to the progressively increasing concentrations of chromate in bodies of water due to
industrial pollution. The biomineralization process of iron by ferritin proteins plays a significant
role in the reduction of toxic chromate compounds, presenting a potential solution to remediate
environmental pollution. This study focused on characterizing the biomineralization of iron and
the subsequent reduction of chromate by L-chain ferritin, specifically investigating its efficacy
compared to conventional ferritin proteins. Iron mineralization of the protein was achieved
through an incremental loading procedure, followed by photochemical batch reactions to assess
iron release and chromate reduction capabilities under simulated solar radiation. Results indicate
that while L-chain ferritin exhibits potential for iron biomineralization, its efficiency in chromate
reduction is limited compared to conventional ferritin proteins.
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