Chinese Scientists Design and Construct a Minimized Synthetic Carbon Fixation Cycle

Researchers from the Institute of Microbiology, Chinese Academy of Sciences (IMCAS) recently reported a minimized synthetic carbon fixation cycle. The cycle only contains four biochemical reactions and is capable of condensing two molecules of carbon dioxide into one molecule of oxalate in each round, according to a research article published in ACS Catalysis on December 28, 2021.

The increasing atmospheric carbon dioxide concentration is one of the major contributors to anthropogenic climate change, which has aroused great concern worldwide. In nature, plants and microbes fix carbon dioxide through carbon fixation pathways. Two synthetic carbon fixation pathways, the CETCH cycle and the ASAP pathway, have also been reported in recent years. However, these pathways usually contain multiple reactions (mostly more than 10 reactions).

Generally, the large number of reactions may reduce the overall carbon fixation efficiency of the pathway. The researchers then asked a question: is it possible to reduce the number of reactions required for a carbon fixation cycle so as to increase the efficiency? and if possible, what is the minimum number of reactions that can constitute a functional carbon fixation cycle?

To address this question, IMCAS researchers designed a minimized synthetic carbon fixation cycle based on thermodynamic and kinetic calculations of enzymatic reactions. This synthetic cycle contains only four reactions and the four enzymes involved are pyruvate carboxylase (PYC), oxaloacetate acetylhydrolase (OAH), acetate-CoA ligase (ACS) and pyruvate synthase (PFOR), so this cycle was named the POAP cycle (Figure 1).

The most challenging reaction in the POAP cycle was the reductive carboxylation of PFOR. Usually, PFOR catalyzes the oxidative decarboxylation of pyruvate. The reductive carboxylation reaction must be driven by a low-potential electron donor. Ferredoxin was selected as the electron donor for this purpose. Among a series of low-potential ferredoxins tested, a ferredoxin from Hydrogenobacter thermophiles was found to efficiently drive the reductive carboxylation of PFOR from Clostridium thermocellum. Subsequently, PYC, OAH and ACS were successfully screened for the construction of POAP cycle.

To construct the POAP cycle, four reactions were divided into two cascades, then the two cascades were assembled together, and the carbon fixation efficiency were tested under anaerobic conditions using ¹³C-labeled NaHCO₃ as the substrate. ¹³C-labeled oxalate produced by the POAP cycle was detected by liquid chromatography - mass spectrometry. The CO₂ fixation rate of the POAP cycle was calculated to be 8.0 ± 1.8 nmol CO₂ min^-1 mg^-1 CO₂-fixing enzymes and the total turnover number was 5 mol/mol enzymes, surpassing that of the synthetic CETCH cycle.

The POAP cycle functions under relatively high temperature (50°C) and anaerobic conditions, so it may provide a novel model for understanding and investigating CO₂ fixation in ancient organisms. The POAP cycle also provides an alternative pathway for biologically converting carbon dioxide into useful chemicals.

This study was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences, the National Science Foundation of China, the Key Research Program of the Chinese Academy of Sciences, and National Key Research and Development Program.

Figure 1  The synthetic POAP cycle (Image by IMCAS)

https://pubs.acs.org/doi/full/10.1021/acscatal.1c04151

Contact:

LI Yin

Institute of Microbiology, Chinese Academy of Sciences

Tel: 86-10-64807485

Email: yli@im.ac.cn