Carbon efficient production of chemicals with yeasts

Abstract

Microbial metabolism offers a wide variety of opportunities to produce chemicals from renewable resources. Employing such processes of industrial biotechnology provides valuable means to fight climate change by replacing fossil feedstocks by renewable substrate to reduce or even revert carbon emission. Several yeast species are well suited chassis organisms for this purpose, illustrated by the fact that the still largest microbial production of a chemical, namely bioethanol is based on yeast. Although production of ethanol and some other chemicals is highly efficient, this is not the case for many desired bulk chemicals. One reason for low efficiency is carbon loss, which decreases the product yield and increases the share of total production costs that is taken by substrate costs. Here we discuss the causes for carbon loss in metabolic processes, approaches to avoid carbon loss, as well as opportunities to incorporate carbon from CO₂ , based on the electron balance of pathways. These aspects of carbon efficiency are illustrated for the production of succinic acid from a diversity of substrates using different pathways.

Keywords: bioeconomy; carbon balance; industrial biotechnology; metabolic engineering.

Figure 1

Pathways involved in the central carbon metabolism of yeasts, highlighting the relation between carboxylation/decarboxylation steps and the change in the degree of reduction of substrates and products. Degree of reduction of the respective substrates, intermediates, and products is indicated by a color code ranging from red (γ = 0) over yellow (γ = 4) to blue (γ = 6).

Figure 2

Production of succinate with incorporation of CO₂ as cosubstrate from the oxidative or reductive branches of the tricarboxylic acid cycle, respectively. For clarity the glyoxylate shunt is not shown in the figure.

Figure 3

Redox neutral succinate production from a combination of the glycolysis alternative high carbon yield cycle (GATHCYC) along with partial tricarboxylic acid cycle and glyoxylate shunt pathways.