Laser kinetic spectroscopy studies of the bimolecular oxygenation of alpha- and beta-subunits within the R-state of human hemoglobin

Bimolecular oxygenation of tri-liganded R-state human hemoglobin (HbA) is described by bi-exponential kinetics with association rate constants k = 27.2 ± 1.3 (M·sec)^-1 and k = 62.9 ± 1.6 (M·sec)^-1. Both the observed processes have been assigned to the bimolecular oxygenation of - and -subunits of the native tetrameric protein by molecular oxygen. The quantum yields of photodissociation within the completely oxygenated R-state HbA are = 0.0120 ± 0.0017 and = 0.044 ± 0.005 for - and -subunits, respectively. The oxygenation reactions of isolated ^PCMB- and ^PCMB-hemoglobin chains are described by mono-exponential kinetics with the association rate constants k = 44 ± 2 (M·sec)^-1 and k = 51 ± 1 (M·sec)^-1, respectively. The quantum yields of photodissociation of isolated ^PCMB- and ^PCMB-chains (0.056 ± 0.006 and 0.065 ± 0.006, respectively) are greater than that observed for appropriate subunits within the R-state of oxygenated HbA.