22Solving environmental and social challenges such as climate change requires a shift 23 from our current non-renewable manufacturing model to a sustainable bioeconomy. 24 To lower carbon emissions in the production of fuels and chemicals, plant biomass 25 feedstocks can replace petroleum using microorganisms as catalysts. The anaerobic 26 thermophile Clostridium thermocellum is a promising bacterium for bioconversion due 27 to its capability to efficiently degrade untreated lignocellulosic biomass. However, the 28 complex metabolism of C. thermocellum is not fully understood, hindering metabolic 29 engineering to achieve high titers, rates, and yields of targeted molecules. In this 30 study, we developed an updated genome-scale metabolic model of C. thermocellum 31 that accounts for recent metabolic findings, has improved prediction accuracy, and is 32 standard-conformant to ensure easy reproducibility. We illustrated two applications of 33 the developed model. We first formulated a multi-omics integration protocol and used 34 it to understand redox metabolism and potential bottlenecks in biofuel (e.g., ethanol) 35 production in C. thermocellum. Second, we used the metabolic model to design mod-36 ular cells for efficient production of alcohols and esters with broad applications as 37 flavors, fragrances, solvents, and fuels. The proposed designs not only feature intuitive 38 push-and-pull metabolic engineering strategies, but also novel manipulations around 39 important central metabolic branch-points. We anticipate the developed genome-scale 40 metabolic model will provide a useful tool for system analysis of C. thermocellum 41 metabolism to fundamentally understand its physiology and guide metabolic engineer-42 ing strategies to rapidly generate modular production strains for effective biosynthesis 43 of biofuels and biochemicals from lignocellulosic biomass. 44 Keywords-Clostridium thermocellum, biofuels, genome-scale model, metabolic model, omics 45 integration, modular cell design, ModCell 46 47Global oil reserves will be soon depleted, 1 and climate change could become a major driver of civil 48 conflict. 2 These challenges to security and the environment need to be addressed by replacing our 49 current non-renewable production of energy and materials for a renewable and carbon neutral ap-50 proach. 3 The gram-positive, thermophilic, cellulolytic, strict anaerobe C. thermocellum is capable 51 of efficient degradation of lignocellulosic biomass to produce biofuels and biomaterial precursors, 52 making this organism an ideal candidate for consolidated bioprocessing (CBP), where saccharifi-53 cation and fermentation take place in a single step. 4 However, its complex and poorly understood 54 metabolism remains the main roadblock to achieve industrially competitive titers, rates, and yields 55 of biofuels such as ethanol 5 and isobutanol. 6
56For the past decade, significant efforts have been dedicated to characterize and manipulate 57 C. thermocellum's central metabolism, due to increasing inter...