Polyhydroxyalkanoate Case Study

1.1 BACKGROUND
Every activity in modern life is influenced by plastics and many depend entirely on plastic products due to their useful material properties and low production costs. Almost every product is constituted of some kind of polymer; however, traditional synthetic polymers (often called plastics), such as polypropylene and polyethylene, are derived from non-renewable petrochemicals and are not readily biodegradable (http://www.plastice.org/ fileadmin/files/Brochure_teachers.pdf). The need for polymers and the products they constitute is ever increasing; however the use of non-renewable fossil fuels for plastic production products not only diminishing fossil fuel stockpiles but eco- ‘unfriendly’ too. The enormous use of petroleum-based
A special class of optically active biopolymers called Polyhydroxyalkanoates (PHAs) shows some of the extraordinary similarities to the well-known synthetic polymers i.e. polypropylene and polyethylene; in addition, their disposal as bio-waste makes them increasingly attractive in the pursuit of sustainable development (Queirós et al., 2014). Polyhydroxyalkanoates comprise a class of polyesters of natural origin accumulated by a variety of microorganisms. These bioplastic compounds are intracellularly accumulated in the form of intracellular granules and stored in response to an environmental stress or nutrient limitation as a reserve of carbon, energy, and reducing power (Li et al., 2014). Under the conditions of carbon source starvation, PHAs are degraded by intracellular depolymerases and subsequently metabolized as a carbon and energy sources (Singh et al., 2013). Structurally, R-hydroxyalkonic acids act as the monomeric form of PHAs. The structure of PHAs, composed of 3-hydroxy fatty acids is shown in Figure
The first person who noticed these structures is said to be Beijerinck; in 1888 he observed the granules in the microscope as highly refractive inclusions. However, the first example of PHA came in existence until almost four decades later in 1926 when a French microbiologist Maurice Lemoigne discovered that a gram negative bacterium Bacillus megaterium accumulated intracellular granules of a polyester called polyhydroxy-3-butyrate (PHB) (Amara et al., 2011). PHA accumulation ability has been reported for many microorganisms including Gram-negative and Gram-positive species (i.e. autotrophic, heterotrophic and phototrophic microorganisms, aerobes and anaerobes) as well as for a number of archaea strains. Homo-polymer PHB, the best characterized member of wide family of polyhydroxyalkanoates, is not only present in microorganisms as storage material but is also very ubiquitous in nature in various roles (Poirier et al., 1995). In recent years the very wide distribution of PHB as a low molecular weight oligomer (120-200 monomers units) has been discovered in microorganisms, plants and animal, including humans (Shrivastav et al., 2013).