act as antimutagens, anticarcinogens [291, 292], and antimicrobial agents [292–294]. Despite their demonstrated bioactive properties, the action of polyphenols on biological systems is complex and disputed because it is affected by bioavailability, doses, metabolism, and other biotransformations. Bioavailability is strictly related to their structures, like degree of glycosylation and conjugation with other polyphenols. For example, polymers such as proanthocyanidins may have direct effects on the stomach [292] and intestinal mucosa, protecting these tissues from oxidative stress or carcinogen action [277]. In addition, non-glycosylated phenolic compounds may be absorbed directly into the small intestine [277]. In turn, to become absorbed in the intestine, polyphenols present in the form of esters, glycosides, or polymers require hydrolyzation by enterocyte enzymes or through the action of colonic microbiote [295]. The antioxidant activity of polyphenols is arguable. On the one hand, studies in cell line cultures have shown that polyphenols are able to reduce oxidative stress and activate the antioxidant response of the cells. On the other hand, they could decrease cell viability and proliferation and induce cell apoptosis by acting as prooxidants and generating free radicals [296]. The effects of polyphenols in cell cultures (either protective antioxidant or prooxidant/cytotoxic) will depend on several factors such as their concentration, their ability to oxidize, their lipophilicity, the content of other antioxidants and metals, and the oxidative stress level of the cell culture [297–299]. Therefore, to safely use polyphenols as bioactive compounds it is necessary to carefully study aspects such as the specific dose, the delivery vehicle, the nutritional and health history, and the characteristics of the microbe.
2.9 Biopolymer Type Number 8: Polythioesters
In 2001, Lütke-Eversloh et al. published the first report on microbial polythioesters (PTEs). PTEs were synthesized by the same polymerase which synthesizes PHAs make PTEs unique biopolymers [220, 221, 300]. The first examples of PTEs were copolymers containing 3HB and 3-mercaptopropionate (3MP), which were obtained from the PHA accumulating bacterium Ralstonia eutropha strain H16 – a “model organism” in PHA research. Adding different precursor substrates is a tool for accelerating the polymerization process or for incorporating another monomric forma of PMAs. Polythioester production also includes incorporating monomeric formes of 3-hydroxybutyrate and 3-mercaptobutyrate, poly(3HB-co-3MB). The total polymer yield by R. eutropha, when 3-mercaptobutyric acid was fed as a carbon source in addition to gluconate contributed to up to 31% of the cellular dry weight. Mutants of R. eutropha with defective PHA synthase were not able to synthesize these copolymers. This demonstrated that the PHA synthase is responsible for the incorporation of 3MP and generally for biosynthesis of PTEs. If R. eutropha was cultivated in the presence of either 3-mercaptopropionic acid, 3,3’ -thiodipropionic acid (TDP) or 3,3’ -dithiodipropionic acid (DTDP), the copolymer poly- (3HB-co-3MP) was accumulated comprising molar fractions of 3MP of up to 54 mol-%. None of the sulfur-containing precursor substrates was utilized as sole carbon source for growth, thus, a second carbon source such as sodium gluconate was provided additionally to enable bacterial growth. The copolymer composition and polymer content referring to the cellular dry weight could be influenced by varying cultivation conditions and feeding regimes. When carbon sources, which are metabolized to acetyl coenzymeA(acetyl-CoA) were present in the culture medium of R. eutropha, the molar ratios of 3MP were usually less than 5 mol-%. It was observed that the total polymer yield decreased simultaneously to increasing 3MP content. However, this is not a strict rule, because other factors like the duration of fermentation also influenced the molecular weights of the accumulated polymers. 3-mercaptovalerate (3MV) were identified as constituents of PTE copolymers isolated from R. eutropha, extending the group of PTE constituents, which were referred to as 3-mercaptoalkanoates (3MA) [220, 221, 300].
2.10 Conclusion
The biopolymers contain eight groups based on their chemical structure. A ninth group could be added to include the monomers that are produced naturally and polymerized chemically such as polylactic acid. The polymeric forms of the inorganic structures are in need of more investigation. There are some unique criteria for biopolymers because they are derived from nature. They are degradable, bioavailable, compatible, renewable, safe, etc., so they are “green”. They are the main income resources for a number of countries. Their applications are diverse. Some have not yet been commercialize due to petroleum oil-based synthetic polymers. Some species of the biopolymer have attracted attention in different times. For example, polyhydroxyalkanoates attract attention after the oil crises in 1973. Being produced by different types of biological cells their control through biochemical engineering or through different molecular tools enabled better management of their production in their mother wild type host cells or in foreigner cells. We should keep the different schools of biopolymer active. The ones which not being focused on today might be in great demand in the future.
Acknowledgement
The author acknowledges his mentors Professor Dr. Alexander Steinbüchel and Professor Dr. Bernd Rhem and the entire membership of the institute of Molecular Mikrobiologie und Biotechnologie, Mathematish-Naturwissenschaftlichen Fakultät der Westfälische Wilhelms-Universitüt Münster, Germany. Special thanks to the members of lab 06. The author acknowledges the DAAD for the grant provided as a PhD scholarship.
Conflict of Interest
The author declares that there is no any kind of conflict with any concerning this chapter
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