Probably most of us are aware that plants take sunlight and use it to “fix” carbon from carbon dioxide (CO2) in the atmosphere into sugar compounds by the process of photosynthesis. In fact, natural photosynthesis removes about 100 billion tonnes of CO2 from the atmosphere every year. The natural release and absorption of CO2 is balanced – but humans are releasing over 30 billion tonnes per year on top of that, which is increasing the CO2 in the atmosphere and causing global warming; this exceeds the ability of plants to remove it. But what if we could find a way to make carbon fixation more efficient? Now, a team of researchers have done just that.

A future green economy requires the development of efficient strategies to convert CO2 into multicarbon compounds that can then be used for other purposes. The authors describe a synthetic cycle for the continuous fixation of CO2 in vitro, i.e. in the lab.

If you learned the basics of photosynthesis at school, then you might remember the simple equation that sums up the process by which water and carbon dioxide are converted to glucose and oxygen by the energy from sunlight:

6H2O + 6CO2 + energy → C6H12O6 + 6O2

This belies the incredible complexity of the process. There are six naturally evolved carbon fixation pathways; this is a seventh, the catchily named crotonyl–coenzyme
A (CoA)/ethylmalonyl-CoA/hydroxybutyryl-CoA (CETCH) cycle.

Science Magazine
Figure 1, Gan & Lui, Science Vol 345, Issue 6314, pp. 830-831 [opinion]
The long version of the name should give you a clue that the paper itself is one for the dedicated biochemist, to be frank, but even a layperson can boggle at the technical skill that went into it.

The CETCH cycle is a reaction network of 17 enzymes…drafted by metabolic retrosynthesis, established with enzymes originating from nine different organisms of all three domains of life, and optimized in several rounds by enzyme engineering and metabolic proofreading. In addition, the cycle features three reactions that were created by rational active-site engineering of existing enzyme scaffolds to catalyze the desired activities, such as Mco and Pco….Notably, CETCH 5.4 relies solely on the reductive carboxylation of enoyl-CoA esters. Although ECRs belong to the most efficient CO2- fixing group of enzymes known to date, they were not selected for autotrophic CO2 fixation during evolution, as far as we know. In addition, the cycle features three reactions that were created by rational active-site engineering of existing enzyme scaffolds to catalyze the desired activities, such as Mco and Pco. The reallocation of reductive carboxylation as a key reaction for a synthetic autotrophic CO2 fixation cycle thus goes beyond simply improving or reshuffling naturally existing autotrophic CO2 fixation reactions and pathways.

Oof. (For the interested, I’ve included the outline of the cycle at the bottom of this post).

Why is it better? Well, primarily, it’s more efficient, requiring 6.2 ATP equivalents per CO2 fixed, as opposed to 6.8 – 8.2 for natural cycles (ATP being the energy currency of cells). It doesn’t sound like a huge difference but it is when scaled up over thousands upon thousands of molecules. Moreover, most of the CO2 fixation that occurs naturally is converted to plant biomass; with this you could in theory go directly to the synthesis of a compound you actually want. Moreover, a lot of the enzymes (biological catalysts) in the process suffer from inefficiencies; Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase), for example, is a slow catalyst that additionally has a strong side reaction with oxygen, which unfortunately leads to the loss of fixed carbon and thus photosynthetic energy by up to 30%  (a process called photorespiration). There have been many attempts to improve Rubisco via genetic engineering but with very little success.

There are a number of potential applications. These could include transplanting this synthetic process into photosynthetic organisms, paving the way to improve CO2 fixation, e.g. in crops, or using it make artificial leaves or photovoltaic cells. it could also be used in the design of a self-sustained, completely synthetic carbon metabolism in artificial or cells for use as chemical manufactories.

The CETCH cycle:

From Schwander et al, 2016: Science Vol. 354, Issue 6314, pp.900-904 [original paper]



Schwander et al, 2016: “A synthetic pathway for the fixation of carbon dioxide in vitro”. Science Vol. 354, Issue 6314, pp. 900-904 DOI: 10.1126/science.aah5237

Gong & Li Science 18 Nov 2016: Vol. 354, Issue 6314, pp. 830-831 DOI: 10.1126/science.aal1559

Image: Sunlight on Leaves, Pam Fray, Wikimedia Commons.

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