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Polyhydroxybutyrate

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Polyhydroxybutyrate (PHB) is a <a href="Polyhydroxyalkanoates" title="Polyhydroxyalkanoates">polyhydroxyalkanoate</a> (PHA), a <a href="Polymer" title="Polymer">polymer</a> belonging to the <a href="Polyester" title="Polyester">polyesters</a> class that are of interest as bio-derived and <a href="Biodegradable_plastic" title="Biodegradable plastic">biodegradable plastics</a>.<a href="#cite_note-1">[1]</a> 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 <a href="Copolymer" title="Copolymer">copolymers</a>.

<div class="mw-heading mw-heading2"><h2 id="Biosynthesis">Biosynthesis</h2></div>

PHB is produced by <a href="Microorganisms" class="mw-redirect" title="Microorganisms">microorganisms</a> (such as <a href="Cupriavidus_necator" title="Cupriavidus necator">Cupriavidus necator</a>, <a href="Methylobacterium_rhodesianum" class="mw-redirect" title="Methylobacterium rhodesianum">Methylobacterium rhodesianum</a> or <a href="Bacillus_megaterium" class="mw-redirect" title="Bacillus megaterium">Bacillus megaterium</a>) apparently in response to conditions of physiological stress;<a href="#cite_note-AckermannMüller1995-2">[2]</a> mainly conditions in which nutrients are limited. The polymer is primarily a product of <a href="Carbon" title="Carbon">carbon</a> assimilation (from <a href="Glucose" title="Glucose">glucose</a> or <a href="Starch" title="Starch">starch</a>) 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 <a href="Condensation_reaction" title="Condensation reaction">condensation</a> of two molecules of <a href="Coenzyme_A" title="Coenzyme A">acetyl-CoA</a> to give acetoacetyl-CoA which is subsequently reduced to hydroxybutyryl-CoA. This latter compound is then used as a monomer to polymerize PHB.<a href="#cite_note-Biopolymers-3">[3]</a> PHAs granules are then recovered by disrupting the cells.<a href="#cite_note-4">[4]</a>

<div class="mw-heading mw-heading2"><h2 id="Thermoplastic_polymer">Thermoplastic polymer</h2></div>

Most commercial plastics are synthetic polymers derived from <a href="Petrochemical" title="Petrochemical">petrochemicals</a>. They tend to resist <a href="Biodegradation" title="Biodegradation">biodegradation</a>. PHB-derived plastics are attractive because they are <a href="Compost" title="Compost">compostable</a> and derived from renewables and are bio-degradable.

<a href="Imperial_Chemical_Industries" title="Imperial Chemical Industries">ICI</a> had developed the material to <a href="Pilot_plant" title="Pilot plant">pilot plant</a> 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 <a href="Polypropylene" title="Polypropylene">polypropylene</a>. Some bottles were made for Wella's "Sanara" range of shampoo; an example using the tradename "Biopol" is in the collection of the <a href="Science_Museum%2C_London" title="Science Museum, London">Science Museum</a>, 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".<a href="#cite_note-5">[5]</a> However, Monsanto's rights to Biopol were sold to the American company <a href="Metabolix" class="mw-redirect" title="Metabolix">Metabolix</a> 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.<a href="#cite_note-Poirier-6">[6]</a> But now with so much media attention on GM crops, there has been little news of Monsanto's plans for PHB.<a href="#cite_note-CouldEat-7">[7]</a>

Biopol is currently used in the medical industry for <a href="Surgical_suture" title="Surgical suture">internal suture</a>. It is nontoxic and biodegradable, so it does not have to be removed after recovery.<a href="#cite_note-8">[8]</a>

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 <a href="Surgical_suture#Absorbability" title="Surgical suture">absorbable sutures</a>.<a href="#cite_note-9">[9]</a>

<div class="mw-heading mw-heading2"><h2 id="Properties">Properties</h2></div>

<ul><li>Water-insoluble and relatively resistant to hydrolytic degradation. This differentiates PHB from most other currently available <a href="Biodegradable_plastic" title="Biodegradable plastic">biodegradable plastics</a>, which are either water-soluble or moisture-sensitive.</li>

<li>Good oxygen permeability.</li>

<li>Good ultra-violet resistance but poor resistance to acids and bases.</li>

<li>Soluble in chloroform and other chlorinated hydrocarbons.<a href="#cite_note-10">[10]</a></li>

<li>Biocompatible and hence is suitable for medical applications.</li>

<li>Melting point 175 °C., and glass transition temperature 2 °C.</li>

<li>Tensile strength 40 <a href="Pascal_(unit)" title="Pascal (unit)">MPa</a>, close to that of polypropylene.</li>

<li>Sinks in water (while polypropylene floats), facilitating its anaerobic biodegradation in sediments.</li>

<li>Non-toxic.</li>

<li>Less 'sticky' when melted.</li></ul>

<div class="mw-heading mw-heading2"><h2 id="History">History</h2></div>

Polyhydroxybutyrate was first isolated and characterized in 1925 by French <a href="Microbiologist" title="Microbiologist">microbiologist</a> Maurice Lemoigne.<a href="#cite_note-11">[11]</a>

<div class="mw-heading mw-heading2"><h2 id="Biodegradation">Biodegradation</h2></div>

Firmicutes and proteobacteria can degrade PHB. Bacillus, Pseudomonas and Streptomyces species can degrade PHB. Pseudomonas lemoigne, <a href="Comamonas" title="Comamonas">Comamonas sp.</a> Acidovorax faecalis, <a href="Aspergillus_fumigatus" title="Aspergillus fumigatus">Aspergillus fumigatus</a> and <a href="Variovorax_paradoxus" title="Variovorax paradoxus">Variovorax paradoxus</a> are soil microbes capable of degradation. <a href="Alcaligenes_faecalis" title="Alcaligenes faecalis">Alcaligenes faecalis</a>, <a href="Pseudomonas" title="Pseudomonas">Pseudomonas</a>, and Illyobacter delafieldi, are obtained from anaerobic sludge. <a href="Comamonas_testosteroni" title="Comamonas testosteroni">Comamonas testosteroni</a> and <a href="Pseudomonas_stutzeri" title="Pseudomonas stutzeri">Pseudomonas stutzeri</a> were obtained from sea water. Few of these are capable of degrading at higher temperatures; notably excepting thermophilic <a href="Streptomyces" title="Streptomyces">Streptomyces sp.</a> and a thermophilic strain of <a href="Aspergillus" title="Aspergillus">Aspergillus sp.</a><a href="#cite_note-12">[12]</a>

<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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<li id="cite_note-11"><a href="#cite_ref-11">^</a> <cite id="CITEREFLemoigne1926" class="citation journal cs1 cs1-prop-foreign-lang-source">Lemoigne, M (1926). "Produits de dehydration et de polymerisation de l'acide ß-oxobutyrique" [Dehydration and polymerization product of β-oxy butyric acid]. Bull. Soc. Chim. Biol. (in French). 8: 770–82.</cite>

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<div class="mw-heading mw-heading2"><h2 id="External_links">External links</h2></div>

<ul><li><a rel="nofollow" class="external text" href="https://web.archive.org/web/20061002105411/http://www.epa.gov/greenchemistry/pubs/pgcc/winners/sba05.html">Abstract of award for PHAs</a></li></ul></div><div><div class="zim-footer">

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