Rubisco. Computer graphic of a molecule of the enzyme Rubisco (ribulose bisphosphate carboxylase/ oxygenase) from a plant. Rubisco is thought to be the most abundant and important protein found in nature. It occurs in all plants and fixes carbon dioxide during photosynthesis. Here, polypeptide chains making up the skeleton of Rubisco are seen. Two types of subunits are seen arranged around a four-fold axis: four small subunits (red); four large subunits (green, blue) where active sites attach the enzyme to its substrate. Behind this structure is a mirror image completing the Rubisco molecule with its eight subunits of each type.