Photosynthesis · enzyme kinetics · evolution
Why is Rubisco so slow?
Rubisco fixes carbon slowly and sometimes reacts with oxygen. Across species, its kinetics reveal why.
Rubisco brings CO₂ into the chemistry of photosynthesis. It is slow. It also accepts O₂, sending carbon through photorespiration. Plants compensate by making large amounts of the enzyme. The usual question is why evolution has not produced a faster, more selective Rubisco.
Carbon enters through Rubisco
Rubisco—ribulose-1,5-bisphosphate carboxylase/oxygenase—acts in the Calvin cycle. With CO₂, it converts RuBP into two molecules of 3-phosphoglycerate. This is the entry point of inorganic carbon into the pathway that builds sugars and biomass.
The oxygenation branch costs energy and releases some previously fixed carbon. At the same time, typical plant Rubiscos turn over only a few CO₂ molecules per active site each second. Slow catalysis and imperfect discrimination therefore appear together.
The hidden assumption is that speed, affinity, and specificity can be improved independently.
Natural Rubiscos lie on a narrow kinetic path
With Yonatan Savir, Elad Noor, and Ron Milo, we compared Rubiscos from photosynthetic bacteria, cyanobacteria, algae, and C₃ and C₄ plants. Each enzyme is described by several kinetic parameters: catalytic rate, affinity for CO₂ and O₂, and discrimination between them.
If these traits varied independently, natural enzymes would fill a broad kinetic space. They do not. The parameters follow strong correlations. In the complete four-parameter subset, one principal component accounts for about 91% of the observed variation.
A constrained kinetic landscape
The environment selects a position
The central tradeoff is between catalytic speed and CO₂ affinity. Faster Rubiscos tend to bind CO₂ less tightly. Specificity and oxygen affinity are coupled to the same constrained variation. The desirable corner—fast, high-affinity, highly selective—is therefore not freely accessible.
The environment chooses the operating point
A constraint tells us which enzymes are accessible. It does not tell us which one is best. That depends on the environment around Rubisco.
At low CO₂, affinity is valuable. When CO₂ is concentrated around the enzyme, a faster, lower-affinity Rubisco can perform better. C₃ plants, C₄ plants, and organisms with carbon-concentrating mechanisms therefore occupy different parts of the same kinetic landscape.
Optimizing net photosynthesis along the measured constraint predicts this shift. The observed Rubiscos lie close to the positions expected for their physiological CO₂ environments.
Slow does not mean poorly adapted.
C₄ photosynthesis changes the environment of the enzyme
C₄ plants do not improve Rubisco in isolation. They first concentrate CO₂ near it. This suppresses oxygenation and changes which point on the kinetic path gives the best performance.
The enzyme and its biochemical environment therefore evolve together. A molecular property that looks poor in isolation can be appropriate once the surrounding machinery is included.
Why is Rubisco difficult to engineer?
Rubisco is an obvious target for improving photosynthesis and crop productivity. But its kinetic traits are coupled. Increasing catalytic rate can alter affinity or specificity; changing one parameter does not leave the others fixed.
Engineering therefore confronts the same geometry as evolution. The problem is not only to move toward a better point, but to change or escape the underlying constraint.
Constraint comes before optimality
Natural selection does not choose among all imaginable proteins. It acts on proteins that molecular structure can realize. Rubisco makes this visible because variation across species traces the accessible kinetic landscape itself.
Evolution optimizes within the possible.