KINETICS OF OXYGEN AND CARBON MONOXIDE REACTIONS 287 



7.2. Comparison of Kinetics of Myohemoglobin 

 and Hemoglobin 



Roughton and Millikan {2308) raised several criticisms of Pauling's 

 hypothesis on the basis of the kinetics of myohemoglobin. The basis for 

 part of their criticism disappears if one takes the more generally accepted 

 value of 17,000 for the molecular weight of myohemoglobin rather than the 

 value 34,000 which they accepted; heme-heme interaction is therefore absent 

 in myohemoglobin. 



The ratio of the velocities with which oxygen and carbon monoxide 

 react with myohemoglobin and hemoglobin are collected in Table 

 IV, taken from the publications of Millikan {1952a). It can be seen 

 that, with the exception of the dissociation of myooxyhemoglobin, all 

 the myohemoglobin reactions are faster than the corresponding 

 hemoglobin reactions. 



TABLE IV 

 Comparison of Myohemoglobin and Hemoglobin Kinetics" 



" According to Millikan {1952a). 



Steric considerations might account for the increase in rate of 

 association reactions. If the hemes are held between "hemaffine" 

 groups on the surface of hemoglobin molecule, the effective target area 

 may well be much less than in the case of myohemoglobin, where the 

 heme is probably attached to only one hemaffine group. The insen- 

 sitivity of myooxyhemoglobin dissociation to pH changes between 

 6.2 and 8.6 indicates the absence of a heme-linked group whose 

 pK value lies in this range. This would also be expected to influence 

 the kinetic differences between the two compounds. 



7.3. Differences between the Kinetics of Oxygen 

 and Carbon Monoxide Reactions 



7.3.1. Effect of pH. We have referred in Section 6.2.5. to the unexpected 

 insensitivity of the replacement reactions to changes in pH. Since the 



