The enzyme known as RuBisCO, or Ribulose-1,5-bisphosphate carboxylase/oxygenase, plays a foundational role in life on Earth. It serves as the primary gateway for atmospheric carbon dioxide to enter the biological world, converting inorganic carbon into organic matter. This protein is widely recognized as the most abundant protein on the planet, reflecting its widespread presence and importance in photosynthesis across plants, algae, and cyanobacteria. Its function underpins the global carbon cycle.
The Role in Carbon Fixation
RuBisCO’s primary function is carbon fixation, occurring within the Calvin Cycle inside chloroplasts. The enzyme catalyzes the attachment of atmospheric carbon dioxide (CO2) to a five-carbon sugar, ribulose-1,5-bisphosphate (RuBP). This creates an unstable six-carbon intermediate.
This unstable intermediate quickly splits into two molecules of 3-phosphoglycerate (3-PGA). These 3-PGA molecules proceed through the Calvin Cycle, forming glucose and other complex carbohydrates. This capture of CO2 is the first and often rate-limiting step in creating sugars, powering plant growth.
The Problem of Photorespiration
Despite its role, RuBisCO has a drawback: it can react with oxygen (O2) in addition to carbon dioxide. This alternative reaction, oxygenation, leads to photorespiration. When oxygen levels are high, especially in hot, dry conditions, RuBisCO can bind O2 to RuBP instead of CO2.
Oxygenation produces 3-phosphoglycerate (3-PGA) and 2-phosphoglycolate. Unlike 3-PGA, 2-phosphoglycolate cannot be used in the Calvin Cycle and must be recycled through a pathway that consumes energy and releases CO2. This undoes some carbon fixation, reducing photosynthetic efficiency and limiting plant growth.
Plant Adaptations to Inefficiency
Some plant species have evolved strategies to mitigate the inefficiency caused by photorespiration. One notable adaptation is C4 photosynthesis, found in plants like corn, sugarcane, and switchgrass. C4 plants spatially separate initial carbon fixation from the Calvin Cycle. They use PEP carboxylase to capture CO2 in mesophyll cells, forming a four-carbon compound transported to bundle sheath cells.
In bundle sheath cells, the four-carbon compound releases CO2, creating a high concentration of CO2 around RuBisCO. This elevated CO2 level outcompetes oxygen for RuBisCO’s active site, significantly reducing photorespiration. Another adaptation is Crassulacean Acid Metabolism (CAM) photosynthesis, common in succulents and cacti. CAM plants temporally separate carbon fixation by opening stomata to collect CO2 at night, storing it as a four-carbon acid.
During the day, when stomata are closed to conserve water, stored CO2 is released internally and fed into the Calvin Cycle. This temporal separation ensures RuBisCO operates in a CO2-rich environment during daylight hours, minimizing oxygenation. Both C4 and CAM pathways represent evolutionary workarounds to RuBisCO’s oxygenase activity, allowing these plants to thrive in environments where photorespiration would otherwise limit their growth.
Engineering a More Efficient Enzyme
Given RuBisCO’s inefficiencies, particularly photorespiration, scientists are exploring genetic engineering approaches to improve its performance. The primary motivation is to enhance the photosynthetic efficiency of major food crops, which could lead to increased yields and help address global food security challenges. Current research focuses on two objectives: increasing the enzyme’s catalytic speed and improving its specificity for carbon dioxide over oxygen.
Researchers are investigating RuBisCO variants from different organisms, including algae and cyanobacteria, which sometimes exhibit higher catalytic rates or better CO2 specificity. Introducing these modified or “faster” versions of RuBisCO into crop plants is a complex endeavor, as the enzyme is composed of multiple subunits and interacts with various cellular components. Altering such a fundamental and ancient enzyme without causing unintended negative consequences for plant health or development remains a significant challenge.