Potential toxicity of transition metals like Hg, Cu and Cd are well known and their affinity toward proteins is of great concern. This work explores the selective nature of interactions of Cu2+, Hg2+ and Cd2+ with the heme proteins leghemoglobin, myoglobin and cytochrome C. The binding profiles were analyzed using absorbance spectrum and steady-state fluorescence spectroscopy. Thermodynamic parameters like enthalpy, entropy and free energy changes were derived by isothermal calorimetry and consequent binding parameters were compared for these heme proteins. Free energy (DG) values revealed Cu2+ binding toward myoglobin and leghemoglobin to be specific and facile in contrast to weak binding for Hg2+ or Cd2+. Time correlated single photon counting indicated significant alteration in excited state lifetimes for metal complexed myoglobin and leghemoglobin suggesting bimolecular collisions to be involved. Interestingly, none of these cations showed significant affinity for cytochrome c pointing that, presence of conserved sequences or heme group is not the only criteria for cation binding toward heme proteins, but the microenvironment of the residues or a specific folding pattern may be responsible for these differential conjugation profile. Binding of these cations may modulate the conformation and functions of these biologically important proteins. © 2016, Higher Education Press and Springer-Verlag Berlin Heidelberg.