Expression of biocatalysts and their use in monomer synthesis for biodegradable polymers from acetone and CO2

Abstract

CO₂ fixation, a key to solving global warming, can convert CO₂ into various compounds, unlike CO₂ reduction; however, it is thermodynamically challenging to fix CO₂ to a simple organic molecule. This study has developed a new synthesis of β-hydroxybutyrate, a monomer of a biodegradable polymer, from CO₂ and acetone using the effective biocatalysts acetone carboxylase (AC) and β-hydroxybutyrate dehydrogenase (HBDH). AC is an attractive biocatalyst that catalyzes the introduction of CO₂ to acetone to form acetoacetate in the presence of the co-enzyme ATP. To make the produced unstable acetoacetate more profitable, HBDH was introduced to catalyze acetoacetate reduction to β-hydroxybutyrate in the presence of the co-enzyme NADH. Rhodobacter capsulatus SB1003, a purple nonsulfur photosynthetic bacterium, successfully expressed both AC and HBDH in the acetone–bicarbonate medium under photoheterotrophic conditions. One-pot β-hydroxybutyrate synthesis from CO₂ and acetone combining AC for CO₂ fixation to acetone and HBDH for acetoacetate reduction to β-hydroxybutyrate has thus been developed using a cell extract including AC and HBDH from Rb. capsulatus. The optimal reaction conditions for the one-pot β-hydroxybutyrate synthesis were found based on the enzymatic activities of AC and HBDH, and the reaction rate was successfully increased by up to three times.