Conclusion On Xenobiotics In The Environment

The degradation of polymers was at first expected to be catalyzed solely by extracellular enzymes, given the assumption that macromolecules are never incorporated into cells, but this was disproved by the periplasmic degradation of PEG and PVA, as described above. These are surely incorporated into the periplasm through the outer membranes of Sphingomonads and are metabolized by periplasmic enzymes, although the mechanism of macromolecule uptake has not been well characterized. Many enzymes related to the microbial degradation of polymers have been elucidated genetically. They show unique features: PEG-DH creates a small branch in GMC flavoprotein oxidoreductases (Zamocky et al. 2004) and PEG-aldehyde dehydrogenase was the first nicotinoprotein aldehyde dehydrogenase. Ether bond-splitting enzymes are not catalyzed by ether bond-specific enzymes, but by various divergent enzymes such as monooxygenase, oxidase, dehydrogenase, hydrolase, and lyase. PVA-DHs form a unique group of quinohemoprotein dehydrogenases. Research on nylon oligomer-degrading enzymes has verified that xenobiotic polymer-degrading enzymes can readily evolve by spontaneous mutation from ancestor enzymes that originally recognized natural compounds analogous to the target polymers. A PEG-DH gene from a Sphingomonad has been distributed and conserved among different genera in the 35 years since PEG-degrading Sphingomonads were isolated in 1975 and a gene for PEG-DH was cloned in 2001, suggesting a significant role for a large plasmid harboring a PEG-degradative gene cluster in circulation for degradation ability among microorganisms. Since a pva operon is on a large plasmid of Sphingopyxis sp. strain 113P3, the PVA-DH gene and the operon structure have probably been distributed among microorganisms. This strongly suggests that the short history of xenobiotic polymers to date has been sufficient for degrading enzymes to evolve since the prototype enzymes existed and the microorganisms had to adapt to an environment contaminated by xenobiotics. In addition, megaplasmids must have sped up the distribution rate of degrading ability. The existence of Sphingomonads that degrade various xenobiotic polymers, such as PEG and PVA, via intracellular enzymes suggest that they have a means of taking macromolecules into the periplasm as well as metabolic enzymes adapted to respond to these macromolecules. Polymer-degrading microorganisms and their enzymes for polymer degradation have been well described. Many examples of symbiotic polymer degradation appear to suggest that symbiotic degradation occurs in an ecosystem when new artificial compounds are introduced. At the same time, non-metabolic polymer degradation suggests that the ecosystem has the disposal potential and versatility in terms of new artificial compounds.

However, information is still limited with regard to the operonic structures of the genes related to degradation and to the regulation of individual genes and operons, except for those involved in PEG and PVA degradation. The peg operon is expressed in the presence of PEG, but expression is triggered by oligomers having the size of more than tetramer. The pva operon is constitutively expressed, but expression is remarkably enhanced by PVA. The size of PVA that triggers promotion of the pva operon remains to be determined. Since PVA-DH and oxidized PVA-hydrolyzing enzymes are periplasmic enzymes, it is likely that depolymerized oligomeric PVA can be incorporated into the cytoplasm where it promotes the expression of the pva operon. Extracellular PLA-degrading proteases from PLA-degrading microorganisms have been induced with proteinaceous materials. Since it is probably impossible for hard, solid PLA to penetrate cells, the microorganisms must have selected another strategy for degradation, one that employs extracellular excretion of proteases induced by proteinous materials including alanine, analogously to lactate. An aliphatic-aromatic-co-polyester-degrading enzyme from T. alba strain AHK119 was expressed constitutively (unpublished data), as the target polyester can never be incorporated into cells. The polyester-degrading enzymes belong to the lipase family and are distinguished from true lipases in the sense that they have no lid covering the active site, which is prerequisite to the interfacial activation, characteristic of true lipases. They must have acquired the ability to recognize and hydrolyze solid macromolecules, but the details of the mechanism are still under study. Thus, microorganisms have employed a variety of adaptation strategies in relation to novel artificial compounds in a short time. Although oligomeric structures are biodegradable, some polymers, such as PE, polystylene, and polyacrylonitrile, are not practically biodegradable. This suggests limitations in the biodegradation of solid polymers with regard to their physico-chemical properties, such as MW, Tg, and mp. In other words, we can design biodegradable polymers and non-biodegradable polymers based on their intended uses.

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