Acrylamide (CH2=CHCONH2) is a well-known bifunctional monomer, appearing as a white odorless flake-like crystal. It is soluble in water, methanol, ethanol, dimethyl ether, and acetone, but insoluble in benzene and heptane. Acrylamide is incompatible with acids, bases, oxidizing agents, irons and iron salts. It decomposes non-thermally to form ammonia while thermal decomposition produces carbon monoxide, carbon dioxide, and oxides of nitrogen .
As a commercial conjugated reactive molecule, acrylamide has been used worldwide for the synthesis of polyacrylamide and other polymers [2, 3]. It has also been used as a binding, thickening, or flocculating agent in grout, cement, sewage, wastewater treatment, pesticide formulation, cosmetics, sugar manufacturing, and to prevent soil erosion. Polymers of this compound have been used in ore processing, food packaging, plastic products, and in scientific and medical laboratories as solid support for the separation of proteins by electrophoresis . Acrylamide monomer is also widely used as an alkylating agent for the selective modification of sulfhydryl proteins and in fluorescence studies of tryptophan residues in proteins. In 2002, there was an alarming report of the occurrence of acrylamide at high levels up to 3 mg/kg in plant-derived foods and thought to form during cooking allowing the formation of Maillard browning products . Many reports have suggested that acrylamide seems to be found in foods that have been processed by heat-treatment methods other than boiling . One possible pathway to the formation of acrylamide is via the Maillard reaction between amino acids, particularly asparagines, and reducing sugars at high temperatures [5, 6]. Some reports suggest acrylamide could form by acrolein (2-propenal, CH=CHCHO), a three-carbon aldehyde, by either the transformation of lipids or the degradation of amino acids, proteins and carbohydrates [7-12].
Acrylamide could be absorbed through unbroken skin, mucous membranes, lungs, and the gastrointestinal tract. Human exposure to acrylamide is primarily occupational from dermal contact with the solid monomer and inhalation of dust and vapor. Although it is not toxic in polymer form, the monomer can cause peripheral neuropathy. Residual monomer in polymers is also of health concern . Primary exposure occurs during the handling of monomers. Two acrylamide manufacturing factories showed breathing zone concentrations of 0.1 to 3.6 mg/m3 . During normal operations, workers at another plant were exposed to not more than 0.3 mg/m3. Aside from occupational exposure, probable exposure to the general public is through consumption of certain foods . Another source of acrylamide exposure to the general public could be through drinking water treated with polyacrylamide flocculants . Acrylamide may not be completely removed in many water treatment processes with some remaining after flocculation with polyacrylamides probably due to its water solubility and is not absorbed by sediment .
Acrylamide is evidentially a neurogenic, terratogenic or carcinogenic toxicant in animals . The neurotoxic properties of acrylamide have been studied for humans in relation to occupational exposures and, experimentally, in laboratory animals. Understanding of acrylamide-induced neuropathies is quite advanced, a consequence of more than 30 years of research on the possible mechanisms of action . The mechanism underlying the neurotoxic effects of acrylamide as with other toxins are interference with the kinesin-related motor proteins in nerve cells or with fusion proteins in the formation of vesicles at the nerve terminus and eventual cell death . Neurotoxicity and resulting behavioral changes in acrylamide-exposed laboratory animals can reduce reproductive fitness. Further, kinesin motor proteins are important in sperm motility, which could alter reproductive parameters. Effects on kinesin proteins could also explain some of the genotoxic effects on acrylamide. These proteins form the spindle fibers in the nucleus that function in the separation of chromosomes during cell division. This could explain the clastogenic effects of the chemical noted in a number of tests for genotoxicity and assays for germ cell damage .
