Meet Rubisco!

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

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.

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.

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 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).

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.

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.