In over 85% of plant species, the carbon dioxide (CO2) that enters a leaf is converted by the enzyme Rubisco into a carbohydrate made up of three carbon atoms, known as 3-phosphoglycerate (PGA). These plants are called C3 plants and include rice, cassava, all legumes, and wheat. Rubisco is an inefficient enzyme because it cannot distinguish between CO2 and oxygen molecules. Around 35% of the time, Rubisco binds with oxygen instead of CO2, resulting in wasted energy and reduced photosynthesis.
Cyanobacteria (blue-green algae) have overcome the limitation of Rubisco through the evolution of mini-organelles called carboxysomes, which allow the elevation of carbon dioxide around Rubisco, but requiring the participation of active molecular pumps for bicarbonate accumulation in the cell. Concentrations of carbon dioxide near the site of Rubisco are so high inside carboxysomes that oxygen cannot generally bind with the enzyme, thereby suppressing photorespiration. Photosynthesis in our crops takes place in small organelles within the cells of leaves, called chloroplasts, which evolved from cyanobacteria. Our mathematical modeling suggests that a large increase in photosynthesis could be achieved by re-engineering the active bicarbonate pumps and carboxysome structures into modern plant chloroplasts. RIPE is attempting this re-engineering, as well as sourcing bicarbonate pumps from the green micro-alga, Chlamydomonas. Many proteins are required to form carboxysomes and active pumps, making this is a particularly high-risk strategy, but one that could pay maximum dividends.