Rubisco stands for ribulose-1,5-bisphosphate carboxylase/oxygenase, and it's the enzyme responsible for the first step of carbon fixation in photosynthesis.

Here's why it's the most abundant:

* Photosynthesis is essential: Photosynthesis is the process that sustains life on Earth by converting sunlight into chemical energy. Rubisco is the key enzyme for this process.

* Widely distributed: Plants, algae, and some bacteria use Rubisco. This makes it ubiquitous in the biosphere.

* High turnover rate: While Rubisco is a very slow enzyme, its concentration is very high to compensate for its slowness.

While Rubisco might not be the most familiar protein, its vital role in life on Earth makes it incredibly abundant.