Dynamic analysis of efficient heme rotation in myoglobin by NMR spectroscopy

Sperm whale myoglobin was reconstituted with 1,4,5,8-tetramethylhemin. The hyperfine-shifted proton NMR signals from the prosthetic group exhibit remarkable pattern changes around 15 degrees C, while the globin resonances are normal to obey the Curie law. The NMR anomaly specifically observed for the heme signals suggests a slow to rapid rotational transition of the hemin about the iron-histidine bond. The temperature-dependent pattern changes were quantitatively analyzed by a dynamic NMR method. Two sets of analyses with the heme-methyl and pyrrole-proton lines consistently afforded delta H not equal to = 16.3 kcal/mol, delta S not equal to = 14.0 e.u., delta G not equal to = 12.1 kcal/mol at 298 K, and a frequency of 90 degrees heme rotation 5600 s-1 at 20 degrees C. The relatively large activation entropy suggests that structural rearrangements at the direct heme vicinity are involved and that efficient heme rotation is accomplished by a number of fluctuative local heme-globin contacts within a conserved crevice structure.