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Fumaric acid (C₄H₄O₄) is an intermediate of TCA cycle. It is white crystal solid acid with no odour and is used in plastic industry and in manufacture of sizing resins in paper industry. Being nontoxic in nature it is used in corn tortillas, wheat, sour dough, fruit juice, rye breads, refrigerated biscuit dough, nutraceutical drinks, gelatin desserts, pie fillings gelling aids, wine, and as supplement in animal feed (Khan et al., 2017). In 2014, the global market size of fumaric acid was 245.4 kilo tons and expanding at a compound annual growth rate of 6.1% from 2015 to 2022 (Fumaric acid market analysis by application, https://www.grandviewresearch.com/industry‐analysis/fumaric‐acid‐market).

It was first isolated by a plant Fumaria. After that many microorganisms’ strains such as Rhizopus, Mucor, Cunninghamella, and Circinella are used for the production of fumaric acid, and among them Rhizopus nigricans, Rhiozopus arrhizus, R. oryzae, and Rhiozopus formosa give higher yield (Singh et al., 2017). Yu et al. irradiated A. oryzae by femtosecond laser twice, and the mutant strains were selected for fumaric acid production (Yu et al., 2012). There was an increase in yield of 36.6% in comparison to the wild‐type strain. Expressing exogenous gene phosphoenolpyruvate carboxylase and overexpressing endogenous gene pyruvate carboxylase increase the carbon flux to oxaloacetate and hence fumarate (Zhang et al., 2012). Fumarase overexpression leads to decrease in fumaric acid as fumarase does hydration of fumarate to malate (Zhang and Yang, 2012). Site‐saturation engineering of proline 474 in pyruvate carboxylase from R. oryzae was done to improve fumaric acid production in S. cerevisiae (Xu et al., 2012). Ethanol yield was lower down by deleting fumarate hydratase gene. The generated strain gives a yield of 314.5 ± 3.8 mg/l.

Similarly, E. coli can also be engineered by overexpressing and deleting the abovementioned genes. The iclR gene encoding a repressor of glyoxylate shunt operon was deleted to direct carbon flux to glyoxylate shunt (Song et al., 2013). All three fumarase genes were also deleted enhancing fumaric acid production to 1.45 g/l. in a strain with all three fumarase genes deleted with the overexpression of phosphoenolpyruvate carboxylase gene gives yield of 3.65 g/l fumaric acid (Li et al., 2014).