TCP, or 1,2,3-trichloropropane, is a stubborn contaminant that has triggered concern among environmental scientists. The possible carcinogen often leaches into groundwater, and scientists don’t have an effective way to remove it. But now chemists report a three-enzyme combination that can turn the troubling compound into benign glycerol (Environ. Sci. Technol. 2014, DOI: 10.1021/es500396r).
Manufacturers produce about 50,000 tons of TCP each year for use as an industrial solvent and a precursor to fumigants. Because studies have shown TCP can cause cancer in animals, and scientists have started to find it in drinking water, the Environmental Protection Agency is considering regulating TCP levels in drinking water.
Unfortunately, there’s no way to break down TCP in an economically feasible way, says Zbyněk Prokop, an environmental chemist at Masaryk University, in the Czech Republic. And it doesn’t decompose naturally: Despite numerous search attempts, scientists have found no organism that can metabolize TCP. This is because the molecule contains very strong carbon-halogen bonds and is extremely toxic to microbes, he says.
Prokop’s group designed a five-step process to convert TCP to glycerol with the help of three enzymes from two types of bacteria found in soils. In the first step, haloalkane dehalogenase from Rhodococcus rhodochrous replaces one of the chlorines in TCP with a hydroxyl, forming 2,3-dichloropropane-1-ol. The last four steps involve haloalcohol dehalogenase and epoxide hydrolase from Agrobacterium radiobacter. These enzymes remove the remaining two chlorine atoms via the temporary formation of epoxides, eventually producing glycerol.
In a one-pot reaction, the enzyme trio completely converted 5 mmol of TCP to glycerol in 30 hours. The researchers also immobilized the enzymes by incorporating them into particles of polyvinyl alcohol hydrogel. They packed these particles into a bed reactor. In the reactor, 10 g of TCP took 2.5 months to totally break down.
The researchers produced the enzymes by having Escherichia coli synthesize each separately, although it is possible to have all three enzymes expressed in one organism, Prokop says. But he thinks that using the enzymes outside of a microbe is best, because it avoids the problem of TCP toxicity on the organism. Also, the U.S. and other governments restrict the release of such genetically modified organisms into the environment, Prokop says.
This reaction pathway is nicely crafted and very clever, says Paul G. Tratnyek, an environmental chemist at Oregon Health & Science University. The group is “not just degrading [TCP], but turning it into a product that might be useful.”
The researchers are now working on scaling up the process for use in real-world conditions, Prokop says.