Fig. 1

Metabolic engineering strategies for enhancing citric acid production. The central metabolism pathway of A. niger was streamlined for citric acid production through engineering of targets for carbon utilization improvement, biosynthesis and precursor enhancement, by-product removal and feedback inhibition reduction and respiratory chain improvement. The Red Crosse represented corresponding gene (green) is deleted. The genes in red represented the targets required to be enhanced. The red dashed line with vertical bar represented the feed-back inhibition, i.e. T6P inhibited the activity of Hxk. STA starch, SUCR sucrose, GLC glucose, GLA gluconic acid, FRU fructose, G6P glucose-6-phosphate, T6P trehalose-6-phosphate, F6P fructose-6-phosphate, F-1,6-BP fructose-1,6-bisphosphate, PEP phosphoenol-pyruvate, PYR pyruvate, OAA oxaloacetate, AcCOA acetyl-CoA; MAL malate, OA oxalic acid, CIT citric acid, ICIT isocitric acid, 2-OG 2-oxoglutarate, SucCOA succinic CoA, SUCC succinate, FUM fumarate, AmyA amylase, GlaA glucoamylase, Inv inverase, AgdA alpha-1,4-glucosidase, GoxC glucose oxidase, Hxk hexokinase, GgsA trehalose-6-P synthase, Pfk1 phosphofructokinase, Pki pyruvate kinase, Pyc pyruvate decarboxylase, OahA oxaloacetate acetylhydrolase, Mdh malate dehydrogenase, Fum fumarase, Frds fumarate reductase, CitA citrate synthase, AOX1 alternative mitochondrial oxidoreductase