The charge distribution and Fukui function of the coordination complexes of the heme and small molecules were calculated in terms of an atom-bond electronegativity equalization method (ABEEM). It can be seen that charge transfers from iron ion to the coordinated atoms when the heme and small molecules combine, including O2, H2O, CO and NO. Fukui function of the iron ion is bigger than those of the coordinated oxygen atoms in O2 and H2O, but smaller than those of the coordinated carbon and nitrogen atoms in CO and NO. It can be obtained from the Fukui function that it is difficult for CO and NO to dissociate from the CO and NO coordination complexes of heme, respectively. Whereas it is easy for O2 and H2O to dissociate from the O2 and H2O coordination complexes of heme. Moreover heme is able to combine with other ligands. When the heme combines with the potassium cyanide (KCN) and sodium azide (NaN3) inhibitors, the Fukui function of the iron is smaller than those of the C and N coordinated atoms, and thus the activities of heme in cytohrome c peroxidase are weakened or inhibited.

Key words: atom-bond electronegativity equalization method, heme, charge distribution, Fukui function, coordination complex