Polyhydroxybutyrate (PHB) is a

polyhydroxyalkanoate Polyhydroxyalkanoates or PHAs are polyesters produced in nature by numerous microorganisms, including through bacterial fermentation of sugars or lipids. When produced by bacteria they serve as both a source of energy and as a carbon store. More ...

(PHA), a

polymer A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...

belonging to the

polyester Polyester is a category of polymers that contain the ester functional group in every repeat unit of their main chain. As a specific material, it most commonly refers to a type called polyethylene terephthalate (PET). Polyesters include natural ...

s class that are of interest as bio-derived and

biodegradable plastic Biodegradable plastics are plastics that can be decomposed by the action of living organisms, usually microbes, into water, carbon dioxide, and biomass. Biodegradable plastics are commonly produced with renewable raw materials, micro-organisms, ...

s. 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 their

copolymer In polymer chemistry, a copolymer is a polymer derived from more than one species of monomer. The polymerization of monomers into copolymers is called copolymerization. Copolymers obtained from the copolymerization of two monomer species are some ...

Biosynthesis

PHB is produced by

microorganisms A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...

(such as ''

Cupriavidus necator ''Cupriavidus necator'' is a Gram-negative soil bacterium of the class Betaproteobacteria. Taxonomy ''Cupriavidus necator'' has gone through a series of name changes. In the first half of the 20th century, many micro-organisms were isolated ...

'', ''

Methylobacterium rhodesianum Methylorubrum rhodesianum is a species of alphaproteobacteria. It has been found on the International Space Station The International Space Station (ISS) is the largest modular space station currently in low Earth orbit. It is a multinat ...

'' or '' Bacillus megaterium'') apparently in response to conditions of physiological stress; mainly conditions in which nutrients are limited. The polymer is primarily a product of

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...

assimilation (from

glucose Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, using ...

starch Starch or amylum is a polymeric carbohydrate consisting of numerous glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants for energy storage. Worldwide, it is the most common carbohydrate in human diets ...

) and is employed by microorganisms as a form of energy storage molecule to be metabolized when other common energy sources are not available. Microbial biosynthesis of PHB starts with the

condensation Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapor to ...

of two molecules of

acetyl-CoA Acetyl-CoA (acetyl coenzyme A) is a molecule that participates in many biochemical reactions in protein, carbohydrate and lipid metabolism. Its main function is to deliver the acetyl group to the citric acid cycle (Krebs cycle) to be oxidized for ...

to give acetoacetyl-CoA which is subsequently reduced to hydroxybutyryl-CoA. This latter compound is then used as a monomer to polymerize PHB. PHAs granules are then recovered by disrupting the cells.

Thermoplastic polymer

Most commercial plastics are synthetic polymers derived from

petrochemical Petrochemicals (sometimes abbreviated as petchems) are the chemical products obtained from petroleum by refining. Some chemical compounds made from petroleum are also obtained from other fossil fuels, such as coal or natural gas, or renewable sou ...

s. They tend to resist

biodegradation Biodegradation is the breakdown of organic matter by microorganisms, such as bacteria and fungi. It is generally assumed to be a natural process, which differentiates it from composting. Composting is a human-driven process in which biodegrada ...

. PHB-derived plastics are attractive because they are

compost Compost is a mixture of ingredients used as plant fertilizer and to improve soil's physical, chemical and biological properties. It is commonly prepared by decomposing plant, food waste, recycling organic materials and manure. The resulting m ...

able and derived from renewables and are bio-degradable. ICI had developed the material to pilot 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 of

polypropylene Polypropylene (PP), also known as polypropene, is a thermoplastic polymer used in a wide variety of applications. It is produced via chain-growth polymerization from the monomer propylene. Polypropylene belongs to the group of polyolefins and ...

. Some bottles were made for Wella's "Sanara" range of shampoo; an example using the tradename "Biopol" is in the collection of the

Science Museum A science museum is a museum devoted primarily to science. Older science museums tended to concentrate on static displays of objects related to natural history, paleontology, geology, industry and industrial machinery, etc. Modern trends in mu ...

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 "Biopo

However, Monsanto's rights to Biopol were sold to the American company

Metabolix Yield10 Bioscience (formerly Metabolix, Inc.) is a company developing new technologies to achieve improvements in crop yield to enhance global food security. History Founded in 1992, with the help of a licensing agreement with Massachusetts Inst ...

in 2001 and Monsanto's fermenters producing PHB from bacteria were closed down at the start of 2004. Monsanto began to focus on producing PHB from plants instead of bacteria. But now with so much media attention on GM crops, there has been little news of Monsanto's plans for PHB. Biopol is currently used in the medical industry for internal suture. It is nontoxic and biodegradable, so it does not have to be removed after recovery. 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 as absorbable sutures.

