Meet Rubisco!

Rubisco is found in the chloroplasts of plant cells where it plays a key role in photosynthesis.

Diagram of a plant cell showing the chloroplasts colored in pink.
Rubisco works in the chloroplasts of plant cells

Remember that photosynthesis is the process plants (and some bacteria) use to build glucose sugar molecules using CO2 and energy from light. Plants can’t just stitch a bunch of raw CO2 molecules together into a glucose, though. So, Rubisco’s important job is to collect CO2 molecules and put them into a form that other enzymes can use.

Sketch of photosynthesis. Water is shown entering the plant and being converted to oxygen by sunlight. Carbon dioxide is shown entering the plant, going through a cycle, and becoming glucose.

Rubisco is a nickname for Ribulose-1,5-Bisphosphate Carboxylase-Oxygenase. Perhaps that’s an intimidating fancy official science name, but Rubisco’s name tells us a lot of information about its specific function. Let’s break it down and translate it:

  • Whenever we see “-ase” at the end of a protein’s name, we know that it must be an enzyme. Enzymes always facilitate a chemical reaction.
  • The words containing “-ase” tell us which kinds of chemical reactions this enzyme facilitates. Rubisco has two names, which indicates that it does two things. “Carboxylase” means that the enzyme adds a CO2 molecule to something. “Oxygenase” means that the enzyme adds an oxygen molecule to something.
  • But what does Rubisco add CO2 and oxygen to? That is what the beginning of the name tells us. “Ribulose-1,5-Bisphosphate” is a type of sugar molecule.
Visual breakdown of Rubisco's "long name." "Ribulose-1,5-Bisphosphate" is a sugar molecule and is the input of Rubsico. "Carboxylase" indicates that Rubisco adds carbon dioxide to something. "Oxidase" indicates that Rubisco adds oxygen to something.  "-ase" indicates that Rubisco is an enzyme (that is, a protein that helps with a chemical reaction).

This name makes a whole lot of sense when we see its function in photosynthesis: adding a CO2 to ribulose-1,5-bisphosphate. This ultimately makes two molecules of 3-phosphoglycerate, which other enzymes can use to build glucose later in photosynthesis.

Rubisco adds carbon dioxide to ribulose-1,5-bisphosphate to generate two molecules of 3-phosphoglycerate.

Rubisco is made up of 8 large pieces and 8 little pieces. Each large piece can facilitate one reaction, so one Rubisco can facilitate 8 reactions at once. This is helpful because Rubisco works slowly. It works so slowly that the enzymes working after it in photosynthesis typically have to wait on Rubisco before they can do their own jobs. To make up for how slow Rubisco works, plants produce a lot of it (and by “a lot” I mean that Rubisco is estimated to be the most abundant protein in the world).

Three dimensional structure model of Rubisco. 
Left panel: top view. Four units are visible, each containing one molecule of ribulose-1,5-bisphosphate. This view shows two planes of symmetry. 
Right panel: side view. All eight units are visible, but are not distinguishable from each other from this perspective. Eight molecules of ribulose-1,6-bisphosphate are counted. Two more planes of symmetry are shown.

Not only is Rubisco a slow worker, it is also an inaccurate worker. Remember the two names “carboxylase” and “oxygenase” that describe two different reactions? The carboxylase reaction is the one used in photosynthesis, but sometimes Rubisco accidentally adds oxygen instead and facilitates the oxygenase reaction. This accidental reaction makes one 3-phosphoglycerate and one phosphoglycolate molecule.

Rubisco adds oxygen to ribulose-1,5-bisphosphate to generate one molecule of 3-phosphoglycerate, and one molecule of phosphoglycolate (which is a waste product).

Unlike 3-phosphoglycerate, phosphoglycolate is not useful for photosynthesis. In fact, it is even toxic to the plant. A group of other enzymes can “salvage” phosphoglycolate, but the salvaging process wastes energy and CO2 molecules.

This wasteful process is posing a big question for plant scientists:

Why is this old, abundant, important, extensively-evolved enzyme still so horribly inefficient?

Some scientists think that Rubisco still has some evolving to do, but even human-engineered versions of Rubisco either work faster and make more mistakes (or work even slower and make the same amount of mistakes anyway). Many think CO2 and O2 are just so similar that it is impossible for Rubisco to tell the difference between them. Still others think that the wasteful reaction might have a purpose that we haven’t figured out yet. Perhaps the true answer is a combination between all of these.

To see Rubisco’s job in more context, check out this video on photosynthesis: Photosynthesis: Crash Course Biology #8

For (a lot) more about Rubisco engineering: Sharwood, R. E. (2016). Engineering chloroplasts to improve Rubisco catalysis: Prospects for translating improvements into food and fiber crops. New Phytologist, 213(2), 494-510. doi:10.1111/nph.14351

Berg, J. M., Tymoczko, J. L., Stryer, L., & Stryer, L. (2015). Biochemistry. New York: W.H. Freeman.

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