Salt-Loving Bacteria Has Affinity For Radioactive Metals

April 20, 2015- A collaboration between an international team of researchers has lead a finding that may have a large impact on the radioactive cleanup efforts at the Fukushima Dai-ichi Nuclear Power Plant.

A recent In the Lab post on Asian Scientist's website looks at the intriguing possibilities of how a recently discovered highly acidic protein from salt-loving bacteria could be used to remove radioactive strontium (Sr2+) and cesium (Cs+) ions from contaminated soil.

Here are the highlights of "Bacteria Could Help Clean Up Radioactive Soil":

• A team from the Japan Atomic Energy Agency, Kyushu Synchrotron Light Research Center, Kagoshima University and Florida State University published the X-ray structure of a beta-lactamase enzyme from one such microbe, the halophile Chromohalobacter sp. 560, reveals to have highly selective cesium binding sites.

• The researchers found a 1.8 to 2.9 angstrom resolution structure for this enzyme. The anomalous X-ray diffraction also revealed binding sites in the protein for Sr2+ and Cs+ ions. The findings were reported in Acta Crystallographica Section D.

• The scientists were able to locate cesium ions in a specific site within the protein even in the presence of a nine-fold molar excess of sodium ions, which would normally out-compete any binding site. The post also notes that, intriguingly, the presence of strontium and cesium ions does not diminish the activity of the enzyme determined using isothermal titration calorimetry. The team tells AsianScientist.com, "The observation of a selective and high-affinity cesium-binding site provides important information that is useful for the design of artificial cesium-binding sites that may be useful in the bioremediation of radioactive isotopes."

• Proteins from salt-loving bacteria have an abundance of acidic amino acids and present an acidic surface for interaction with a range of metal ions. The Protein Data Bank records twelve types of such enzymes that can bind to calcium, sodium, magnesium, potassium, iron, zinc, strontium and cadmium ions. Indeed, the post notes the presence of these materials in various enzymes is usually a prerequisite for their structure and functionality.

• This metal affinity pointed to proteins from halophiles might become molecular mops for separating precious metals from mixtures or in remediation when toxic metals ions must be extracted selectively from a site. The researchers view the proteins as models for artificial reagents to be used in this context.

• In the Fukushima incident, radioactive cesium deposited on the land at the site, amounting to 2.4 petabequerels (PBq) of radioactivity, and became fixed in soil particles, comprising weathered biotite, a micaceous mineral found in many igneous and metamorphic rocks.

• Most of the soil has been removed, but the issue of extracting the radioactive elements for safe disposal has not been addressed. The remaining soil is also contaminated and no cost-effective method for extracting the leeching cesium has been demonstrated.

• "The team suggests that protein absorbents related to the beta-lactamase from Chromohalobacter might be designed using the techniques of synthetic biology, the most likely approach being to engineer a native protein to make the affinity site described by the team," AsianScientist.com writes. "The genes for such an agent might then be engineered into new breeds of plant that could be grown on the site."

• Once the protein absorbents are expressed in plant roots, cesium could be extracted from the soil efficiently, the plants harvested and disposed of safely, leaving behind improved soil.

• The post concludes with team member Ryota Kuroki mentioning one social issue involved, "Although the removal of cesium is an important theme for us, public acceptance for the use of genetically engineered plants is not strong enough here in Japan, so we are going to shift our theme for finding useful sites to gather other rare materials using engineered proteins derived from the structural information of the halophilic proteins."

Read the full AsianScientist.com post here.

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