Biopolymers microbial-synthesized biopolymer

The situation is different for aqueous species of humic substances, the organic matter in soil that is not identifiable as unaltered or partially altered biomass or as conventional biomolecules.21 Humic substances comprise organic compounds that are not synthesized directly to sustain the life cycles of the soil biomass. More specifically, they comprise polymeric molecules produced through microbial action that differ from biopolymers because of their molecular structure and their long-term persistence in soil. This definition of humic substances implies no particular set of organic compounds, range of relative molecular mass, or mode of chemical reactivity. What is essential is dissimilarity to conventional biomolecular structures and biologically refractory behavior. 

Most microbial exopolysaccharides are apparently synthesized in-tracellularly. However, with various Leuconostoc, Streptococcus, and Bacillus species, such exopolysaccharides as dextran and levan can be formed by adding proper substrates that do not penetrate the cell membrane.216,217 Surprisingly little information is available about the biosynthesis of biopolymers of commercial value. However, as most of them are probably formed intracellularly, the process by which substrates enter microbial cells, where they are modified by various enzymic reactions and finally excreted in polymerized form into the medium, bears attention. Even with a lack of complete biosynthetic information, the results of research on related micro-organisms may be extrapolated to form a reasonable hypothesis for the biosynthesis of polysaccharides. 

Polylactic acid (PLA) is the second common biopolymer that is produced by microbial fermentation. It is thermoplastic aliphatic polyester that can be synthesized from biologically produced lactic acid polymerized by ring opening polymerization. Lactic acid is a chiral molecule existing as two stereoisomers, L- and D-lactic acid, which can be produced by different ways, i.e., biologically or chemically synthesized [Averous, 2008). 

Figure 1 Biopolymer recyclable ecosystem Involving biological and/or chemical syntheses and biological degradation. The ecosystem consists of upstream process for blorefinety and bioprocesses such as microbial factory and plant factory for blopolymer production. Representative biopolymers are PHA and PLA. Reproduced with permission from Taguchi, S. Tsuge, T. In Protein Engineering Handbook, Lutz, S., Bomschuer, U. T., Eds. Wiley-VCH Weinheim, 2008 p 877, published by Wiley.

Figure 10 shows the inherit advantage of lactic acid produced by the universal metabohc pathway shared by most living organisms from humans, to plants, to microbes. Through the anaerobic glycolysis pathway, two lactic acid molecules (A/ =90) are produced from one glucose (M =180) with a 100% theoretical yield (i.e., no loss at all). For the anaerobic fermentation of ethanol, the theoretical yield is around 51% with two CO M =AA) molecules lost per glucose. After ethanol has been dehydrated to ethylene and polymerized into bio-PE, the theoretical yield from glucose is only around 31% compared with 80% for PLA (Fig. 10). For PHBV-like biopolymer synthesized in microbial cells, the building block is typically acetyl-CoA with around 51% theoretical yield after the loss of CO (decarboxylation) from...

Polyhydroxyalkanoates (PHAs) such as poly(4-hydroxybutyric acid) (P(4HB)), poly(4-hydroxyvalerate) (P(4HV)), and their copolymers are bacterial polyesters synthesized by microbial fermentation. They have been described in detail in the earlier section on hydrolytically resorbable biopolymers. They are also found to be resorbable by enzymatic action in vivo. This particular family of bacterial polyesters is one of the most promising biomaterials currently under investigation. 

Like polymers, biodegradable polymers have been classified using distinct methods, including methods based on polymer origin (synthesis), renewability content, and biodegradability level. The most prominent and well-established method is the classification based on synthetic procedmes. In broad terms, these polymers are classified into two main categories (i) agropolymers (obtained from biomass) and (ii) biopolyesters (natural or synthetic biopolymers). Biopolyesters is itself a broad term that includes three subclasses (a) microbially produced polymers (b) polymers synthesized... 

Depending on their origins, biopolymers used as matrices in composite materials are divided into three main classes agropolymer based (renewable sources), microbially derived, and chemically synthesized. Some authors also mention a fourth class, which consists of blends of the aforementioned three classes. 

Renewable sources Chemical synthesized Microbial aynthealzed Biopolymer blends... 

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