Improved succinate production by metabolic engineering

Abstract

Succinate is a promising chemical which has wide applications and can be produced by biological route. The history of the biosuccinate production shows that the joint effort of different metabolic engineering approaches brings successful results. In order to enhance the succinate production, multiple metabolical strategies have been sought. In this review, different overproducers for succinate production, including natural succinate overproducers and metabolic engineered overproducers, are examined and the metabolic engineering strategies and performances are discussed. Modification of the mechanism of substrate transportation, knocking-out genes responsible for by-products accumulation, overexpression of the genes directly involved in the pathway, and improvement of internal NADH and ATP formation are some of the strategies applied. Combination of the appropriate genes from homologous and heterologous hosts, extension of substrate, integrated production of succinate, and other high-value-added products are expected to bring a desired objective of producing succinate from renewable resources economically and efficiently.

Figure 1

Succinate production pathway from (a) the reductive branch of the TCA cycle. Succinate accumulates derived from phosphoenolpyruvate, via some intermediate, including oxaloacetate, malate, and fumarate. (b) The glyoxylate pathway. The glyoxylate pathway operates as a cycle to convert 2 mol acetyl CoA to 1 mol succinate. (c) The oxidative TCA cycle. This pathway converts acetyl-CoA to citrate, isocitrate, and succinate and subsequently converted to fumarate by succinate dehydrogenase. Under aerobic conditions, the production of succinate is not naturally possible, and to realize succinate accumulation under aerobic condition, inactivation of sdhA gene to block the conversion of succinate to fumarate in TCA cycle is necessary.

Figure 2

Fermentation of glucose to succinate by genetically engineered central anaerobic metabolic pathway. Bold vertical bar means that the relative gene was inactivated. Bold arrows show overexpression. (1) The PEP-dependent glucose uptake system is replaced with ATP-dependent phosphorylation, (2) the activation of the glyoxylate pathway, (3) the knockouts of lactate (lactate dehydrogenase), (4) the knockouts of formate pathway (pyruvate formate-lyase), (5) and (6) the knockouts of acetate pathway (acetate kinase, phosphate acetyltransferase), (7) the knockouts of ethanol pathway (alcohol dehydrogenase), (8) overexpressed phosphoenolpyruvate carboxylase, (9) overexpressed malic enzyme, (10) overexpressed pyruvate carboxylase.