Cell-free metabolic engineering: Biomanufacturing beyond the cell

Industrial biotechnology and microbial metabolic engineering are poised to help meet the growing demand for sustainable, low-cost commodity chemicals and natural products, yet the fraction of biochemicals amenable to commercial production remains limited. Common problems afflicting the current state-of-the-art include low volumetric productivities, build-up of toxic intermediates or products, and byproduct losses via competing pathways. To overcome these limitations, cell-free metabolic engineering (CFME) is expanding the scope of the traditional bioengineering model by using in vitro ensembles of catalytic proteins prepared from purified enzymes or crude lysates of cells for the production of target products. In recent years, the unprecedented level of control and freedom of design, relative to in vivo systems, has inspired the development of engineering foundations for cell-free systems. These efforts have led to activation of long enzymatic pathways (>8 enzymes), near theoretical conversion yields, productivities greater than 100 mg L^-1 h^-1, reaction scales of >100 L, and new directions in protein purification, spatial organization, and enzyme stability. In the coming years, CFME will offer exciting opportunities to: (i) debug and optimize biosynthetic pathways; (ii) carry out design-build-test iterations without re-engineering organisms; and (iii) perform molecular transformations when bioconversion yields, productivities, or cellular toxicity limit commercial feasibility. Cell-free metabolic engineering (CFME) is the practice of using in vitro ensembles of catalytic proteins prepared from purified enzymes or crude lysates of cells for the production of useful products. By avoiding constraints associated with product toxicity, focusing substrates towards the biosynthesis of a single product, and separating cellular growth objectives from engineering process objectives, CFME complements existing in vivo strategies and expands the scope of metabolic engineering. CFME offers exciting opportunities to debug and optimize enzymatic pathways and perform molecular transformations when bioconversion yields, productivities, or cellular toxicity limit commercial feasibility of whole-cell fermentation.