High-yield anaerobic succinate production by strategically regulating multiple metabolic pathways based on stoichiometric maximum in Escherichia coli

BACKGROUND Succinate has been identified by the U.S. Department of Energy as one of the top 12 building block chemicals, which can be used as a specialty chemical in the agricultural, food, and pharmaceutical industries. Escherichia coli are now one of the most important succinate producing candidates. However, the stoichiometric maximum succinate yield under anaerobic conditions through the reductive branch of the TCA cycle is restricted by NADH supply in E. coli. RESULTS In the present work, we report a rational approach to increase succinate yield by regulating NADH supply via pentose phosphate (PP) pathway and enhancing flux towards succinate. The deregulated genes zwf243 (encoding glucose-6-phosphate dehydrogenase) and gnd361 (encoding 6-phosphogluconate dehydrogenase) involved in NADPH generation from Corynebacterium glutamicum were firstly introduced into E. coli for succinate production. Co-expression of beneficial mutated dehydrogenases, which removed feedback inhibition in the oxidative part of the PP pathway, increased succinate yield from 1.01 to 1.16 mol/mol glucose. Three critical genes, pgl (encoding 6-phosphogluconolactonase), tktA (encoding transketolase) and talB (encoding transaldolase) were then overexpressed to redirect more carbon flux towards PP pathway and further improved succinate yield to 1.21 mol/mol glucose. Furthermore, introducing Actinobacillus succinogenes pepck (encoding phosphoenolpyruvate carboxykinase) together with overexpressing sthA (encoding soluble transhydrogenase), further increased succinate yield to 1.31 mol/mol glucose. In addition, removing byproduct formation through inactivating acetate formation genes ackA-pta and heterogenously expressing pyc (encoding pyruvate carboxylase) from C. glutamicum led to improved succinate yield to 1.4 mol/mol glucose. Finally, synchronously overexpressing dcuB and dcuC encoding succinate exporters enhanced succinate yield to 1.54 mol/mol glucose, representing 52 % increase relative to the parent strain and amounting to 90 % of the strain-specific stoichiometric maximum (1.714 mol/mol glucose). CONCLUSIONS It's the first time to rationally regulate pentose phosphate pathway to improve NADH supply for succinate synthesis in E. coli. 90 % of stoichiometric maximum succinate yield was achieved by combining further flux increase towards succinate and engineering its export. Regulation of NADH supply via PP pathway is therefore recommended for the production of products that are NADH-demanding in E. coli.