A protein is a polypeptide, a molecular chain of amino acids. Polypeptides are, indeed, the building blocks of your body. And, the most abundant protein in your body is collagen. However, the world's most abundant protein is RuBisCO, an enzyme that catalyzes the first step in carbon fixation.RuBisCO, whose full scientific name is "ribulose-1,5-bisphosphate carboxylase/oxygenase, which is found in plants, algae, cyanobacteria, and certain other bacteria. Carbon fixation is the main chemical reaction responsible for inorganic carbon entering the biosphere.In plants, this is part of photosynthesis, in which carbon dioxide is made into glucose .Typical enzymes can process a thousand molecules per second, but RuBisCO fixes only about three carbon dioxide molecules per second. Plant cells compensate for this slow rate by building lots of the enzyme. Chloroplasts are filled with RuBisCO, which comprises half of the protein. This makes RuBisCO the most plentiful single enzyme on the Earth.