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Polyhydroxybutyrate (PHB) is apolyhydroxyalkanoate (PHA), apolymer belonging to thepolyesters class that are of interest as bio-derived andbiodegradable plastics.[1] The poly-3-hydroxybutyrate (P3HB) form of PHB is probably the most common type of polyhydroxyalkanoate, but other polymers of this class are produced by a variety of organisms: these include poly-4-hydroxybutyrate (P4HB), polyhydroxyvalerate (PHV), polyhydroxyhexanoate (PHH), polyhydroxyoctanoate (PHO) and theircopolymers.
PHB is produced bymicroorganisms (such asCupriavidus necator,Methylobacterium rhodesianum orBacillus megaterium) apparently in response to conditions of physiological stress;[2] mainly conditions in which nutrients are limited. The polymer is primarily a product ofcarbon assimilation (fromglucose orstarch) and is employed by microorganisms as a form of energy storage molecule to be metabolized when other common energy sources are not available.[citation needed]
Microbial biosynthesis of PHB starts with thecondensation of two molecules ofacetyl-CoA to give acetoacetyl-CoA which is subsequently reduced to hydroxybutyryl-CoA. This latter compound is then used as a monomer to polymerize PHB.[3] PHAs granules are then recovered by disrupting the cells.[4]
Most commercial plastics are synthetic polymers derived frompetrochemicals. They tend to resistbiodegradation. PHB-derived plastics are attractive because they arecompostable and derived from renewables and are bio-degradable.
ICI had developed the material topilot plant stage in the 1980s, but interest faded when it became clear that the cost of material was too high, and its properties could not match those ofpolypropylene. Some bottles were made for Wella's "Sanara" range of shampoo; an example using the tradename "Biopol" is in the collection of theScience Museum, London.
In 1996, Monsanto (who sold PHB as a copolymer with PHV) bought all patents for making the polymer from ICI/Zeneca including the trademark "Biopol".[5] However, Monsanto's rights to Biopol were sold to the American companyMetabolix in 2001 and Monsanto's fermenters producing PHB from bacteria were closed down at the start of 2004.
The first report of PHB production in transgenic plants was in 1992, inArabidopsis. This involved inserting genes for two enzymes fromAlcaligenes eutrophus so that enzymes were produced continuously to synthesise PHB fromacetoacetyl-CoA.[6] Systems to produce PHB by plant cells in culture were also developed.[7] Monsanto had interest in producing PHB from plants instead of bacteria. But with media attention on GM crops, there was little news of Monsanto's plans for PHB after 2005.[8]
Biopol is currently used in the medical industry forinternal suture. It is nontoxic and biodegradable, so it does not have to be removed after recovery.[9]
TephaFLEX is a bacterially derived poly-4-hydroxybutyrate, manufactured using a recombinant fermentation process by Tepha Medical Devices, intended for a variety of medical applications that require biodegradable materials such asabsorbable sutures. It was first marketed in 2008.[10][11] In 2021 the parent company Tepha, Inc was bought byBD, an international medical technology company.[12]
Polyhydroxybutyrate was first isolated and characterized in 1925 by FrenchmicrobiologistMaurice Lemoigne.[14]
Firmicutes and proteobacteria can degrade PHB.Bacillus,Pseudomonas andStreptomyces species can degrade PHB.Pseudomonas lemoigne,Comamonas sp.Acidovorax faecalis,Aspergillus fumigatus andVariovorax paradoxus are soil microbes capable of degradation.Alcaligenes faecalis,Pseudomonas, andIllyobacter delafieldi, are obtained from anaerobic sludge.Comamonas testosteroni andPseudomonas stutzeri were obtained from sea water. Few of these are capable of degrading at higher temperatures; notably excepting thermophilicStreptomyces sp. and a thermophilic strain ofAspergillus sp.[15]