A method of generating a protein with an improved functional property, the method comprising: (a) identifying at least one Target amino acid Residue in a first protein, wherein said Target amino acid Residue is associated with said functional property; (b) comparing at least one homologous second protein from the same or a different phylogenetic branch as the first protein with the first protein and identifying at least one Variant amino acid Residue between the first protein and the second protein; (c) selecting at least one Candidate amino acid Residue from the Variant amino acid Residue identified in (b) on the basis of said Candidate amino acid Residue affecting said Target amino acid Residue with respect to said functional property; (d) forming at least one Candidate Mutant protein in silico or producing at least one Candidate Mutant protein in vitro in which said at least one Candidate amino acid Residue from the second protein substitutes a corresponding residue in the first protein; and (e) screening said at least one Candidate Mutant protein produced in (d) to identify a protein having said improved functional property; a Rubisco protein and its large subunit polypeptide produced by the method; and polynucleotides encoding the same.