Inactivation of Malic Enzymes Improves the Anaerobic Production of Four-Carbon Dicarboxylic Acids by Recombinant Escherichia coli Strains Expressing Pyruvate Carboxylase

The genes maeA and maeB, encoding NADH- and NADPH-dependent malic enzymes, have been deleted in a recombinant Escherichia coli strain with inactivated mixed-acid fermentation pathways and a modified system of glucose transport and phosphorylation upon the heterological expression of the pyruvate carboxylase gene. During anaerobic glucose utilization, the parental strain synthesized malic, fumaric, and succinic acids as the main fermentation end products, while pyruvic acid was accumulated as the main by-product resulting from the functioning of the pyruvate–oxaloacetate–malate–pyruvate futile cycle. Upon individual deletions of the maeA and maeB genes, the mutant strains converted glucose into four-carbon dicarboxylic acids with increased efficiency still secreting notable amounts of pyruvic acid. The combined inactivation of both malic enzymes in the constructed strain significantly elevated the portion of malic, fumaric, and succinic acids among the fermentation end products with a concomitant decrease in the secretion of pyruvic acid and other by-products due to the abolishment of the action of the futile cycle competing with the target biosynthetic processes.