RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) constitutes approximately 50% of total leaf soluble protein and about 25–50% of total chloroplast protein — making it the most abundant protein on Earth. Some sources cite specific percentages within chloroplasts varying by species and conditions. The answer listed here (19%) reflects the value given in the original paper, which may refer to a specific measurement of stromal protein composition.