Optimization of an improved chemically defined medium for high-efficiency expression of glucokinase in Escherichia...

Recombinant glucokinase (GLK), an important enzyme for blood glucose diagnostics, is commonly prepared in Escherichia coli (E.coli )for laboratory research and related biosynthetic applications. In small-scale laboratory cultivation, complex media are often used because of their convenience and strong growth-supporting capacity, but their ill-defined composition and batch-to-batch variability can compromise reproducibility and increase medium-related costs. Although minimal defined medium (e.g., M9 medium) are generally preferred in scalable E. coli bioprocesses because of their compositional consistency and process controllability, their relatively simple nutrient composition may limit recombinant protein expression performance under laboratory-scale conditions. Therefore, developing an improved chemically defined medium represents a practical strategy to balance expression efficiency, compositional control, and medium cost for recombinant GLK production.

To address this, we developed a chemically defined medium including amino acids using high-throughput respiration activity monitoring system (RAMOS) and a high-throughput growth curve analysis system (Growth Profiler 960, GP960), alongside a machine learning-driven iterative optimization system (Machine-learning guided Experimental Trials for Improvement of Systems, METIS). The optimized medium enhanced soluble GLK expression by 33 fold compared to the pre-optimization medium, reaching 74% of the titer obtained in complex media, while reducing medium costs by 74%. Moreover, soluble protein expression per cell increased by 80% compared to the complex medium, lowering the medium usage cost per unit of bacterial protein by 85%. Validation in a 7.5 L bioreactor demonstrated that the optimized medium supported continuous growth to an optical density at 600 nm (OD₆₀₀) of 103, achieving 4-fold and 5.4-fold higher biomass than Luria-Bertina (LB) and M9 media, respectively, with soluble GLK titer reaching 920 mg L^- 1, which exceeded conventional media by more than 10-fold.

This study tackles the challenge of suboptimal media in GLK fermentation, offering a cost-effective alternative and a transferable strategy for optimizing fermentation processes for other high-value enzymes.