Bacteria manufacture polyesters as energy reserves. They do so at ambient temperatures and pressures in water. By contrast, industrial polyester manufacturing processes may require some added heat, pressure changes, and/or organic solvents. Although polyesters are not soluble in water, bacteria are able to store small polyester granules in their watery cellular environment by coating each granule with water-soluble proteins.
Enzymes secreted by microbes facilitate the formation of “ester” bonds between organic molecules at normal temperatures and pressures (the process is called a “dehydration” reaction because water is a by-product of bond formation) (1). More starting material is joined to the compound via ester bonds, hence the term polyester (2). Microbes store their polyester granules in water for later use as an energy and carbon source. Artist: Emily Harrington. Copyright: All rights reserved. See gallery for details.
“Biopolyester (PHAs = polyhydroxyalkanoates) composed of hydroxy fatty acids represent a rather complex class of storage polymers synthesized by various eubacteria and archaea and are deposited as water-insoluble cytoplasmic nano-sized inclusions. These spherical shell-core particles are composed of a polyester core surrounded by phospholipids and proteins.” (Rehm 2007:41)
“The biologically produced biopolyesters comprise a complex class of polyoxoesters that are synthesized by most genera of Eubacteria and even members of the family Halobacteriaceae of the Archaea. The majority of prokaryotes synthesize poly(3-hydroxybutyric acid) (PHB) and/or other PHAs composed of medium-chain length (R)-3-hydroxyfatty acids (6–14 carbon atoms) as reserve material. These polyesters are deposited as spherical water-insoluble inclusions in the cytoplasm. The biopolyester constitutes the core of the granule. Meanwhile, more than 150 different hydroxyalkanoic acids are now known to occur as constituents of PHAs implying that the respective CoA thioester are accepted as substrates by the polyester synthases. These water-insoluble PHAs crystallize after solvent extraction and exhibit rather high molecular weights (ranging from about 5 × 10^5 to 5 × 10^6), thermoplastic and elastomeric properties and some other interesting physical and material properties.” (Rehm 2007:42)