Polyethers

Poly(alkylene glycol)s have a common structural formula: HO[R-O]nH [R=CH2CH2 for PEG, CH3CHCH2 for polypropylene glycol (PPG), a polymer of 1,2-propylene oxide, (CH2)4 for polytetramethylene glycol (PTMG), and C2H5(CHCH2) for polybutylene oxide (PBO), a polymer of 1,2-butylene oxide], where n represents the average range of units. The physical properties of PEGs vary from viscous liquids to waxy solids based on their molecular sizes, although every PEG from oligomers up to polymers with a molecular weight (MW) of a few million is completely water-soluble. Commercially available PPG can be divided into two groups, the diol and triol types, based on the straight or branched chain structure of the polymer. The water solubility of PPGs is lost when the MW is increased to more than approximately 700 (triol type) and 1,000 (diol type) due to the inclusion of a methyl group in each monomer unit. Therefore, copolymers of PEG and PPG are used as detergents, where PEG is a hydrophilic constituent and PPG a hydrophobic one. Another copolymer is also used as a water-soluble flame-resisting pressure liquid, where ethylene oxide and propylene oxide are randomly copolymerized. PBO is an oily polymer due to its pendant ethyl groups. In general, PTMG is a waxy substance, from which water-soluble oligomers have been removed as impurities. PEG was the first member of the polyether group to be manufactured in large quantities and to be used as a commodity chemical in various industrial fields. The most common hydrophilic moieties in the nonionic surfactants are ethylene oxide polymers. The majority of PEGs produced are used in the production of nonionic surfactants, very important groups of industrial products with applications from domestic detergents to agrochemicals, food emulsifiers and other industrial preparations. These products ultimately constitute a significant burden on domestic and industrial wastewater systems. Therefore, their biodegradability characteristics have been observed over the past 50 years, which were reviewed by Kawai (1987, 2002, 2010b). Because of low toxicity and skin irritation, PEGs are widely used in the pharmaceutical industry in the preparation of ointments, suppositories, tablets, and solvents for injection, and also for the preparation of cosmetics, such as creams, lotions, powders, cakes, and lipstick. They are also used as intermediates in the production of resins, such as alkyd resin and polyurethane resin, and as components in the manufacture of lubricants, antifreeze agents, wetting agents, printing inks, adhesives, shoe polish, softening agents, sizing agents, and plasticizers. Furthermore, this material has been used in making resin gels to immobilize enzymes or microbial cells and in the chemical modification of enzymes. Although PEGs appear to be metabolically inert and nontoxic, they are sulfated in vitro by the rat and guinea pig liver (Roy et al. 1987), and repeated topical application of a PEG-based antimicrobial cream to open wounds in rabbits and burn patients has been found to cause a syndrome related to the metabolism of PEGs to various compounds, including mono- and diacids (Herald et al. 1989). Furthermore, the possibility, that PEG 400 and PEG oligomers are toxic, has also been suggested (Gordienko and Kudokotseva 1980). Biodegradation of PEG might pose an additional risk due to metabolite production. Chemically unsubstituted PPG is used in solvents for drugs and in paints, lubricants, inks, and cosmetics, but is mostly transformed to polyurethanes or surface-active agents. PBO is an oily material used in sizing agents, cleaning agents, and dispersants. PTMG is used exclusively as a constituent of polyurethane.

PEGs with different MWs have been produced and have been used in industrial and domestic applications for more than 60 years. Some of them are included as non-toxic and biodegradable segments of copolymers, and their larger parts are transformed into neutral detergents and liberated into streams after use.

Various types of PEG-degraders that are able to assimilate a variety of molecular sizes have been isolated since the first report of PEG 400 by Payne (1963). Although PEGs with MW higher than 1,000 were long considered to be biore-sistant, those up to 20,000 or more have since been found to be biodegradable. PEGs with a high MW, from 4,000 to 20,000, are assimilated by a limited number of species: Pseudomonas aeruginosa (up to 20,000) (Haines and Alexander 1975), soil bacteria (up to 6,000) (Hosoya et al. 1978), Pseudomonas stutzeri (up to 13,500) (Obradors and Aguilar 1975), and Sphingomonas species (up to 20,000); the strains were originally identified as Flavobacterium species (Ogata et al. 1975). Sphingomonads include sphingolipids in their outer membranes instead of the lipopolysaccharides found in most Gram-negative bacteria. Various lipophilic xenobiotic-assimilating bacteria are included in this genus (Kawai 1999). Most recently, a Gram-positive actinomycete, Pseudonocardia sp. strain K1, originally isolated as a tetrahydrofuran degrader, was also found to grow on PEG 4,000 and 8,000 (Kohlweyer et al. 2000).

