Motions in myoglobin

When the structure of myoglobin was first revealed by x-ray crystallography, it was discovered that there was no clear evidence of a pathway for ligands to enter the protein to bind at the heme iron. Motions within the protein are necessary for the protein to function. Pressure release and flash photolysis experiments help characterize some of these protein motions.Sperm whale carbonmonoxymyoglobin exhibits three spectroscopically distinguishable carbon monoxide stretch bands between 1910 $cm\sp{-1}$ and 1990 $cm\sp{-1}$ labeled $A\sb0$, $A\sb1$, and $A\sb3$. Pressure release measurements reveal three relaxations in the protein which are measured by: (1) the shifting in frequency of $A\sb0$, and the exchanges (2) $A\sb1 \to A\sb3$, and (3) $A\sb{0} \to A\sb{1} + A\sb{3}$. Each relaxation is non-Arrhenius and solvent viscosity dependent.The rebinding of the A substates was observed after flash photolysis by monitoring the Soret band and the A substates. The low-temperature rebinding (20K-160K) exhibit increasing rebinding rates with increasing temperature. The three A substates show different rebinding rates. From $\sim$170K to $\sim$220K, the rebinding rate decreases with increasing temperature in each A substate. Between 220K and $\sim$300K, the absorbance change of $A\sb0$ is non-monotonic in time due to an interconversion between $A\sb0$ and $A\sb{1} + A\sb3$ during rebinding. This interconversion rate matches well with the extrapolated interconversion rate deduced from pressure release measurements. Since the three A substates have different rebinding rates, it is possible that the binding process is controlled in part by the interconversion between A substates.The maximum entropy method applied to the rebinding data reveals five peaks in the distribution of rebinding rates, labeled (from fast to slow) 1,2,3,E, and S. Peak 1 is due to geminate rebinding. Peaks 2 and 3 are well characterized, but not well understood. Peak 2 is present in all the A substates (except possibly $A\sb3$) with the same rate as peak 2 monitored in the Soret. Peak 3 is the same in the Soret, $A\sb1$ and likely $A\sb3$, however, it is absent in $A\sb0$. Peak 3 matches closely with the extrapolated interconversion between $A\sb1$ and $A\sb3$ and the shifting of peak frequency of $A\sb0$ measured in pressure release experiments. Peak E is due to the interconversion $A\sb{0} \to A\sb{1} + A\sb{3}$ and peak S arises from the rebinding of CO from the solvent.U of I OnlyETDs are only available to UIUC Users without author permissio