Microbial Upgrading of Acetate into Value-Added Products-Examining Microbial Diversity, Bioenergetic Constraints and Metabolic Engineering Approaches

Abstract

Ecological concerns have recently led to the increasing trend to upgrade carbon contained in waste streams into valuable chemicals. One of these components is acetate. Its microbial upgrading is possible in various species, with Escherichia coli being the best-studied. Several chemicals derived from acetate have already been successfully produced in E. coli on a laboratory scale, including acetone, itaconic acid, mevalonate, and tyrosine. As acetate is a carbon source with a low energy content compared to glucose or glycerol, energy- and redox-balancing plays an important role in acetate-based growth and production. In addition to the energetic challenges, acetate has an inhibitory effect on microorganisms, reducing growth rates, and limiting product concentrations. Moreover, extensive metabolic engineering is necessary to obtain a broad range of acetate-based products. In this review, we illustrate some of the necessary energetic considerations to establish robust production processes by presenting calculations of maximum theoretical product and carbon yields. Moreover, different strategies to deal with energetic and metabolic challenges are presented. Finally, we summarize ways to alleviate acetate toxicity and give an overview of process engineering measures that enable sustainable acetate-based production of value-added chemicals.

Keywords: Escherichia coli; acetate; acetate metabolism; acetate tolerance; acetate-derived chemicals; bioenergetic constraints; metabolic engineering; process engineering.

Figure 1

Acetate metabolism in E. coli, V. natriegens, and P. aeruginosa; Arrows indicate the preferred direction of reactions. Gray arrows indicate reactions only present in P. aeruginosa. Enzymes are represented by numbers in green circles (cf. Table 2).

Figure 2

Acetate metabolism in Y. lipolytica. Arrows indicate the preferred direction of reactions. Enzymes are represented by numbers in green circles (cf. Table 3).

Figure 3

Acetate and ethanol utilization in C. kluyveri; Arrows indicate the preferred direction of reactions. Enzymes are represented by numbers in green circles (cf. Table 4).

Figure 4

Metabolic adaptations of sulfate-reducing bacteria to utilize acetate: (a) modified citric acid cycle as used by Desulfobacter postgatei; (b) reverse Wool–Ljungdahl pathway as employed by organisms like Desulfobacter autotrophicum; arrows indicate the preferred direction of reactions. Enzymes are represented by numbers in green circles (cf. Table 5). “H₂” indicates the generation of two protons and two electrons, which may result in the formation of reduced ferredoxin, NADH, or NADPH.

Figure 5

Central carbon metabolism of E. coli for acetate, glucose, and glycerol as carbon sources under aerobic conditions. A list of the depicted enzymes is given in Table 8.

Figure 6

Overview of all products from acetate in E. coli listed in Table 7 and their ties into the central carbon metabolism. Red arrows indicate engineered pathways; green labels constitute end-products of engineered pathways.