Picosecond phase grating spectroscopy of hemoglobin and myoglobin: energetics and dynamics of global protein motion

Phase grating spectroscopy has been used to follow the optically triggered tertiary structural changes of carboxymyoglobin (MbCO) and carboxyhemoglobin (HbCO). Probe wavelength and temperature dependencies have shown that the grating signal arises from nonthermal density changes induced by the protein structural changes. The material displaced through the protein structural changes leads to the excitation of coherent acoustic modes of the surrounding water. The coupling of the structural changes to the fluid hydrodynamics demonstrates that a global change in the protein structure is occurring in less than 30 ps. The global relaxation is on the same time scale as the local changes in structure in the vicinity of the heme pocket. The observed dynamics for global relaxation and correspondence between the local and global structural changes provides evidence for the involvement of collective modes in the propagation of the initial tertiary conformational changes. The energetics can also be derived from the acoustic signal. For MbCO, the photodissociation process is endothermic by 21 +/- 2 kcal/mol, which corresponds closely to the expected Fe-CO bond enthalpy. In contrast, HbCO dissipates approximately 10 kcal/mol more energy relative to myoglobin during its initial tertiary structural relaxation. The difference in energetics indicates that significantly more energy is stored in the hemoglobin structure and is believed to be related to the quaternary structure of hemoglobin not present in the monomeric form of myoglobin. These findings provide new insight into the biomechanics of conformational changes in proteins and lend support to theoretical models invoking stored strain energy as the driving force for large amplitude correlated motions.