Vol. 76, No. 15
Biosynthesis and Biodegradation of 3-HydroxypropionateContaining Polyesters
Bjorn Andreeßen and Alexander Steinbuchel* ¨ ¨
Institut fur Molekulare Mikrobiologie und Biotechnologie,Westfalische Wilhelms-Universitat Munster, D-48149 Munster, Germany ¨ ¨ ¨ ¨ ¨ 3-Hydroxypropionate (3HP) is an important compound in the chemical industry, and the polymerized 3HP can be used as a bioplastic. In this review, we focus on polyesters consisting of 3HP monomers, including the homopolyester poly(3-hydroxypropionate) and copolyesters poly(3-hydroxybutyrate-co-3-hydroxypropionate),poly(3-hydroxypropionate-co-3-hydroxybutyrate-co-3-hydroxyhexanoate-co-3-hydroxyoctanoate), poly(4-hydroxybutyrate-co-3-hydroxypropionate-co-lactate), and poly(3-hydroxybutyrate-co-3-hydroxypropionate-co-4-hydroxybutyrate-co-lactate). Homopolyesters like poly(3-hydroxybutyrate) are often highly crystalline and brittle, which limits some of their applications. The incorporation of 3HP monomers reducesthe glass transition temperature, the crystallinity, and also, at up to 60 to 70 mol% 3HP, the melting point of the copolymer. This review provides a survey of the synthesis and physical properties of different polyesters containing 3HP. Bacterial polyhydroxyalkanoates (PHAs) are natural biodegradable thermoplastics produced by various microorganisms as intracellular energy and carbon storagecompounds. PHAs have attracted increased attention as possible alternatives to petroleum-based polymers. They are biodegradable, insoluble in water, nontoxic, biocompatible, piezoelectric, thermoplastic, and/or elastomeric. These features make PHAs suitable for several applications in the packaging industry, medicine, pharmacy, agriculture, and food industry, as raw materials for the production ofenantiomerically pure chemicals, and for the production of paints (2, 44). The best characterized PHA is poly(3-hydroxybutyrate) [poly(3HB)], which is synthesized by many bacteria (28, 31, 38, 41). Unfortunately, poly(3HB) is a highly crystalline and brittle polymer with a low elongation-to-break factor, which has prevented its use in a wide range of applications. To obtain bacterial PHAs withimproved physical and mechanical properties, previous studies have demonstrated the biosynthesis of copolyesters consisting of 3-hydroxybutyrate (3HB) and a second constituent. Pathways for the biosynthesis of such PHA copolyesters, like poly(3hydroxybutyrate-co-3-hydroxyvalerate) [poly(3HB-co-3HV)], poly(3-hydroxybutyrate-co-4-hydroxyvalerate) [poly(3HB-co4HV)], andpoly(3-hydroxybutyrate-co-3-hydroxypropionate) [poly(3HB-co-3HP)], occur naturally in many bacteria or have been engineered (5, 40). Some of these polyesters exhibit material characteristics comparable to those of petrochemical-derived polymers. However, in contrast to petrochemical-based polymers, PHAs are completely biodegradable to CO2 and water. Another advantage is that they can be produced from renewable resources.Unfortunately, PHA production by bacterial fermentation is costly and, due to inefﬁcient utilization of the resources, not necessarily environmentally convenient in all cases (13). An example of a bacterium-synthesized copolymer which is competitive with polymers produced from petrochemicals in bulk is poly(3HB-co-3HV). Cupriavidus necator (32), formerly Ralstonia eutropha or Alcaligenes eutrophus,accumulates this copolyester when fed with glucose and propionic acid in a phosphate-depleted batch culture (30). The physical properties of poly(3HB-co-3HV) resemble those of polyethylene and polypropylene (18). Poly(3HB-co-3HV) has been commercially produced through fermentation using a glucose-utilizing mutant of C. necator that requires cofeeding of propionic acid for 3-hydroxyvalerate formation. This...