We have isolated various PEG-utilizing bacteria with various degradabilities towards PEG 400-20,000 (Ogata et al. 1975). Isolates able to degrade PEG 4,000 and 20,000 were identified as Sphingomonads, and based on the newest taxonomy, they have been renamed and designated type species of Sphingopyxis macrogol-tabida and Sphingopyxis terrae, respectively (Takeuchi et al. 2001). Interestingly, S. terrae can grow on PEG as a symbiotic mixed culture with a concomitant associate (Kawai and Yamanaka 1986; Kawai 1996). Another focus of PEG degradation studies is the biochemical mechanism of degradation. Several reports have suggested different mechanisms (Kawai 2002), but the most probable metabolic pathway is an exogenous metabolic one based on repeated oxidation steps. PEG is oxidized by alcohol dehydrogenases linked with a dye or NAD. PEG-dehydrogenases (PEG-DHs) from PEG-utilizing Sphingomonads have been cloned and characterized as FAD-including alcohol dehydrogenases (Sugimoto et al. 2001; Ohta et al. 2006). PEG-aldehyde dehydrogenase was cloned from PEG-utilizing Sphingomonads and characterized as a NADP-containing nicotinoprotein PEG-aldehyde dehydrogenase (Ohta et al. 2005), the first report of a nicotinoprotein aldehyde dehydrogenase. The ether bond-splitting enzyme involved in the PEG metabolism was perhaps a glycolic acid oxidase or glycolic acid dehydrogenase active on carboxylated PEG (Yamanaka and Kawai 1991; Enokibra and Kawai 1997). All the metabolic enzymes included in PEG degradation have been localized in the membrane and are thought to work in the periplasm, in accordance with the fact that PEG and its metabolites were detected in the periplasmic fraction (unpublished data), suggesting that PEG is taken up into the periplasm and metabolized there. We cloned the genes involved in PEG degradation, and found that the peg operon consisted of five genes and was expressed by PEG through induction of an araC-type regulator (Charoenpanich et al. 2006; Tani et al. 2007, 2008), as shown in Fig. 16.2. This was the first report on the regulation of degradative genes by a macromolecule. Two genes coding PEG-DH and PEG-aldehyde dehydrogenase are involved in the peg operon. The role of other genes in the peg operon was suggested with regards to the PEG

Fig. 16.2 The operonic structure of genes involved in PEG degradation and its regulation by PEG

metabolism, as summarized in Fig. 16.2 (Tani et al. 2007). A gene coding PEG-carboxylate dehydrogenase was detected in the downstream region of the peg operon, and was found to act as an ether bond-splitting enzyme (Somyoonsap et al. 2008). Anaerobic biodegradation of PEG has been well investigated as compared to other polymers. Schink's group reported that higher MW PEG (up to PEG 40,000) is degraded by anaerobes rather than by aerobes (Strass and Schink 1986; Schink and Stieb 1983). They proposed an anaerobic metabolic route for PEG, and suggested that acetaldehyde is produced from PEG by a diol dehydrorase-like enzyme (PEG acetaldehyde lyase), but they failed to purify the enzyme (Strass and Schink 1986; Frings et al. 1992). Schink's group also demonstrated conversion of 2-phenoxyethanol to phenol and acetaldehyde, in a way similar to a diol hydratase reaction, by a strictly anaerobic Gram-positive bacterium, Acetobacterium strain LuPhet1, but could not rule out an alternative pathway for the production of acetaldehyde (Speranza et al. 2002). Dwyer and Tiedje (1986) obtained a meth-anogenic consortia from sewage sludge that degraded ethylene glycol to PED 20,000. In addition, Alcaligenes faecalis var. denitrificans TEG-5, the first PEG-degrader able to degrade PEG under aerobic conditions, displayed PEG degradation under anaerobic nitrate-reducing conditions (Grant and Payne 1983).

In parallel with PEG degrading bacteria, we isolated PPG- and PTMG-utilizing bacteria (Kawai et al. 1977; Kawai and Moriya 1991). PPG with an MW of up to 4,000 was assimilated. Distinct degradation of PTMG was limited to oligomers up to octamer, since PTMG is insoluble in water and is used exclusively as a constituent of polyurethanes, but oligomers up to octamer can be washed out with water as impurities from polymers, and are found in wastewater. The first attack on PPG and PTMG was considered to be dependent on dehydrogenases (Kawai and Moriya 1991; Tachibana et al. 2002). The presence of several different PPG dehydrogenases (PPG-DHs), localized in the membrane, the periplasm, and the cytoplasm respectively, has been suggested for PPG-utilizing Stenotrophomonal maltophilia (Tachibana et al. 2002) from which pyrroloquinoline quinone (PQQ)-dependent PPG-DH was purified and characterized as a type-I quinoprotein dehydrogenase, localized in the periplasm (Tachibana et al. 2003). Later, cytoplasmic NAD-dependent PPG-DH was characterized and hypothesized to work on low molecular sizes of PPG in the cytoplasm (Tachibana et al. 2008). This is different from the only membrane-bound PEG-DH suggested for PEG-degrading Sphingomonads. PPG might have more affinity with phospholipids, which are the main constituents of the cytoplasmic membrane, and oligomeric PPGs probably can traverse the cytoplasmic membrane and are metabolized in the cytoplasm (Kawai et al. 1985; Hu et al. 2008a). Oligomeric PPG might express genes related to PPG metabolism.

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