2. Release of acrylamide in environment
Acrylamide is a synthetic monomer with a broad spectrum of industrial applications, mainly as a precursor in the production of several polymers, such as polyacrylamide [1, 19]. High molecular weight polymers can be modified to develop nonionic, anionic, or cationic properties for specific uses [1, 20]. Various grades of acrylamide are available with the industrial grade typically with a purity of 98 to 99%. Acrylamide for laboratory use ranges from routine to pure, the former for electrophoresis, the latter for molecular applications . The largest demand for acrylamide polymers in industry is for flocculation of unwanted chemical substances in water arising from mining activities, pulp and paper processing, sewage treatment, and other industrial processes. Applications are based on the principles of colloidal suspensions and used to clean up liquids, particularly aqueous media, either for disposal or human consumption [20, 22-23]. Acrylamide is also used as a chemical intermediate in the production of
In worldwide usage, acrylamide is released into environment as waste during its production and in the manufacture of polyacrylamides and other polymers. Residual acrylamide concentrations in 32 polyacrylamide flocculants approved for water treatment plants ranged from 0.5 to 600 ppm . Acrylamide may remain in water after treatment  and after flocculation with polyacrylamides due to its high solubility and is not readily adsorbed by sediment . Other sources of release to water are from acrylamide-based sewer grouting and recycling of wastepaper. Another important source of contamination is from acrylonitrile-acrylamide production which releases approximately 1 g acrylamide in each liter of effluent . Some reports have indicated that polyacrylamide, in the presence of sunlight and glyphosate, photolytically degrades to acrylamide monomer and this is a direct introduction of acrylamide into agricultural areas [36-38]. The half-life of acrylamide monomer in rivers ranges from weeks to months . However, one report indicates that polyacrylamide does not degrade to acrylamide monomer in the presence of sunlight and glyphosate. Additionally, glyphosate appears to interact with either the acrylamide monomer or polymer, decreasing the rate of monomer degradation . The most important environmental contamination results from acrylamide use in soil grouting . Half-life of acrylamide in aerobic soil increases with decreasing temperature . Under aerobic conditions, acrylamide was readily degraded in fresh water by bacteria with a half-life of 55-70 h, after acclimatization for 33-50 h . Acrylamide has been shown to remain slightly longer in estuarine or salt than fresh water .
Acrylamide releases to land and water from 1987 to 1993 totaled over 18.16 tons of which about 85 percent was to water, according to Toxic Chemical Release Inventory of the U.S. Environmental Protection Agency (EPA) . These releases were primarily from plastic industries which use acrylamide as a monomer. In 1992, discharges of acrylamide, reported to the Toxic Chemical Release Inventory by certain US industries included 12.71 tons to the atmosphere, 4.54 tones to surface water, 1,906.8 tones to underground injection sites, and 0.44 tones to land . In an EPA study of five industrial sites that produce acrylamide and polyacrylamide, acrylamide (1.5 ppm) was found in only one sample downstream from a polyacrylamide producer and no acrylamide was detected in soil or air samples . Concentrations of 0.3 ppb to 5 ppm acrylamide have been detected in terrestrial and aquatic ecosystems near industrial areas that use acrylamide and/or polyacrylamides [42-43]. Cases of human poisoning have been documented from water contaminated with acrylamide from sewer grouting. The acrylamide monomer was found to remain stable for more than 2 months in tap water . Atmospheric levels around six US plants were found on an average of < 0.2 µg/m3 (0.007 ppb) in either vapor or particulate form . The vapor phase chemical should react with photochemically produced hydroxyl radicals (half-life 6.6 h) and be washed out by rain .
3. Microbial degradation of acrylamide
The interest in environmental problems is continuously growing and there are increasing demands to seek the sustainable and controllable process which do not burden the environment significantly. Biodegradation is one of the classic methods for removal of undesired organic compounds to concentrations that are undetectable or below limits established as acceptable by regulatory agencies.
Acrylamide is likely to partially biodegrade in water within approximately 8-12 days . If released on land, acrylamide can be expected to leach readily into the ground and biodegrade within a few weeks. In five surface soils that were moistened to field capacity, 74-94% degradation occurred in 14 days in three soils and 79 to 80% in 6 days in the other two soils . Acrylamide may not be completely degraded in domestic sewage and water treatment facilities if residence times are relatively short [13, 45]. Further degradation through bioremediation of acrylamide to less harmful substances would alleviate environmental concerns.
Since 1982, microbial degradation of acrylamide has been explored extensively with a diversity of isolates (Table 1), mainly
Several acrylamide degraders use a coupling reaction of nitrile hydratase (EC 22.214.171.124) and amidase (EC 126.96.36.199) for biotransformation of acrylonitrile to acrylic acid via acrylamide as an intermediate [46, 56]. For example,
In 1990, Shanker and his colleagues isolated an acrylamide-degrading bacterium,
Many aerobic microorganisms utilize acrylamide as their sole source of carbon and energy including
In domestic wastewater in Thailand, four novel acrylamide-degrading bacteria (
||Soil||Aerobic (Enzymatic degradation)|||
||Soil||Aerobic (Enzymatic degradation)|||
||Soil||Aerobic (Free cells)|||
||Soil||Aerobic (Free cells)|||
||Soil||Aerobic (Immobilized cells)|||
||Soil||Aerobic (Enzymatic degradation)|||
||Soil||Aerobic (Enzymatic degradation)|||
||Soil||Aerobic (Immobilized cells)|||
||Wastewater treatment system||Aerobic (Free cells)|||
||Bovine slaughterhouse||Photoheterotropic (Free cells)|||
||Soil||Aerobic (Free and immobilized cells)|||
||Wastewater treatment system||Anaerobic (Free cells)|||
||Soil||Aerobic (Free cells)|||
||Antarctic soil||Aerobic (Free cells)|||
|Natural microbial populations||Rocky Ford Highline Canal, Colorado USA||Aerobic and anaerobic (Free cells)|||
||Soil||Aerobic (Free cells)|||
||Domestic wastewater||Aerobic (Free and immobilized cells)|||
|Domestic wastewater||Aerobic (Free cells)|||
||Soil||Aerobic (Free cells)|||
||Soil||Aerobic (Free cells)|||
||Filamentous fungi used in food and beverage industries||Aerobic (Free cells)|||
Degradation of acrylamide under anaerobic conditions has been rarely described. Recently a new strain of
4. Metabolism of acrylamide
Until now, we can not deny possible routes for acrylamide other than deamination via amidase [50, 59, 62, 64, 67]. The subsequent fate of acrylate is not well understood but probably involves pathways and enzymes that have been characterized to various degrees for other acrylate utilizing bacteria (Figure 1). Acrylate metabolism is believed to proceed via hydroxylation to β-hydroxypropionate, then oxidized to CO2  or reduced to propionate . Another plausible pathway for mineralization of acrylamide is via formation of acrylyl CoA which eliminates lactate as a final product .
