Metabolic Engineering of Poly(3-Hydroxyalkanoates): From DNA to Plastic

Abstract

SUMMARY Poly(3-hydroxyalkanoates) (PHAs) are a class of microbially produced polyesters that have potential applications as conventional plastics, specifically thermoplastic elastomers. A wealth of biological diversity in PHA formation exists, with at least 100 different PHA constituents and at least five different dedicated PHA biosynthetic pathways. This diversity, in combination with classical microbial physiology and modern molecular biology, has now opened up this area for genetic and metabolic engineering to develop optimal PHA-producing organisms. Commercial processes for PHA production were initially developed by W. R. Grace in the 1960s and later developed by Imperial Chemical Industries, Ltd., in the United Kingdom in the 1970s and 1980s. Since the early 1990s, Metabolix Inc. and Monsanto have been the driving forces behind the commercial exploitation of PHA polymers in the United States. The gram-negative bacterium Ralstonia eutropha, formerly known as Alcaligenes eutrophus, has generally been used as the production organism of choice, and intracellular accumulation of PHA of over 90% of the cell dry weight have been reported. The advent of molecular biological techniques and a developing environmental awareness initiated a renewed scientific interest in PHAs, and the biosynthetic machinery for PHA metabolism has been studied in great detail over the last two decades. Because the structure and monomeric composition of PHAs determine the applications for each type of polymer, a variety of polymers have been synthesized by cofeeding of various substrates or by metabolic engineering of the production organism. Classical microbiology and modern molecular bacterial physiology have been brought together to decipher the intricacies of PHA metabolism both for production purposes and for the unraveling of the natural role of PHAs. This review provides an overview of the different PHA biosynthetic systems and their genetic background, followed by a detailed summation of how this natural diversity is being used to develop commercially attractive, recombinant processes for the large-scale production of PHAs.

Polyhydroxyalkanoates: Properties and chemical modification approaches for their functionalization

“…14 PHAs are generally composed of (R)-b-hydroxy fatty acids, where the pendant group (R) varies from methyl (C1) to tridecyl (C13) (adopted from Ref. 103).…”

Involvement of poly(3‐hydroxybutyrate) synthesis in catabolite repression of tetralin biodegradation genes in Sphingomonas macrogolitabida strain TFA

“…It could be that the repressing signal is the granule itself or, alternatively, the functioning of the PHB biosynthetic pathway. Because many bacteria synthesize polyhydroxyalkanoates as a sink for carbon and reducing equivalents (Madison and Huisman, 1999), and NADPH is required in this pathway to reduce acetoacetyl-CoA to 3-hydroxybutyryl-CoA, it is possible that altering the polymer synthesis had significant consequences on the bacterial physiology that leads to the sensing of carbon limitation when the polymer cannot be accumulated under carbon excess conditions. These results are fully in agreement with those observed in P. putida where the lack of PHA accumulation results in a respiration of the excess of carbon source rather than incorporating it into biomass or other compounds that could be reused as carbon and energy sources (data unpublished, M.A.…”

Biosynthesis of 2‐Hydroxyacid‐Containing Polyhydroxyalkanoates by Employing butyryl‐CoA Transferases in Metabolically Engineered Escherichia coli

“…Bio-based polymers synthesized from renewable resources are promising candidates for overcoming environmental problems such as global warming and environmental pollution caused by wasted and un-degraded synthetic polymers, as they are carbon neutral polymers that can be carbon-negative-footprint polymers by using suitable recycling process and can be biodegradable and biocompatible depending on the types and compositions of monomers in the polymers. [3][4][5]24] The production costs and material properties of bio-based polymers must be competitive to those of chemically synthesized polymers for successful commercialization. Thus, biorefinery processes have been hallii butyryl-CoA transferase cultured in MR medium containing 20 g L À1 glucose supplemented with 1 g L À1 3HB and 1 g L À1 2HB.…”

“…[3,4] More than 150 hydroxycarboxylic acids containing various functional groups have been identified as possible monomers for PHAs, most of which can be synthesized from cellular metabolic intermediates. [3][4][5] PHAs are synthesized in cells and accumulated in the cytoplasmic space as distinct hydrophobic granules. [3,4] Generally, PHA synthesis occurs in microorganisms in two major steps.…”

“…[3][4][5] PHAs are synthesized in cells and accumulated in the cytoplasmic space as distinct hydrophobic granules. [3,4] Generally, PHA synthesis occurs in microorganisms in two major steps. First, hydroxyacyl-CoAs (HA-CoAs) are generated through recombinant or inherent metabolic routes in host microorganisms.…”