What is the most abundant protein on Earth?

Ask a room full of people to name the most common protein on the planet and you will hear collagen, keratin, maybe whatever is in a tub of protein powder. Nobody says the actual winner, because almost nobody has heard of it. It has a clumsy name, it lives inside leaves, and by the standards of enzymes it is famously bad at its job.

The short answer

The prize is generally given to RuBisCO, short for ribulose-1,5-bisphosphate carboxylase/oxygenase. It is the enzyme that plants, algae and many bacteria use to pull carbon dioxide out of the air and fix it into sugar. In 1979 the biochemist R. J. Ellis published a paper in Trends in Biochemical Sciences with the wonderfully blunt title The most abundant protein in the world, estimating the global amount at about 40 trillion grams. That is roughly 40 million tonnes of a single protein.

The real story

Here is the twist: RuBisCO is everywhere precisely because it is so sluggish. Each molecule fixes only about 3 to 10 carbon dioxide molecules per second, which is slow for an enzyme. Plants make up for it with sheer numbers. In many common plants, RuBisCO accounts for around half of all the soluble protein in a leaf, and in so-called C4 plants it is still around 30 percent.

It also has a bad habit. RuBisCO cannot always tell carbon dioxide from oxygen, and sometimes grabs the wrong one. At normal air levels, it performs its useful carbon-fixing reaction roughly four times for every one time it reacts with oxygen instead. That wasteful side reaction, called photorespiration, costs the plant energy and releases some of the carbon it worked to capture.

Even the name is a bit of a joke. According to the standard account, the nickname RuBisCO was coined in 1979 by the biochemist David Eisenberg, who used it humorously at a seminar honouring the retirement of the plant scientist Sam Wildman. It stuck, partly because it sounds suspiciously like a brand of biscuit.

So every salad, every lawn and every forest canopy is quietly packed with the same clumsy enzyme, doing the slow, error-prone work of turning air into the sugars that plants are built from.

Why people get it wrong

We tend to think about proteins in terms of our own bodies and our own shopping trolleys. Collagen is on skincare jars, whey is on gym shelves, and keratin is on shampoo bottles, so those are the names that come to mind. But the planet is not built around humans. By sheer mass, Earth is a green world, and the protein doing the heavy lifting is the one inside leaves that nobody ever puts on a label. It is a good reminder that famous and common are two very different things.

The Sole sums it up: “The most important protein on Earth is slow, clumsy and keeps grabbing the wrong gas. Honestly, it’d fit right in on this show.”