Enzyme fixing carbon 10 times more abundant than we thought

Ribulose-1,5-bisphosphate carboxylase / oxygenase (Rubisco) is a carbon-binding enzyme that is found in all photosynthetic organisms. "More than 90% of the organic carbon found on the planet in the form of biomass is the product of rubisco's action," said Yinon Bar-on of the Weizmann Scientific Institute in Israel, who carried out Analysis with Ron Milo.

Forty years ago, biochemist John Ellis declared that rubisco was the most abundant protein on Earth. But it turns out that his calculations in the background have been shifted by an order of magnitude. Instead of only 0.04 billion tonnes, plants and marine organisms actually contain 0.7 billion tonnes of rubisco.

To estimate the terrestrial rubisco, which represents 90% of the total, Bar-on and Milo calculated the overall leaf mass. The leaves are the only parts of the plant containing rubisco. By combining data from field and remote sensing studies, the team also took into account different plant species containing different amounts of rubisco. Rubisco's calculations for the world's oceans rely on biomass estimates of marine autotrophs, such as algae and cyanobacteria.

The huge difference between Ellis' estimate and that of the Weizmann team stems from a misconception about the effectiveness of rubisco, Bar-on says. "Ellis divided [the annual global carbon fixation rate] by the maximum rate at which rubisco works in the lab – from there you can get an estimate of the amount of rubisco. The main reason his number was much lower than ours is that he used a much higher rate for rubisco. "

In the end, rubisco does not work at its maximum catalytic rate. Marine Rubisco works at 15% of its maximum, while Rubisco in terrestrial plants works at only 1% of its maximum, thus transforming three molecules of carbon dioxide per second.

"Any rubisco can be as effective as it is in vitro," says Hayer-Hartl. However, laboratory studies have shown the effectiveness of rubisco at 25 ° C in bright light and high carbon dioxide content. However, the natural conditions vary, for example, each leaf does not always get full sun.

Understanding the causes of the gap between the maximum and measured effectiveness of rubisco, Bar-on explains, will help scientists take into account the factors that can affect carbon sequestration in the world. Researchers are also working on improving rubisco enzymes to create higher yielding crops.

Since the previous estimate of rubisco was so far removed from reality, Bar-on wonders if there might be another protein even more abundant. "Since the biomass on the planet is mainly made up of plants, the main candidate for the most abundant proteins would be an abundant substance in all plant tissues, not just in leaves like rubisco."

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More information:
Yinon M. Bar-On et al. The overall mass and the average rate of rubisco, Proceedings of the National Academy of Sciences (2019). DOI: 10.1073 / pnas.1816654116