* Rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase) is the enzyme responsible for the first step of carbon fixation in photosynthesis. It's essential for plant life and makes up a significant portion of their total protein content.

* It's estimated that Rubisco makes up around 40% of the total soluble protein in leaves and is present in vast quantities in the plant kingdom, contributing to its abundance.

While other proteins like collagen are also found in high amounts, Rubisco's role in photosynthesis and its prevalence across a massive number of plants make it the most abundant protein on Earth.