The mode-specific vibrational energy relaxation (VER) of the iron-protoporphyrin IX (the heme) was studied using a non-Markovian time-dependent perturbation theory at the UB3LYP/6-31G(d) level. The derived VER time constants of the excited nu(4) and nu(7) modes, 1.2+/-0.1 and 2.1+/-0.1 ps, respectively, agree well with previous experimental results for MbCO (1.1+/-0.6 ps for the nu(4) mode and 1.9+/-0.6 ps for the nu(7) mode). The energy transfer pathways from the excited nu(4) and nu(7) modes were identified. The different symmetries of the nu(4) and nu(7) modes are reflected in distinct relaxation pathways. No direct energy transfer between the nu(4) and nu(7) modes was observed. The overtone of the approximately 350 cm(-1) iron out-of-plane gamma(7) mode was observed to be strongly coupled to the nu(7) mode and essential to its relaxation. The two isopropionate side chains of the heme were found to play an essential role in the VER mechanism for both nu(4) and nu(7) modes, providing the mode-specific level explanation to previous observations. Comparison of the results for VER in iron porphine (FeP) to results for the imidazole-ligated porphine model (FeP-Im) demonstrates that the axial Im ligand has little effect on the nu(4) or nu(7) mode relaxation processes. By considering the VER process as a multistep reaction and the third order Fermi resonance parameters the reaction rate constants, the VER kinetics of FeP was examined by solving the master equation.