Abstract
Polyhydroxyalkanoates (PHAs) are accumulated in many gram-positive and gram-negative bacteria as intracellular carbon and energy storage material under nutrient-limited conditions. Bacterial PHAs are expected to become attractive alternatives for petrochemically based plastics, since they are biodegradable thermoplastics. Typical bacterial PHA, poly-3-hydroxybutyrate [P (3HB) or PHB], is highly crystalline, stiff and brittle. The lack of flexibility limits the range of its applications. The copolyester of (R) -3HB and longer chain (R) -3-hydroxyalkanoate, P (3HB-co-3HA), is less crystalline, more ductile, easier to mold and tougher than P (3HB). Based on the PHA biosynthesis mechanism, control of monomer composition in PHA by genetic and metabolic engineering makes it possible to synthesize a copolyester with good properties.
A low molecular weight PHB, termed OHB, has been identified as complexes with other biomacromolecules in a wide variety of organisms from bacteria, to plants and animals. The supermolecular complex of OHB with calcium ion-polyphosphate was demonstrated to be a prebiotic calcium ion transport channel and to be closely related to genetic competency for transforming Escherichia coli. Further studies on the physiological functions of OHB including calcium ion channel formation would pioneer the stage for future PHB polymer science.
