Polyhydroxybutyrate (PHB) is a polyhydroxyalkanoate (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 natura ...

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 ...

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'', '' Methylobacterium rhodesianum'' or ''

Bacillus megaterium ''Bacillus megaterium'' is a rod-like, Gram-positive, mainly aerobic spore forming bacterium found in widely diverse habitats.De Vos, P. ''et al.'' Bergey's Manual of Systematic Bacteriology: Volume 3: The Firmicutes. ''Springer'' (2009) It has ...

'') 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—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon ma ...

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, u ...

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 die ...

) 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 vapo ...

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 renewabl ...

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 biodegra ...

. 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 ...

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

pilot plant A pilot plant is a pre-commercial production system that employs new production technology and/or produces small volumes of new technology-based products, mainly for the purpose of learning about the new technology. The knowledge obtained is then ...

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 a ...

. 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 ...

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 Ins ...

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 MPA or mPa may refer to: Academia Academic degrees * Master of Performing Arts * Master of Professional Accountancy * Master of Public Administration * Master of Public Affairs Schools * Mesa Preparatory Academy * Morgan Park Academy * Mou ...

, 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 Greek ) is a scientist who studies microscopic life forms and processes. This includes study of the growth, interactions and characteristics of microscopic organisms such as bacteria, algae, fungi, and some types of para ...

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 L ...

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 to ...

'', ''Comamonas'' sp. '' Acidovorax faecalis'', ''

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 i ...

'' 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 ''Alcaligenes faecalis'' is a species of Gram-negative, rod-shaped bacteria commonly found in the environment. It was originally named for its first discovery in feces, but was later found to be common in soil, water, and environments in associa ...

'', ''

Pseudomonas ''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 ...

'', 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 A by-product or byproduct is a secondary product derived from a production ...

'' 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