In these cases, the ultimate products turned out to be non-toxic, i.e., the enzyme
application was justified; however, in general this may not be the case. Another way
of enzymatic detoxication is the formation of polymers. For example, triclosan was
converted by an integrated nano-bio redox process into non-toxic dimer and trimer
products (Bokare et al. 2010).
Enzymes can also be effectively applied to detoxification of cyanide and nitriles.
Although rhodanese can be used to detoxicate cyanide in the presence of thiosulfate
forming less toxic thiocyanate (Cipollone et al. 2006), a more promising way is to
use cyanide hydratases that detoxify cyanide in the sole presence of water. For
example, cyanide hydratases convert cyanide into formamide and then cyanide
dihydratase converts this intermediate into ammonia and formate. Some bacteria,
e.g., Pseudomonas stutzeri, and many fungi, e.g., Fusarium lateritium, Aspergillus
niger, Botryotinia fuckeliana, produce a variety of cyanide (di)hydratases that were
successfully tested for cyanide detoxification (Martínková et al. 2015).
Two main pathways were described for nitrile hydrolysis, either direct hydrolysis
to the corresponding carboxylic acids and ammonia by nitrilase (Nagasawa et al.
2000) or a two-step conversion into their amides by nitrile hydratase and further into
their carboxylic acids and ammonia by amidase (Okamoto and Eltis 2007). The
nitrilase isolated from Rhodococcus rhodochrous J1 was able to catalyze a quick
conversion of both aromatic and aliphatic nitriles, namely benzonitrile into benzoic
acid and acrylonitrile into acrylic acid (Nagasawa et al. 2000). The isolated nitrile
hydratase of Rhodococcus sp. RHA1 effectively converted acetonitrile,
propionitrile, acrylonitrile, and butyronitrile into the corresponding amides. Bacteria
containing both essential enzymes to complete the nitrile group detoxification,
namely both nitrile hydratase and amidase, could be isolated, too (Okamoto and
Eltis 2007). For real-world field scale remediation, however, nitrilases converting
nitriles directly to carboxylic acids are much more practical than nitrile hydratases,
which require a second step and another enzyme to complete the conversion.
24.3.3.2 Detoxification of Heavy Metals
Detoxification of metals is based mainly on their reduction that usually lowers both
their toxicity and solubility. The redox reactions catalyzed by enzymes require an
electron donor, usually NAD(P)H originated from aerobic respiration. The second
common metabolic pathway occurs when metals/metalloids serve as terminal electron acceptors in anaerobic respiration. In either case, the electrons for metal
reduction are produced by complex metabolism. Thus, extracellular reduction by
free enzymes (as opposed to extracellular reduction conducted by externally
bounded membrane reductases) suffers from a deficiency of essential co-factors
(Thatoi et al. 2014). Therefore, the addition of NAD(P)H, a mixture of enzymes or
the use of the whole cell metabolism is essential for most cases of enzymatic metal
reduction. Payne et al. (2002) explored a promising approach when a mixture of
hydrogenase and cytochrome c3 from Desulfovibrio vulgaris was used in combination with lactate, pyruvate, and hydrogen as electron donors for the reduction of
uranium(VI) to uranium(IV). Cytochrome c3 regenerates the hydrogenase using
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M. Halecký and E. Kozliak
application was justified; however, in general this may not be the case. Another way
of enzymatic detoxication is the formation of polymers. For example, triclosan was
converted by an integrated nano-bio redox process into non-toxic dimer and trimer
products (Bokare et al. 2010).
Enzymes can also be effectively applied to detoxification of cyanide and nitriles.
Although rhodanese can be used to detoxicate cyanide in the presence of thiosulfate
forming less toxic thiocyanate (Cipollone et al. 2006), a more promising way is to
use cyanide hydratases that detoxify cyanide in the sole presence of water. For
example, cyanide hydratases convert cyanide into formamide and then cyanide
dihydratase converts this intermediate into ammonia and formate. Some bacteria,
e.g., Pseudomonas stutzeri, and many fungi, e.g., Fusarium lateritium, Aspergillus
niger, Botryotinia fuckeliana, produce a variety of cyanide (di)hydratases that were
successfully tested for cyanide detoxification (Martínková et al. 2015).
Two main pathways were described for nitrile hydrolysis, either direct hydrolysis
to the corresponding carboxylic acids and ammonia by nitrilase (Nagasawa et al.
2000) or a two-step conversion into their amides by nitrile hydratase and further into
their carboxylic acids and ammonia by amidase (Okamoto and Eltis 2007). The
nitrilase isolated from Rhodococcus rhodochrous J1 was able to catalyze a quick
conversion of both aromatic and aliphatic nitriles, namely benzonitrile into benzoic
acid and acrylonitrile into acrylic acid (Nagasawa et al. 2000). The isolated nitrile
hydratase of Rhodococcus sp. RHA1 effectively converted acetonitrile,
propionitrile, acrylonitrile, and butyronitrile into the corresponding amides. Bacteria
containing both essential enzymes to complete the nitrile group detoxification,
namely both nitrile hydratase and amidase, could be isolated, too (Okamoto and
Eltis 2007). For real-world field scale remediation, however, nitrilases converting
nitriles directly to carboxylic acids are much more practical than nitrile hydratases,
which require a second step and another enzyme to complete the conversion.
24.3.3.2 Detoxification of Heavy Metals
Detoxification of metals is based mainly on their reduction that usually lowers both
their toxicity and solubility. The redox reactions catalyzed by enzymes require an
electron donor, usually NAD(P)H originated from aerobic respiration. The second
common metabolic pathway occurs when metals/metalloids serve as terminal electron acceptors in anaerobic respiration. In either case, the electrons for metal
reduction are produced by complex metabolism. Thus, extracellular reduction by
free enzymes (as opposed to extracellular reduction conducted by externally
bounded membrane reductases) suffers from a deficiency of essential co-factors
(Thatoi et al. 2014). Therefore, the addition of NAD(P)H, a mixture of enzymes or
the use of the whole cell metabolism is essential for most cases of enzymatic metal
reduction. Payne et al. (2002) explored a promising approach when a mixture of
hydrogenase and cytochrome c3 from Desulfovibrio vulgaris was used in combination with lactate, pyruvate, and hydrogen as electron donors for the reduction of
uranium(VI) to uranium(IV). Cytochrome c3 regenerates the hydrogenase using
510
M. Halecký and E. Kozliak