Mapping and refactoring pathway control through metabolic and protein engineering: The hexosamine biosynthesis pathway

Microorganisms possess a plethora of regulatory mechanisms to tightly control the flux through their metabolic network, allowing optimal behaviour in response to environmental conditions. However, these mechanisms typically counteract metabolic engineering efforts to rewire the metabolism with a view to overproduction. Hence, overcoming flux control is key in the development of microbial cell factories, illustrated in this contribution using the strictly controlled hexosamine biosynthesis pathway. The hexosamine biosynthesis pathway has recently garnered attention as gateway for the industrial biotechnological production of numerous mono-, oligo- and polysaccharidic compounds, composed of, i.a., glucosamine, N-acetylglucosamine, and neuraminic acid and with a vast application potential in the health, comsetics, and agricultural sector. First, the various alternative pathways in eukaryotes and prokaryotes are discussed. Second, the main regulatory mechanisms on transcriptional, translational and post-translational control, and the strategies to circumvent these pathway bottlenecks are highlighted. These efforts can serve as an inspiration to tackle regulatory control when optimizing any microbial cell factory.