I'm on holiday at the moment, so today's post is another section from my long essay last year, about the potential uses of biorefineries. It was written for a more scientific-based audience so might be a little harder to decipher than my usual posts.
Bioplastics are polyesters that accumulate intracellularly in microorganisms in storage granules. They are usually built up from hydroxyl-acyl CoA derivatives through a range of different pathways in different microorganisms. As they are both biodegradable and biocompatible they have found numerous applications within medical and surgical fields, as well as having a greater environmental advantage over petroleum based plastics. The main disadvantages of bioplastics for commercial use are their high production and recovery costs.
The most widely produced bioplastics are poly(3-hydroxybutyrate) and poly(hydroxyalkanoic acid), referred to as PHB and PHA respectively. These both contain different β-oxidation intermediates as monomers, which are enzymatically polymerised through a condensation reaction. The structure of PHA is shown below (the 'n' indicates that the section show below is repeated multiple times):The first bioplastic to be described was PHB, found in Bacillus megaterium in 1926 by Lemoigne. It is stored in polymer form in granules within the cell.In order to decrease the recovery costs of the PHB granules, several attempts have been made to produce the secreted monomers, for polymerization outside the bacterial system. This has been achieved by expressing recombinant genes in E. coli
There are a large number of PHA polymers, ninety-one of which have been fully characterised. They are produced by both Gram negative and Gram positive bacteria via at least five different metabolic pathways. The main enzyme involved in polymer formation is PHA synthase (of the α/β hydrolase family), which polymerizes the monomers by connecting the coenzyme A thioesters of one monomer to the hydroxyl groups at positions 3, 4, 5 or 6 of the acyl moiety of the second monomer. There are four classes of PHA synthase, which are distinguished by their primary structures, substrate specificity and subunit composition. PHA synthases are found on the surface of the PHA storage granules, along with other proteins, and phospholipids.
(The structure of a PHA granule is shown above, image taken from Rehm 2003)
Engineering of recombinant bacteria that are capable of producing bioplastics requires both the transfer of a functional PHA synthase enzyme (there is no evidence as yet to suggest that any post-translational modifications of the enzyme are important for its function), and the engineering of suitable substrates that provide the enzyme with suitable substrates and sufficient concentrations. While the enzyme has been successfully transferred into model organisms such as E. coli, S. cerevisiae and even some transgenic plants, the provision of substrates is a more difficult problem as it involves dealing with large numbers of interlinked metabolic pathways. Metabolic flux analysis, carried out in transgenic E. coli, has substantially increased the carbon flux towards the production of PHB without detriment to the health of the bacteria, however this form of analysis has not yet been carried out on more complex PHA polymers.
Madison LL, & Huisman GW (1999). Metabolic engineering of poly(3-hydroxyalkanoates): from DNA to plastic. Microbiology and molecular biology reviews : MMBR, 63 (1), 21-53 PMID: 10066830
Steinbuchel, A., & Valentin, H. (1995). Diversity of bacterial polyhydroxyalkanoic acids FEMS Microbiology Letters, 128 (3), 219-228 DOI: 10.1111/j.1574-6968.1995.tb07528.x
Rehm BH (2003). Polyester synthases: natural catalysts for plastics. The Biochemical journal, 376 (Pt 1), 15-33 PMID: 12954080
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