Cell-Free Reaction System for ATP Regeneration from <scp>d</scp>-Fructose

Adenosine triphosphate (ATP)-dependent <i>in vitro</i> bioprocesses, such as cell-free protein synthesis and the production of phosphorylated fine chemicals, are of considerable industrial significance. However, their implementation is mainly hindered by the high cost of ATP. We propose and demonstrate the feasibility of a cell-free ATP regeneration system based on the <i>in situ</i> generation of the high-energy compound acetyl phosphate from low-cost d-fructose and inorganic phosphate substrates. The enzyme cascade chains d-fructose phosphoketolase, d-erythrose isomerase, d-erythrulose phosphoketolase, and glycolaldehyde phosphoketolase activities theoretically enabling production of 3 mol ATP per mol of d-fructose. Through a semirational engineering approach and the screening of nine single-mutation libraries, we optimized the phosphoketolase (PKT) from <i>Bifidobacterium adolescentis</i>, identifying the improved variant Bad.F6Pkt H548N. This mutant exhibited a 5.6-fold increase in d-fructose activity, a 2.2-fold increase in d-erythrulose activity, and a 1.3-fold increase in glycolaldehyde activity compared to the wild-type enzyme. The Bad.F6Pkt H548N mutant was initially implemented in a cell-free reaction system together with an acetate kinase from <i>Geobacillus stearothermophilus</i> and a glycerol kinase from <i>Cellulomonas</i> sp. for the production of glycerol-3 phosphate from ADP and glycerol. We demonstrated the feasibility of ATP regeneration from 25 mM d-fructose with a stoichiometry of 1 mol of ATP per mol of C₆ ketose. Subsequently, the reaction system was enhanced by incorporating d-erythrose isomerase activity provided by a l-rhamnose isomerase from <i>Pseudomonas stutzeri</i>. In the complete system, the ATP yield increased to 2.53 mol mol_fructose^-1 with a maximum productivity of 7.2 mM h^-1.