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Catalysis Looks to the Future
Biodegradation of Organic Waste
Enzymes, like other catalysts, accelerate the rate of reactions. Reactions catalyzed by enzymes include the oxidation and hydrolysis of natural and synthetic organic chemicals regarded as pollutants of soil and groundwater. Enzymes have some natural advantages over other catalysts in the degradation of environmental pollutants. Enzymes are most active against materials at low concentration (micromolar to millimolar range) in the presence of water, and can be simple and inexpensive to manufacture because they are grown along with microorganisms. Enzymes themselves are biodegradable.
Enzymes can exhibit either narrow or broad substrate specificity. The latter characteristic is desirable for enzymes that attack and degrade organic contaminants. For example, methane monooxygenase has an amazingly broad substrate specificity and can catalyze the oxidation of alkenes, ethers, and alicyclic, aromatic, or heterocyclic molecules. This enzyme system can also degrade synthetic organics such as the chlorinated solvents chloroform, dichloroethylene, trichloroethylene, and 1,1,1-trichloroethane. The ability of enzymes to degrade natural organics such as the components of gasoline, crude oil, and most solvents, as well as synthetic organics such as trichloroethylene or polychlorinated biphenyls, means that most, if not all, organic contaminants can be degraded in reactions catalyzed by enzymes.
Chlorinated organics such as dichlorodiphenyl/trichloroethane and its by-product dichlorodiphenyl ethylene, pentachlorophenol, chlorocatechols, and other chlorinated aromatics used as preservatives and pesticides are degraded in oxidation reactions catalyzed by enzymes from bacteria and other microorganisms. Even the most complex halogenated organics, such as polychlorinated biphenyls (PCBs) and chlorinated solvents, are subject to catalytic attack by certain microorganisms.
PCBs were developed earlier in the century as oils for use in electrical equipment and as lubricating fluids in industrial applications because they gave good insulating and lubricating properties without being explosive or flammable. PCBs were later discovered to bioaccumulate and are now classed as environmental hazards. Recently, enzymes have been found in microbes that will reductively dechlorinate PCBs and oxidize them in the presence of molecular oxygen. Even though these enzymes were not evolved to degrade PCBs, they have a broad-enough substrate specificity to catalyze the initial degradation of PCB molecules in the environment. The use of broad substrate specificity oxygenases in bacteria may be the only practical method of treating PCB-contaminated soil and water because of their low cost and adaptability in the environment.
Similarly, chlorinated solvents have been widely adopted because of their excellent solvent properties and lack of flammability. Trichloroethylene