A synthetic biological metabolic pathway fixes CO2 more efficiently than plants

In future, greenhouse gas carbon dioxide could be removed from the atmosphere by deploying a new biological method. A team headed by Tobias Erb, Leader of a Research Group at the Max Planck Institute for Terrestrial Microbiology in Marburg, has developed a synthetic but completely biological metabolic pathway based on the model of photosynthesis that fixes carbon dioxide from the atmosphere 20% more efficiently that plants can photosynthetically. 

"CO2-fixing enzymes of a completely different quality are found in nature," emphasized Erb. Such enzymes, which are faster and more efficient than RuBisCo in plants, work naturally in the metabolism of microorganisms. Erb himself succeeded in isolating one of these enzymes – with the unpronounceable name of "Crotonyl-CoA Carboxylase/Reductase" – from bacteria. This enzyme almost never makes mistakes and is also turbo-charged, so to speak, working at twenty times the speed of its counterpart in the plant world.

Tobias Erb therefore initially designed a new theoretical cycle known as CETCH – which stands for Crotonyl-CoA/Ethylmalonyl-CoA/Hydroxybutyryl-CoA – with potential matching enzymes and all biochemical reactions. From databases containing 40,000 known enzymes, he identified a few dozen candidates which could perform the required tasks.

Erb's team then combined all the enzymes in a test tube to produce an "optimized cycle that performed robustly" in just two years. The researchers continually tested new biocatalysts, which were often genetically modified, and tried out new combinations of enzymes to find the system in which the components performed optimally together.

The end result was a synthetic CO2-fixing cycle, something which, as far as Erb is aware, "nobody has ever achieved before." A total of 17 different enzymes, including three "designer enzymes", are used from nine distinct organisms including human beings. The bottom line is that the CETCH cycle, where the Marburg-based researchers emulate photosynthesis's dark reaction, fixes CO2 at 20% greater efficiency than the Calvin cycle in plants.