The contribution of heme propionate groups to the conformational dynamics associated with CO photodissociation from horse heart myoglobin

Photoacoustic calorimetry and transient absorption spectroscopy were used to study conformational dynamics associated with CO photodissociation from horse heart myoglobin (Mb) reconstituted with either Fe protoporphyrin IX dimethylester (FePPDME), Fe octaethylporphyrin (FeOEP), or with native Fe protoporphyrin IX (FePPIX). The volume and enthalpy changes associated with the Fe–CO bond dissociation and formation of a transient deoxyMb intermediate for the reconstituted Mbs were found to be similar to those determined for native Mb (ΔV1 = −2.5 ± 0.6 ml mol−1 and ΔH1 = 8.1 ± 3.0 kcal mol−1). The replacement of FePPIX by FeOEP significantly alters the conformational dynamics associated with CO release from protein. Ligand escape from FeOEP reconstituted Mb was determined to be roughly a factor of two faster (τ = 330 ns) relative to native protein (τ = 700 ns) and accompanying reaction volume and enthalpy changes were also found to be smaller (ΔV2 = 5.4 ± 2.5 ml mol−1 and ΔH2 = 0.7 ± 2.2 kcal mol−1) than those for native Mb (ΔV2 = 14.3 ± 0.8 ml mol−1 and ΔH2 = 7.8 ± 3.5 kcal mol−1). On the other hand, volume and enthalpy changes for CO release from FePPIX or FePPDME reconstituted Mb were nearly identical to those of the native protein. These results suggest that the hydrogen bonding network between heme propionate groups and nearby amino acid residues likely play an important role in regulating ligand diffusion through protein matrix. Disruption of this network leads to a partially open conformation of protein with less restricted ligand access to the heme binding pocket.