A powerful tool that also enables unraveling acrylamide metabolic pathways is the sequential induction of catabolic enzymes and intermediatary metabolites. Further, insight into degradative pathways is also provided from assaying the probable key proteins that are synthesized at sufficient levels when acrylamide is present. Using proteome analysis, fifteen proteins differentially expressed from
5. Bioremediation of acrylamide and future prospects
Bioremediation is viewed as a sustainable process for wastewater treatment, which under appropriate conditions, can promote an efficient reduction of organic matter with minimal energy requirements and, therefore, low costs. Major limitations are the bioavailability of the organic matter and the finding of efficient biodegraders. Physico-chemical environmental conditions also greatly influence the rate and extent of degradation. In general, degradation efficiency is dependent on three overall factors (i) microorganisms that can degrade the specific chemical structure (ii) environmental conditions that allow the microorganisms to grow and express their degradation enzymes and (iii) good physical contact between the organic substrate and the organism.
Rapid degradation of acrylamide coupled with growth requires not only amidase or microorganism producing amidase, but also a whole pathway, i.e. a set of enzymes that are differentially synthesized in the presence of acrylamide. Although a complete catabolic pathway for acrylamide does not exist, recombination and mutation processes and exchange of genetic information between microorganisms may lead to the development of organisms with improved catabolic activities. Alternatively, microorganisms can cooperate by combining their catabolic potential in mixed cultures and in this way may completely mineralize acrylamide. Wang and Lee elucidated the effectiveness of
Microorganisms typically require sufficient water, inorganic nutrients, carbon sources, and trace elements for maintenance and growth. Besides growth substrates, other specific organic compounds such as vitamins or other growth factors are essential for some microorganisms. Monosaccharides like glucose and fructose have been reported as support elements for the growth and degradation potential of acrylamide-degrading bacteria [53-54]. However, in some cases supplementation of acrylamide containing growth medium with glucose or succinate as additional carbon source demonstrated a severe repression in degrading ability [48, 71-75]. Addition of glutamate or ammonium sulfate as an additional nitrogen source to the growth medium demonstrated an increase in degradation potential compared to the cells grown only on acrylamide . One interesting study found that
Toxic compounds (e.g. heavy metals) should not be present at high concentrations, since they can inactivate essential enzymes. As explained in  iron (<10 mM) enhanced the rates of acrylamide degradation of
Optimum conditions for acrylamide biodegradation are achieved if pH and temperature are in the range of pH 6-8 and mesophilic temperature (15-30ºC), respectively [3, 45-48, 53-55]. Most microorganisms consume considerably less energy for the maintenance of basic functions under neutral conditions. This means that more energy is available for growth. It has been known that metabolic activity of tropical soils typically is high and fosters several processes such as carbohydrate fermentation and carbon dioxide production leading to the lowering of pH. Thus, for successful bioremediation of pollutants including acrylamide pH control may be essential. Addition of an inexpensive chemical such as calcium carbonate to neutralize soil pH during bioremediation can optimize remediation .
Studies on acrylamide biodegradation are mainly concerned with the isolation and identification of suitable microbial strains. Most studies use either free or immobilized cells for acrylamide removal. Of these, immobilized cells are advantageous because the immobilized cells are less likely than free cells to be adversely affected by predators, toxin, or parasites [77-78]. Additionally, they can be reused, saving resources and time. However, the implementation of immobilized cells may be sensitive to pH, temperature and acrylamide concentration. Moreover, large accumulations of the metabolic intermediate, acrylic acid, may affect some microbial activity [3, 51, 60]. Hence, the attempt to biotransform acrylamide with amidase or nitrile-converting enzymes via hydrolysis.
