Modelling the three-dimensional structure and the electrostatic potential field of two Cu,Zn superoxide dismutase variants from tomato leaves

The three-dimensional structure of tomato P31 and T10 Cu,Znsuperoxide dismutases (SODs) were computer modelled using thestructure of the bovine enzyme as a template. The structure-essentialresidues retain in the models the position occupied in the otherCu,Zn SODs of known 3D structure and the overall packing ofthe ß-barrel is maintained. Formation of ‘aromaticpairs’occurs between newly inserted aromatic residues.The number of total charges changes in the two variants andsome charged residues located in the proximity of the activesite in most Cu,Zn SODs disappear in tomato enzymes. Calculationof the electrostatic potential field, carried out by numericallysolving the Poisson-Boltzmann equation, indicates that in bothvariants a negative potential field surrounds all the proteinsurface except the active site areas, characterized by positivepotential values, as already observed in the bovine enzyme.This result confirms that coordinated mutations of charged residueshave occurred in the evolution of this enzyme giving rise toa peculiar electrostatic potential distribution common to allmembers of this protein family.