Occurrence, metabolism, metabolic role, and industrial uses of bacterial polyhydroxyalkanoates

Abstract

Polyhydroxyalkanoates (PHAs), of which polyhydroxybutyrate (PHB) is the most abundant, are bacterial carbon and energy reserve materials of widespread occurrence. They are composed of 3-hydroxyacid monomer units and exist as a small number of cytoplasmic granules per cell. The properties of the C4 homopolymer PHB as a biodegradable thermoplastic first attracted industrial attention more than 20 years ago. Copolymers of C4 (3-hydroxybutyrate [3HB]) and C5 (3-hydroxyvalerate [3HV]) monomer units have modified physical properties; e.g., the plastic is less brittle than PHB, whereas PHAs containing C8 to C12 monomers behave as elastomers. This family of materials is the centre of considerable commercial interest, and 3HB-co-3HV copolymers have been marketed by ICI plc as Biopol. The known polymers exist as 2(1) helices with the fiber repeat decreasing from 0.596 nm for PHB to about 0.45 nm for C8 to C10 polymers. Novel copolymers with a backbone of 3HB and 4HB have been obtained. The native granules contain noncrystalline polymer, and water may possibly act as a plasticizer. Although the biosynthesis and regulation of PHB are generally well understood, the corresponding information for the synthesis of long-side-chain PHAs from alkanes, alcohols, and organic acids is still incomplete. The precise mechanisms of action of the polymerizing and depolymerizing enzymes also remain to be established. The structural genes for the three key enzymes of PHB synthesis from acetyl coenzyme A in Alcaligenes eutrophus have been cloned, sequenced, and expressed in Escherichia coli. Polymer molecular weights appear to be species specific. The factors influencing the commercial choice of organism, substrate, and isolation process are discussed. The physiological functions of PHB as a reserve material and in symbiotic nitrogen fixation and its presence in bacterial plasma membranes and putative role in transformability and calcium signaling are also considered.

Screening for Methane Utilizing Mixed Communities with High Polyhydroxybutyrate (PHB) Production Capacity Using Different Design Approaches

“…Thus, there has been an evolutionary approach to find a more suitable ecofriendly biodegradable polymer alternative, i.e., bio-based plastic, that can be synthesized from nature-acquired resources, such as the plant-based polylactic acid (PLA) and the bacteria-based polyhydroxyalkanoate (PHA). PHA polymers can be produced using sustainable renewable waste materials, unlike petroleum-based plastics, which are produced from nonrenewable fossil fuels [ 4 ]. PHAs are microbially synthesized hydrophobic inclusions that are stored intracellularly under stress conditions of phosphorus, nitrogen, and oxygen limitations accompanied by carbon abundance [ 5 ].…”

The Thermal and Mechanical Properties of Medium Chain-Length Polyhydroxyalkanoates Produced by Pseudomonas putida LS46 on Various Substrates

“…This variability has produced PHAs with drastically different polymer properties (Brandl et al, 1988). PHAs have been compared to polypropylene, low-density polyethylene, rubber and adhesives due to the range of observed PHA properties (Anderson and Dawes, 1990;de Koning et al, 1994;Chen, 2010;Mozejko and Ciesielski, 2014). Possible applications for PHAs include molds and films for replacing single-use plastics, laminates, composites, coatings, adhesives, and biocompatible products for the biomedical (Madison and Huisman, 1999).…”

Wine By-Products as Raw Materials for the Production of Biopolymers and of Natural Reinforcing Fillers: A Critical Review

“…Poly(hydroxyalkanoates) (PHAs) belong to a family of fully biodegradable polyesters that generally consist of 3-, 4-, 5-, and 6-hydroxyl-carboxylic acids. PHAs are usually accumulated by bacterial fermentation [108] or, less often, by transgenic microorganism [109] and plants [110] in intracellular granulates as carbon and energy reserves when nutrient supplies are imbalanced. PHAs are very interesting biopolymers because of their renewability, biodegradability, biocompatibility, and extreme versatility.…”