Abstract

Polyhydroxyalkanoates (PHAs) are a biobased polymer material having diverse structures. In order to expand their range of application, the exploration of way to synthesize new polymers containing new monomer units should be important step. During the past decade, our group has developed engineered microbial platforms that can synthesize a variety of unusual polyesters. In this talk, these recent progress and unsolved questions as follows will be discussed. (i) Biosynthesis of lactate(LA)-enriched P(LA-co-3HB) P[LA-co-3-hydroxybutyrate (3HB)] was produced in recombinant Escherichia coli expressing engineered PHA synthases and propionyl-CoA transferases with broad substrate specificity. The LA fraction has been successfully regulated in the rage of 0-70 mol% by means of metabolic engineering, the engineering of PHA synthase, and the selection of carbon source. P(LA-co-3HB)s have the properties of pliability and stretchiness which distinctly differed from those of the rigid poly(lactic acid) (PLA) and P(3HB) homopolymers. In addition, the biosynthesis of PLA-like polyester was achieved using engineered Corynebacterium glutamicum. The Coryne-PLA had an advantage over chemical PLA in that the biological polymer possessed an extremely high enantiomer excess. (ii) Biomass utilization for PHA productions Lignocellulosic biomass, which is mainly composed of cellulose, hemicellulose and lignin, is a potent feedstock to produce PHAs. We succeeded in synthesizing P(LA-co-3HB) from lignocellulosic biomass-derived sugars, which contained glucose and xylose as major components. In addition, we found an interesting phenomenon that xylose, which is a second abundant component in lignocellulosic biomass, was efficiently converted into P(LA-co-3HB) compared to P(3HB). It is well-known that xylose could be used for fermentation, but normally with lower efficiency than glucose. Therefore, P(LA-co-3HB) is a potent candidate as a target compound of xylose industry. (iii) Expansion of unusual PHAs The biosynthetic system of LA-based polymers has been expanded to biosynthesize glycolate(GL)- and 2-hydroxybutyrate(2HB)-based polymers. The incorporation of GL units into PHA backbone was found to effectively modify the physical properties of PHA. In the case of synthesis of 2HB-based polymers, the strict enantiospecificity of PHA synthase allowed us to produce isotactic (R)-2HB-based polymers, including P[(R)-2HB], from racemic precursors of 2HB. P(2HB) exhibited stretchy property in contrast to PLA. Moreover, 2HB units were useful to modify and modulate the polymer properties. (iv) Biodegradation of unusual PHAs We addressed the degradation of the LA-based copolymer and isolated a P(LA-co-3HB)-degrading bacterium from soil, and the depolymerase contributing to the degradation. This result suggested the potential of the copolymer as a biodegradable material.