Ribulose-1,5-bisphosphate carboxylase/oxygenase, commonly known as Rubisco, is the world’s most abundant protein, representing up to 50% of the soluble protein within plant leaves. This enzyme performs the most widespread biochemical reaction on Earth, initiating the process of photosynthesis that sustains nearly all life forms. By acting as the primary gateway for atmospheric carbon into the biological world, Rubisco establishes the foundation for the entire food chain. The enzyme’s internal mechanics reveal a paradox of both power and limitation.
The Structure of Rubisco
The quantity of Rubisco present in plant tissue compensates for its sluggishness. Compared to typical enzymes that process thousands of molecules per second, Rubisco is slow, fixing only about three to ten molecules of carbon dioxide per second. Plant cells must produce massive amounts of this protein to achieve the necessary rate of carbon fixation. The protein is organized into a complex structure composed of 16 subunits. These include eight large chains containing the active sites and eight small chains serving a regulatory role. Rubisco resides within the stroma, the inner fluid-filled space of the chloroplasts, where carbon fixation occurs.
Primary Function: Carbon Fixation
The enzyme catalyzes the first step of the Calvin cycle, a process known as carbon fixation. In this reaction, Rubisco binds atmospheric carbon dioxide ($\text{CO}_2$) to a five-carbon sugar molecule called ribulose-1,5-bisphosphate (RuBP). This binding creates an unstable six-carbon compound that immediately splits. The cleavage of this intermediate yields two molecules of 3-phosphoglycerate (3-PGA). This carboxylation reaction incorporates inorganic carbon from the atmosphere into an organic molecule. The 3-PGA molecules then proceed through the rest of the Calvin cycle, where they are converted into sugars that serve as the plant’s primary source of chemical energy and building blocks.
The Inefficiency Problem (Photorespiration)
Rubisco possesses a flaw that limits photosynthetic output: its ability to react with molecular oxygen ($\text{O}_2$). This dual function is why its full name includes “carboxylase/oxygenase.” When Rubisco binds to oxygen instead of carbon dioxide, it initiates photorespiration. The oxygenation reaction occurs when the enzyme attaches $\text{O}_2$ to the RuBP molecule, yielding one molecule of 3-PGA and one two-carbon compound called phosphoglycolate. Unlike 3-PGA, phosphoglycolate cannot be used by the Calvin cycle.
The plant must then expend energy, including ATP and NADPH, to salvage the carbon from phosphoglycolate across three different organelles (chloroplasts, peroxisomes, and mitochondria). This salvage operation consumes energy and releases fixed carbon back as $\text{CO}_2$, undoing the work of photosynthesis. Photorespiration is exacerbated by high temperatures, which cause the plant’s stomata to close to conserve water. This leads to a drop in internal $\text{CO}_2$ concentration and a buildup of $\text{O}_2$. Under these conditions, the increased ratio of oxygen to carbon dioxide forces Rubisco to bind to oxygen more frequently, potentially reducing photosynthetic efficiency by up to 25% in many common plant species.
Evolutionary Adaptations to Improve Efficiency
The inefficiency of photorespiration has driven the evolution of specialized photosynthetic pathways that act as carbon concentrating mechanisms to favor the enzyme’s carboxylase function.
C4 Photosynthesis
C4 plants, such as corn and sugarcane, utilize spatial separation known as Kranz anatomy. They first fix $\text{CO}_2$ in outer mesophyll cells using phosphoenolpyruvate (PEP) carboxylase, an enzyme which has no affinity for oxygen. The resulting four-carbon compound (malate) is transported into specialized bundle sheath cells where Rubisco is sequestered. Once inside, the malate is broken down to release a high concentration of $\text{CO}_2$ directly around Rubisco. This ensures Rubisco is saturated with carbon dioxide, suppressing the oxygenase reaction and minimizing photorespiration.
Crassulacean Acid Metabolism (CAM)
Plants adapted to desert environments, like cacti and pineapples, employ Crassulacean Acid Metabolism (CAM) to achieve temporal separation. CAM plants open their stomata only at night to take in $\text{CO}_2$ and fix it into an organic acid using PEP carboxylase. This acid is stored in the cell’s vacuole until the daytime when the stomata are closed to conserve water. The stored acid is then broken down during the day, releasing a burst of $\text{CO}_2$ into the chloroplast, ensuring Rubisco operates in a high-concentration environment and avoiding photorespiration.