Mechanism of ligand binding to hemes and hemoproteins. A high-pressure study

Reversible Hexacoordination of α-Hemoglobin-stabilizing Protein (AHSP)/α-Hemoglobin Versus Pressure

“…The slope of the first phase (low P region) was Ϫ4.5 cm 3 /mol for the complex and Ϫ7 cm 3 /mol for the ␣-chains, whereas the second phase showed a similar slope for both systems with a volume of 5 cm 3 /mol. It has been reported that negative activation volume is characteristic of the bond formation reaction, and the positive activation volume is described as characteristic of CO diffusion reaction into the protein matrix (15,(25)(26)(27). These positive activation volumes for the ligand migration reflect the conformational fluctuations of the proteins for the changes from a "closed" to "open" structure (15,(25)(26)(27).…”

“…It has been reported that negative activation volume is characteristic of the bond formation reaction, and the positive activation volume is described as characteristic of CO diffusion reaction into the protein matrix (15,(25)(26)(27). These positive activation volumes for the ligand migration reflect the conformational fluctuations of the proteins for the changes from a "closed" to "open" structure (15,(25)(26)(27). Thus AHSP binding to ␣-Hb does not modify the volume profile for the CO binding reaction, but the peak of the bell shaped curve (transition for the change in sign of the activation volume) was displaced to higher pressures for the complex (Fig.…”

Unusual Pressure Effects on Ligand Rebinding to the Human Myoglobin Leucine 29 Mutants

“…) (8,9,12,13,16,17). As examined using small compounds and model systems (17), these differences in the sign of the activation volumes have been attributed to the different rate-determining step for the ligand binding process.…”

“…Although various gaseous ligands can bind to the heme iron located inside the protein matrix, x-ray structures of hemoproteins show no explicit pathways for entry and escape of the ligands (5-7), implying that large protein fluctuation would be required for ligand binding reactions. Since the structural fluctuation in proteins is supposed to be perturbed by hydrostatic pressure (1,2,4), the influence of pressure on ligand binding to hemoproteins has been extensively studied by our group (8 -12) and other groups (4,(13)(14)(15)(16)(17). By measurements of pressure dependence of ligand binding kinetics, the activation volume (⌬V ‡ ) of the ligand binding reaction (the volume change of the transition state with respect to the ground state) can be estimated by Equation 1.…”

“…In the carbon monoxide rebinding reactions for hemoproteins, negative activation volumes ranging between Ϫ42 and Ϫ9 cm 3 mol Ϫ1 have been obtained (8,9,11,13,14,17,18), whereas the dioxygen rebinding reactions exhibited small positive activation volumes (0 -8 cm 3 mol…”