Microbial production of atactic PHB

Poly(3-hydroxybutyrate) (PHB) is a versatile biopolymer that can be synthesized chemically from petrochemical feedstock or microbially from renewable raw materials. The arrangement of side chains along a chiral polymer chain (tacticity), plays a crucial role in determining key properties such as melting temperature, brittleness, and tensile strength. Consequently, PHB properties can be tailored by altering tacticity. Tailoring the tacticity of PHB has only been achieved for the chemical synthesis route despite microbial synthesis being the industrial standard. Microbially produced PHB is purely isotactic and therefore has an unfavorably high melting temperature and is very brittle, making it unsuitable for many applications. Atactic PHB with random stereoconfiguration has considerably lower melting temperature, significantly higher tensile strength, and improved elasticity than isotactic or pure syndiotactic PHB.

The invention is based on an Escherichia coli strain genetically engineered to carry a synthetic operon for the synthesis of atactic PHB. This operon encodes enzymes that control the metabolic flux from acetyl-CoA to atactic PHB. In addition, the strain contains two genomic knockouts that prevent the degradation of an intermediate and thus increase productivity. The result is a system that can achieve different material properties by fine-tuning the tacticity and can be transferred to other PHB producing strains.

The invention details the microbial synthesis of atactic PHB, which exhibits reduced crystallinity and enhanced biodegradability compared to isotactic PHB. Furthermore, atactic PHB is easier to process due to the greater difference between its melting and decomposition temperatures, preventing the polymer from decomposing during thermal processing and making its properties closer to polypropylen.

• Sustainable and microbial fermentative production of atactic PHB, which can be applied to any industrial microorganism
• Compared to natural microbial PHB, atactic microbial PHB is less crystalline and brittle
• Improved recyclability compared to isotactic PHB
• Various carbon sources can be used such as carbohydrates, hydrogen, CO2, and methane

TRL 4.5