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Bacterial Enzyme “Juggles” Substrates, Catalysing Key Processes in Photosynthesis 20 Times Faster Than Plant Enzymes
The discovery of a bacterial enzyme that converts carbon dioxide into other carbon compounds 20 times faster than plant enzymes during photosynthesis opens up alternative approaches to designing artificial photosynthesis or carbon capture technology.

Plants depend on carbon fixation, a process that transforms gaseous carbon dioxide into essential carbon-rich biomolecules, for their survival. This process is achieved through photosynthesis, which also plays an important role in ensuring the delicate balance of carbon throughout the wider ecosystem as it is one of the key processes in the fast carbon cycle.

CO2 fixation involves the formation of a C–C bond between an acceptor substrate and a CO2molecule, of which the CO2 molecule is the electrophile. For the ease of C–C bond formation, most carboxylases activate their respective nucleophilic substrate by converting them into an enol or enolate form. They are highly reactive strong nucleophiles, so tight catalytic control over the enol or enolate form and the CO2 molecule is essential to reduce the yield of unwanted products from side reactions.

However, despite their importance, plant enzymes like ribulose-1,5-bisphosphate-carboxylase/oxygenase (RuBisCO), are not the most efficient biological catalysts to catalyse CO2fixation. It has a slow catalytic capacity, with a lower turnover number (Kcat) compared to other enzymes.

Furthermore, some side reactions result in RuBisCO fixing an oxygen (O2) molecule instead of CO2, producing 2-phosphoglycolate. This is toxic to the cell, and thus has to be broken down in an energy-intensive process, making the overall process rather inefficient.

Fortunately, a breakthrough by an international research team, comprising members from the Max Planck Institute for Terrestrial Microbiology, has discovered a bacterial enzyme that carries out this key step in carbon fixation 20 times faster than enzymes in plants.

These enzymes, enoyl-CoA carboxylases/reductases (ECRs), are naturally produced by the soil bacteria Kitasatospora setae, which not only has carbon-fixing abilities but can also produce antibiotics.

This enzyme comes in pairs of molecules that work in tandem to catalyse the reaction faster than regular plant enzymes that can only bind to one substrate at a time. The pair of molecules can be likened to “juggling” the substrates, as one pair opens wide to attach to the substrate while the other pair closes over the bound substrate and catalyses the carbon-fixing reaction, in a continuous cycle.

A small amount of molecular “glue” holds each pair of molecules together such that they can open and close in a coordinated manner while the twisting motion of the enzyme helps to load and unload substrates and products for an increased rate of reaction. The combination of the “glue” and twisting motionsresults in a carbon fixation rate of100 times per second.

Demystifying the enzyme’s form, function and action would help scientists to engineer processesthat mimic photosynthesis. “This ‘photosynthesis 2.0’ could take place in living or synthetic systems such as artificial chloroplasts – droplets of water suspended in oil,” saidTobias Erb of the Max Planck Institute for Terrestrial Microbiology in Germany, whose research interests lie in designing bioreactors for artificial photosynthesis as a potential pathway to recycle carbon dioxideinto biofuels, fertilisers, antibiotics, and other useful carbon-derived products.

“This bacterial enzyme is the most efficient carbon fixer that we know of, and we came up with a neat explanation of what it can do,” said Soichi Wakatsuki, a professor at SLAC National Accelerator Laboratory, Stanford and one of the senior leaders of the study.

Source: DeMirci et al. (2022). Intersubunit Coupling Enables Fast CO2-Fixation by Reductive Carboxylases, ACS Central Science

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