Polyhydroxybutyrate

Polyhydroxybutyrate (PHB) is a polyhydroxyalkanoate (PHA), a polymer belonging to the polyesters class that was first isolated and characterized in 1925 by French microbiologist Maurice Lemoigne. PHB is produced by micro-organisms (like Alcaligenes eutrophus or Bacillus megaterium) apparently in response to conditions of physiological stress. The polymer is primarily a product of carbon assimilation (from glucose or starch) and is employed by micro-organisms 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 of two molecules of acetyl-CoA to give acetoacetyl-CoA which is subsequently reduced to hydroxybutyryl-CoA. This latter compound is then used as a monomer to polymerize PHB.

The poly-3-hydroxybutyrate (P3HB) form of PHB is probably the most common type of polyhydroxyalkanoate, but many 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 copolymers.



Polyhydroxybutyrate has attracted much commercial interest as a plastic material because its physical properties are remarkably similar to those of polypropylene (PP), even though the two polymers have quite different chemical structures. While PHB appears stiff and brittle, it also exhibits a high degree of crystallinity, a high melting point of about 180 °C, but, most importantly, PHB is rapidly biodegradable, unlike PP.

Two major factors inhibiting widespread use of PHB lie in its production costs, which are a lot higher than plastics produced from petrochemicals, and its brittleness, since PHB as it is currently produced cannot handle high impact. There are also some concerns of how large quantities of PHB would affect the environment. In the future research using genetic technology may be able to produce a better bacteria-based plastic that has more desirable properties and is cheaper to produce. For example PHB production may become cheaper if researchers could find a way to make bacteria produce larger amounts of polymer within shorter time spans. Once PHB becomes as cheap as plastics produced from petrochemicals, then it will probably become widely used, since it has the potential to be employed for packaging products like bottles, bags, wrapping film and disposable nappies. PHB is being also evaluated as a material for tissue engineering scaffolds and for controlled drug-release carriers owing to its biodegradability, optical activity and isotacticity.

Given the wide interest into polyhydroxyalkanoates, research on PHB has been quite extensive both in academic and industrial centres. Within the last decade, a team at the DoE Plant research lab at Michigan State University genetically modified plants to enable them to produce PHB. In 1992 they took two genes from PHB-making bacteria and inserted them directly into two cress plants and then crossed them. Some of the offspring plants incorporated both the new genes and produced PHB in their leaves. They had managed to create a plant that could grow plastic. After further refinining of the process they managed to increase production within the plant without affecting growth: 14% of the dry weight of the genetically modified leaves is PHB. During the 1980s, by means of genetic engineering, the three genes responsible for PHB production in A. eutrophus were successfully transferred to the common bacterium E. coli as well, paving the way for recombinant bacterial synthesis. In 1996 Monsanto (who sold PHB under the trade name Biopol) bought all patents for making the polymer from ICI/Zeneca. However, Monsanto's rights to Biopol were sold to the American company Metabolix in 2001 and Monsanto's fermenters producing PHB from bacteria were closed down at the start of 2004. The focus is now 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. In June 2005, a US company received the "Presidential Green Chemistry Challenge Award" (small business category) for their development and commercialisation of a cost-effective method for manufacturing PHAs in general, including PHB.