Nuclear magnetic resonance studies of xenon-129 with myoglobin and hemoglobin.

Nuclear magnetic resonance studies of 129Xe are consistent with one kinetically distinguishable binding environment in methemoglobin and two in metmyoglobin. The Xe binding site in methemoglobin is assigned to a cavity formed by the A-B and G-H corners of the globin chain [Schoenborn, B.P. (1965) Nature (London) 208, 760-762]. The small differences between alpha-hemoglobin and beta-hemoglobin are not resolved by the NMR experiments. The Xe association rate constant at 18 degrees C with methemoglobin is greater than 6 X 10(7) M-1 s-1 with an activation barrier of approximately 13 kcal/mol. One of the binding sites in metmyoglobin associated with a cavity on the proximal side of the porphyrin ring, opposite the O2 binding site [Schoenborn, B.P., Watson, H.C., & Kendrew, J.C. (1965) Nature (London) 207, 28-30]. An estimate of the association rate constant of Xe at 18 degrees C is 1 X 10(7) M-1 s-1 with an activation barrier of approximately 16 kcal/mol. The second metmyoglobin binding site has similar NMR and kinetic properties of those for methemoglobin.