Biology / Science

Two steps forward, one step back

I read that the biggest store of protein on Earth is in plants. What is it?

Question via email.

All living things use proteins for an almost endless array of purposes. Our muscles are made of protein, the hormones that regulate our bodily processes are made of protein, and all kinds of cell functions rely on proteins. It’s fair to say that life couldn’t exist without protein, but there is one candidate that stands above all the rest of them for sheer importance. That protein is ribulose-1,5-bisphosphate carboxylase/oxygenase — Rubisco, to its friends.

One way of visualising a molecule of Rubisco.

Rubisco is one of the proteins involved in carbon fixation; in other words, it helps plants make everything they need from carbon dioxide, water and sunlight. Without Rubisco, there could be no us, because pretty much all the energy we use comes ultimately from plants. Even if you eat only meat (not really advised), the animals producing that meat got their energy from plants, possibly via eating other animals. It can be a long chain back to Rubisco, but it’s an unbroken one.

That alone sounds like a good reason for it to be the most numerous protein on Earth, but actually, there’s a problem. Rubisco is really inefficient, at the current level of CO2 in the air. For one thing, it’s slow. Some enzymes can catalyse thousands of reactions a second, but Rubisco only manages 3-10. And it’s also inefficient because about 25% of the time, it goes through what — from a plant’s perspective, and ours too — is the wrong reaction. It carboxylates instead of oxygenates, totally wasting energy.

You’d think that a more efficient Rubisco would be a good idea, but most plants have pretty much gone the other way: they just have more Rubisco. That’s why Rubisco is the most numerous protein on Earth: it’s one of the most important, but it’s also one of the least effective.

2 thoughts on “Two steps forward, one step back

  1. What about fungi? Are there any fungi that don’t feed on plants? Not that I think that a fungus only diet would be healthy…

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    • Hm, not sure why my reply never posted here! Answer: I don’t believe there are any autotrophic fungi, so they all feed on some kind of organic matter. (There are three categories of fungi I know of: saprotrophic, feeding on already dead things; necrotrophic, which kill cells before digesting them; and biotrophy, in which they feed on living tissue — like Candida albicans.)

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