The ferriheme protein metmyoglobin (metMb) in buffer solution at physiological pH 7.4 reversibly
binds the biomessenger molecule nitric oxide to yield the nitrosyl adduct (metMb(NO)). The kinetics of the
association and dissociation processes were investigated by both laser flash photolysis and stopped-flow kinetics
techniques at ambient and high pressure, in three laboratories using several different sources of metMb. The
activation parameters $\Delta$H⧧, $\Delta$S⧧, and $\Delta$V⧧ were calculated from the kinetic effects of varying temperature and
hydrostatic pressure. For the "on" reaction of metMb plus NO, reasonable agreement was found between the
various techniques with $\Delta$$H_{on}$⧧, $\Delta$$S_{on}$⧧, and $\Delta$$V_{on}$⧧ determined to have the respective values $\sim65$ kJ $mol^{-1}$, $\sim60$ J $mol^{-1}$ $K^{-1}$, and $\sim20$ $cm^{3}$ $mol^{-1}$. The large and positive $\Delta$S⧧ and $\Delta$V⧧ values are consistent with the operation of a limiting dissociative ligand substitution mechanism whereby dissociation of the $H_{2}O$ occupying the sixth distal coordination site of metMb must precede formation of the Fe-NO bond. While the activation enthalpies
of the "off" reaction displayed reasonable agreement between the various techniques (ranging from 68 to 83
kJ $mol^{-1}$), poorer agreement was found for the $\Delta$$S_{off}$⧧ values. For this reason, the kinetics for the "off" reaction were determined more directly via NO trapping experiments, which gave the respective activation parameters
$\Delta$$H_{off}$⧧ = 76 kJ $mol^{-1}$, $\Delta$$S_{off}$⧧ = $\sim41$ J $mol^{-1}$ $K^{-1}$, and $\Delta$$V_{off}$⧧ = 20 $cm^{3}$ $mol^{-1}$), again consistent with a
limiting dissociative mechanism. These results are discussed in reference to other investigations of the reactions
of NO with both model systems and metalloproteins.