Properties

*Water-insoluble and relatively resistant to hydrolytic degradation. This differentiates PHB from most other currently available

s, which are either water-soluble or moisture-sensitive. *Good oxygen permeability. *Good ultra-violet resistance but poor resistance to acids and bases. *Soluble in chloroform and other chlorinated hydrocarbons. *Biocompatible and hence is suitable for medical applications. *Melting point 175 °C., and glass transition temperature 2 °C. *Tensile strength 40 MPa, close to that of polypropylene. *Sinks in water (while polypropylene floats), facilitating its anaerobic biodegradation in sediments. *Nontoxic. *Less 'sticky' when melted

History

Polyhydroxybutyrate was first isolated and characterized in 1925 by French

microbiologist A microbiologist (from Ancient Greek, Greek ) is a scientist who studies microscopic life forms and processes. This includes study of the growth, interactions and characteristics of Microorganism, microscopic organisms such as bacteria, algae, f ...

Maurice Lemoigne Maurice may refer to: People *Saint Maurice (died 287), Roman legionary and Christian martyr *Maurice (emperor) or Flavius Mauricius Tiberius Augustus (539–602), Byzantine emperor *Maurice (bishop of London) (died 1107), Lord Chancellor and Lo ...

Biodegradation

Firmicutes and proteobacteria can degrade PHB. ''Bacillus'', ''Pseudomonas'' and ''Streptomyces'' species can degrade PHB. ''

Pseudomonas lemoigne ''Pseudomonas'' is a genus of Gram-negative, Gammaproteobacteria, belonging to the family Pseudomonadaceae and containing 191 described species. The members of the genus demonstrate a great deal of metabolic diversity and consequently are able t ...

'', ''Comamonas'' sp. ''

Acidovorax faecalis ''Acidovorax'' is a genus of within the family Comamonadaceae The Comamonadaceae are a family of the Betaproteobacteria.Willems A., J. De Ley, M. Gillis, and K. Kersters. ''Comamonadaceae, a New Family Encompassing the Acidovorans rRNA Comple ...

'', ''

Aspergillus fumigatus ''Aspergillus fumigatus'' is a species of fungus in the genus ''Aspergillus'', and is one of the most common ''Aspergillus'' species to cause disease in individuals with an immunodeficiency. ''Aspergillus fumigatus'', a saprotroph widespread in ...

'' and ''

Variovorax paradoxus ''Variovorax paradoxus'' is a gram negative, beta proteobacterium from the genus ''Variovorax''. Strains of ''V. paradoxus'' can be categorized into two groups, hydrogen oxidizers and heterotrophic strains, both of which are aerobic. The genus na ...

'' are soil microbes capable of degradation. '' Alcaligenes faecalis'', '' Pseudomonas'', and '' Illyobacter delafieldi'', are obtained from anaerobic sludge. ''

Comamonas testosteroni ''Comamonas testosteroni'' is a Gram-negative soil bacterium. Strain I2''gfp'' has been used in bioaugmentation trials, in attempts to treat the industrial byproduct 3-chloroaniline. This species can also digest plastic Plastics are a wide ...

'' and ''

Pseudomonas stutzeri ''Pseudomonas stutzeri'' is a Gram-negative soil bacterium that is motile, has a single polar flagellum, and is classified as bacillus, or rod-shaped. While this bacterium was first isolated from human spinal fluid, it has since been found in man ...

'' were obtained from sea water. Few of these are capable of degrading at higher temperatures; notably excepting thermophilic ''Streptomyces'' sp. and a thermophilic strain of ''Aspergillus'' sp.{{cite journal , doi=10.3390/ijms10093722 , pmid=19865515 , pmc=2769161 , title=Biodegradability of Plastics , journal=International Journal of Molecular Sciences , volume=10 , issue=9 , pages=3722–42 , year=2009 , last1=Tokiwa , first1=Yutaka , last2=Calabia , first2=Buenaventurada P. , last3=Ugwu , first3=Charles U. , last4=Aiba , first4=Seiichi , doi-access=free

References

External links

Abstract of award for PHAs
Polyesters Biodegradable plastics Thermoplastics Bioplastics