Microbial degradation of nitriles proceeds through two enzymatic pathways. Nitrilase (EC 188.8.131.52) catalyzes the direct cleavage of nitriles to the corresponding acids and ammonia, and nitrile hydratase (NHase) catalyzes the hydration of nitriles to amides. Both nitrile-converting enzymes have increasingly attracted attention as catalysts for processing many organic chemicals [79-81]. Nitrile hydratase is commonly used as the catalyst in the production of acrylamide and is known as one of the most important industrial enzymes [82-83]. Generally, the gene operon of nitrile hydratase consists of the genes for the alpha and beta subunits of NHase, the NHase activator and amidase. The amides produced by NHase are degraded to their corresponding free carboxylic acids and ammonia by the action of amidases . Thus, nitrile-converting enzymes are of broad use as alternatives for acrylamide biotransformation.
Acrylic acid, the intermediate product in acrylamide catabolism, is a commodity chemical with an estimated annual production capacity of 4.2 million metric tons . Acrylic acid and its esters can be used in paints, coatings, polymeric flocculants, paper and so on. It is conventionally produced from petrochemicals. Currently, most commercial acrylic acid is produced by partial oxidation of propene which produces undesirable by-products and large amount of inorganic wastes . Currently, there is an innovative manufacturing method using nitrile-amide converting enzymes. For acrylamide degraders, it is initially degraded to ammonia and acrylic acid (acrylate), a process catalyzed by amidase. Then acrylate is reduced to generate energy for growth. Until now, the acrylate-utilizing enzyme has not been well characterized but believed to be acrylate reductase [48, 57]. The identification of the gene encoding this enzyme remains a challenge. Moreover, from an economic aspect, the acrylate reductase-deficient strains created by a gene-disruption method, lead to acrylic acid accumulation in wastewater and are recommended for acrylamide bioremediation in the future.
Sequence similarities have been identified using computer methods for database searches and multiple alignment, between several nitrilases, cyanide hydratase, β-alanine synthase and the first type of aliphatic amidases which hydrolyze only short-chain aliphatic amides . All these enzymes involving the reduction of organic nitrogen compounds and ammonia production exhibited several conserved motifs. One of which contains an invariant cysteine that is part of the catalytic site in nitrilases. Another highly conserved motif includes an invariant glutamic acid that might also be involved in catalysis. Sequence conservation over the entire length of these enzymes, as well as the similarity in the reactions constitutes a definite family which points to a common catalytic mechanism . Chemical mutagenesis and X-ray crystallography have been analyzed for three-dimensional structures of amidases. Only a few crystal structures of nitrilase-related amidases have been reported with
Acrylamide amidases have similar sequences with nitrilases and seem to have descended from a common ancestry along with members of the sulfhydryl enzyme family. In these amidases an invariant cysteine residue was reported to act as the nucleophile in the catalytic mechanism and is confirmed by the three dimensional structural model of the amidase of
Development of thermostable amidase is also important. Based on the three-dimensional structure of amidase, additional disulfide bridges can be engineered by site-directed mutagenesis for enzyme stabilization. Novel amidases that show broad substrate specificity may be developed to biodegrade the toxic environmental pollutants, acrylamide and amides. Random approaches such as directed evolution, reverse engineering and site-directed mutagenesis could be applied to achieve such ends.
Our understanding of the biochemistry and molecular biology of amidase is advancing rapidly and already providing information that is of use today. Moreover, recent developments in amidase studies have broadened the scope of potential applications of the enzyme in acrylamide bioremediation as well as that of acrylic acid production. I predict that these developments combined with progress in genetic engineering and enzyme crystallography will have a major effect on the practical applications of acrylamide bioremediation.
6. Concluding remarks
A huge demand for acrylamide as an ubiquitous monomer for industry led to its environmental presence, however the International Agency for Research on Cancer has classified this compound as a probable human carcinogen. Bioremediation seems to be the only efficient and environmentally friendly process to decompose this monomer. The first step in developing acrylamide bioremediation is to choose high potent microorganisms. Choice of microorganisms is challenging owing to the large scale degradation of acrylamide and elucidation of the intermediate in catabolic pathways is the first important step. Nevertheless, the main problem is the rapid conversion of intermediate acrylic acid to other metabolites. Research on the relationship between degradation mechanisms and membrane structure of acrylamide-utilizing bacteria awaits further characterization. It is noteworthy that successful remediation of acrylamide depends on the ability of microbes to adapt to new environmental conditions and the availability of active and stable chemical degrading bacteria. Indigenous predators, parasites and toxicants are known to severely restrict biodegradation and should be a concern.
The author is grateful to Dr. N. Kurukitkoson for his encouragement to write this review and would like to thank F.W.H. Beamish for proofreading the manuscript.
E: Glutamic acid
D: Aspartic acid
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