Biocatalysis for the application of CO2 as a chemical feedstock

Abstract

Biocatalysts, capable of efficiently transforming CO2 into other more reduced forms of carbon, offer sustainable alternatives to current oxidative technologies that rely on diminishing natural fossil-fuel deposits. Enzymes that catalyse CO2 fixation steps in carbon assimilation pathways are promising catalysts for the sustainable transformation of this safe and renewable feedstock into central metabolites. These may be further converted into a wide range of fuels and commodity chemicals, through the multitude of known enzymatic reactions. The required reducing equivalents for the net carbon reductions may be drawn from solar energy, electricity or chemical oxidation, and delivered in vitro or through cellular mechanisms, while enzyme catalysis lowers the activation barriers of the CO2 transformations to make them more energy efficient. The development of technologies that treat CO2-transforming enzymes and other cellular components as modules that may be assembled into synthetic reaction circuits will facilitate the use of CO2 as a renewable chemical feedstock, greatly enabling a sustainable carbon bio-economy.

Keywords: CO2 transformation; RuBisCO; biocatalysis; carboxylase; formate dehydrogenase.

Figure 1

Biocatalytic routes for conversion of CO₂ into compounds with carbon in the reduced oxidation states indicated at the top. FDH: formate dehydrogenase, FaldDH: formaldehyde dehydrogenase, ADH: alcohol dehydrogenase, CODH: carbon monoxide dehydrogenase, RuBisCO: ribulose-1,5-bisphosphate carboxylase oxygenase, CA: carbonic anhydrase, R: H, CH₃.

Figure 2

Carbonic anhydrase-catalysed rapid interconversion of CO₂ and HCO₃⁻ in living systems.

Scheme 1

The Calvin cycle for fixation of CO₂ with RuBisCO.

Scheme 2

The reductive TCA cycle with CO₂ fixation enzymes designated.

Scheme 3

The Wood–Ljungdahl pathway for generation of acetyl-CoA through reduction of CO₂ to formate and CO. FDH: formate dehydrogenase, CODH: CO dehydrogenase, ACS: acetyl-CoA synthase, FH₄: tetrahydrofolate.

Scheme 4

The acyl-CoA carboxylase pathways for autotrophic CO₂ fixation. ACC: acetyl-CoA/propionyl-CoA carboxylase, PEPC: phosphoenolpyruvate carboxylase.

Figure 3

RuBisCO CO₂-fixing bypass installed in E. coli and S. cerevisiae to increase carbon flux toward products of interest. PRK: phosphoribulosekinase.

Scheme 5

Integrated biocatalytic system for carboxylation of phosphoenolpyruvate (19), using PEPC and carbonic anhydrase.

Scheme 6

PEPC and pyruvate carboxylase catalysed carboxylation of pyruvate backbone for the generation of oxaloacetate (9) and other dicarboxylates.

Scheme 7

Decarboxylase catalysed carboxylation of (a) phenol derivatives, (b) indole and (c) pyrrole.

Figure 4

Formate dehydrogenase (FDH) catalysed reversible reduction of CO₂ to formate with electron donor regeneration through hydrogenase-catalysed H₂ oxidation (red arrow) or electrochemical reduction at a cathode (blue arrow).

Figure 5

Sequential generation of formate, formaldehyde and methanol from CO₂ using reducing equivalents sourced through electrochemical cells or photocatalysts. ED: electron donor, FDH: formate dehydrogenase, FladDH: formaldehyde dehydrogenase, ADH: alcohol dehydrogenase.

Figure 6

Hydrogen storage as formic acid through biocatalytic hydrogenation of CO₂ and subsequent on-demand release through zeolite catalysed dehydrogenation.

Figure 7

Schematic showing required flow of reducing equivalents for CO₂ fixation through biotechnological applications.