A closeup view of a CO2 modified lysine residue (Lys-CO2) within the active site of a protein.

Cyanobacteria are “excellent little factories for developing CO2-capture technologies” to help reduce greenhouse gas emissions, says Simon Fraser biochemist Dustin King. The problem, however, is that the cellular mechanisms it and other organisms use to sense and respond to CO2 are a bit of a mystery.

King and his colleagues recently came up with a strategy to shed light on how these water-borne blue-green algae fix CO2. They hope their research will lead to a better understanding of how cyanobacteria regulate their carbon capture system so others can

genetically engineer them to store even more CO2. Genetically modified cyanobacteria are already being used at power stations and factories in some parts of the world to soak up CO2 and King hopes his team’s work will help to boost their efficiency.

Cyanobacteria pull CO2 from the atmosphere so they can use the carbon to create essential nutrients they need to live. They respond to low environmental CO2 levels by actively concentrating it within specialized cellular compartments where it is directly added onto cellular metabolites. However, the mechanism through which cells ‘sense’ low CO2 levels is unknown.

King proposes that CO2 binds to particular proteins and changes their chemical properties. He reasons that this binding serves as a sort of CO2 responsive chemical signal that the cell uses to regulate its CO2-concentrating system. Artificially triggering that signal could lead to increased CO2 storage.

But this process has been hard to study because CO2 binds to proteins and then immediately falls off before scientists can detect it. This means they have no idea what part of the protein is involved in triggering the signal.

King and his colleagues, however, have come up with “a chemical trick” to see these elusive CO2 binding sites on cyanobacteria proteins. In a paper published in July in Nature Chemical Biology, they show how they did it.

The first step was to use a molecule called isocyanic acid that mimics CO2’s behaviour as it binds to a protein. “Isocyanic acid is able to modify the same parts on these proteins that CO2 does, except, as opposed to CO2, it forms a nice, stable adduct,” says King. “Where it gets a bit complicated is how do you now detect CO2 binding using this analogue?”

The answer is to expose the protein-isocyanic acid adduct to CO2. The CO2 then competes with the isocyanic acid to modify the protein, which lessens the amount of isocyanic acid in the adduct.

In the process of competing, the CO2 now forms a stable bond and the resulting adduct can be examined with a mass spectrometer to discover the binding site on the protein. King and his colleagues went a step further by using the synchrotron at the University of Saskatchewan’s Canadian Light Source to study the adduct in three dimensions.

University of Virginia chemist Ku-Lung Hsu calls it “an elegant and powerful method to study the lysine carboxylome.” As he writes in an accompanying commentary, “the ability to quantify Lys-CO2 PTMs on proteins with amino acid resolution enables the assignment of CO2-sensing function not only at the protein level but also to specific protein domains that mediate the response.”

King hopes his team’s discovery will provide a springboard for other, yet-to-be-discovered advances in human and environmental health. “Now we’re beginning to explore what effect changing the function of this protein has, not just in cyanobacteria but in other life forms,” he says.