RuBisCO

 RuBisCO is the most abundant protein on Earth. When plants evolved, high CO2 and Low O2 favoured the Calvin cycle, but now there is more O2 than CO2 Photorespiration is selected, meaning that that must be the most evolutionary pressured protein in the world.

Photosynthesis is the fundamental cornerstone of nearly all life forms on our planet. Through the remarkable utilization of solar energy, plants achieve the extraordinary task of transforming water drawn from the soil and carbon dioxide (CO2) extracted from the air into a broad spectrum of life, ranging from delicate mosses adorning damp rocks and logs to vast stretches of grasslands to the majestic grandeur of towering redwoods. This process, by which carbon is converted into the living fabric of our natural world, is made possible by the action of an enzyme called Rubisco—an enzyme that could very well be the most abundant protein globally.

Photosynthesis is the conduit through which most carbon enters Earth's biosphere. As a consequence, each and every carbon atom within our own bodies can be traced back to a point in time when Rubisco pulled it from the atmosphere, marking a remarkable achievement for this single enzyme.

However, within the magnificence of Rubisco lies a fundamental vulnerability that countless aeons of evolution have yet to resolve. When the levels of atmospheric CO2 dip in comparison to molecular oxygen (O2), Rubisco loses its ability to differentiate between the two gases and can mistakenly latch onto O2 molecules. This misstep triggers a process known as photorespiration, which plants must engage in to avert the accumulation of unwanted byproducts resulting from O2 fixation. Regrettably, photorespiration exacts a toll on the plant's energy reserves and causes a loss of previously captured CO2. Particularly in the context of the Earth's current atmospheric conditions—an environment characterized by relatively low CO2 and high O2 levels over the last 30 million years—photorespiration can significantly diminish the efficiency of photosynthesis.

The evolutionary landscape thus places substantial pressure on the emergence of novel adaptations that address or sidestep this inherent flaw in Rubisco.