(a) The active site region of dUTPase in complex with dUTP. The protein is represented by its molecular surface colored according to its electrostatic potential (negative, red; positive, blue, and near neutral, white). The dUTP is shown in ball-and-stick form with its N, O, and P atoms blue, red, and yellow Mg ions that have been modeled into the structure are represented by green spheres. Note the complementary fit of the uracil ring into its binding pocket, particularly the close contacts that discriminate against a methyl group on C5 of the pyrimidine ring and a 2'-OH group on the ribose ring.