GLOBIN AND HEMOGLOBIN AS PROTEINS 245 



The interpretation of the diflference between the base bound to globin and 

 to hemoglobin is difficult, since {cf. Section 4.) the coupling of hematin to 

 globin also involves the aggregation of two globin units, of molecular weight 

 34,000, into the hemoglobin molecule of molecular weight 68,000. 



The minimum difference between the hem/globiri and the globin was two 

 equivalents per Hlifner unit (molecular weight 17,000) at pH 11.33. The 

 maximum difference was '2.67 equivalents at pH 9.5, falling to 2.2 equivalents 

 at pM 7.7 and rising again to 2.6 equivalents at pH 5.5. Theorell did not 

 attempt to interpret his results between pH 5.5 and pH 8. Between pH 8 

 and pH 11.3 he accounted for the difference in base bound between hem/- 

 globin and globin by the presence in hem/globin of two fully ionized carboxyls 

 and the ionization of the iron taking place with a />K value of 8.5 at 0° C, 

 while in the globin he found evidence for the presence of a group which 

 dissociated with a pK value of 10 at 0° C. and which could not be detected 

 in the hemoglobin. 



In view of the difficulty in interpreting the pK value of 10 as well as the 

 results obtained between pH 5.5 and 8, the results cannot be said to have 

 provided unequivocal evidence for the presence of basic groups in the protein 

 which form bonds with the hematin carboxyls. 



3.4. Other Oxygen-Carrying Pigments 



In the discussion of the problem of the linkage of prosthetic group to 

 protein in hemoglobin it has been necessary to ignore the differences which 

 exist between the globins of different species (Chapter VII). It would appear, 

 however, that the general conclusions are applicable to most mammalian 

 hemoglobins. There is evidence {cf. Sections 2.2.5.. 6.2.8., and 7.2.) that the 

 linkage between hematin and globin in myohemoglobin is different from 

 that in hemoglobin in that the hematin iron appears to be attached to only 

 one histidine imidazole. The invertebrate oxygen carriers (Chapter VII) 

 have not been investigated in great detail but from the presence of the 

 sigmoid dissociation curve (Section 5.1.9.) it may be concluded that "heme- 

 linked" groups are present and that the heme may lie between two histidine 

 imidazoles as in the mammalian hemoglobins. 



4. GLOBIN AND HEMOGLOBIN AS PROTEINS 

 4.1. Molecular Weight 



4.1.1. Hemoglobin. The iron content of hemoglobin established 

 the equivalent weight on the basis of the iron atom as 16,700. This, 

 however, provided only a minimum estimate of the size of the mole- 

 cule. The early measurements of the osmotic pressure gave results 

 in which the number of subunits of equivalent weight 16,700 in a 

 molecule of hemoglobin varied. 



In 1924 Adair (3,4) first succeeded in obtaining reproducible values 



