248 VI. HEMOGLOBIN 



In a personal communication to Taylor (cited in 2747) Svedberg suggests 

 that one cause of the discrepancy between the results of Theorell and Poison 

 was a species difference, the latter worker using myohemoglobin from cow 

 heart muscle. 



4.2. Shape of the Hemoglobin Molecule and Arrangement 



of the Hemes 



4.2.1. Shape »f the Hemoglobin Molecule. If the molecular 

 weight of hemoglobin is determined from measurement of osmotic 

 pressure or from measurement of the sedimentation equilibrium, the 

 results may be used in conjunction with measurements of the sedi- 

 mentation velocity or the diffusion constant to gain information as 

 to the shape of the molecule. The sedimentation velocity or the diffu- 

 sion constant, found by experiment, is compared with that calculated 

 for a spherical molecule of the correct molecular weight. The differ- 

 ence between the predicted and found value has been interpreted in 

 terms of the asymmetry of the molecule, and it has been concluded 

 that hemoglobin may be an ellipsoidal molecule whose major axis may 

 be three or four times as great as the minor axis (530,1622,204-4,3167, 

 2721). This result has resently become suspect. The value for the 

 partial specific volume used by Svedberg has been criticized by Adair 

 and Adair (9), who concluded that, if allowance is made for hydration, 

 the molecular volume in solution becomes some 46% greater than 

 that calculated from the dry protein. In consequence the molecule 

 becomes somewhat less asymmetric than was previously assumed. 



By x-ray analysis of single crystals of hemoglobin and oxyhemo- 

 globin, Perutz {325,2133,2135,2136) has produced data which go far 

 to clear up the gross shape. He investigated wet and dry crystals 

 and concluded that the molecule was 64 A long, 48 A wide and 36 A 

 thick. [PerUtz recently, in a personal communication, revised his 

 conclusions and now considers the molecule to be a circular disc, 57 A 

 diameter, 34 A thick (c/. 324a)]. The molecule is symmetrical about 

 a diad axis which is parallel to the h axis of the crystal. A similar 

 structure was found for both oxyhemoglobin and hemiglobin. The 

 protein molecules in the crystal form rigid and coherent layers which 

 alternate with layers of liquid of crystallization. The actual mole- 

 cules of hem/globin appear to be impenetrable to liquid as their 

 structure is not affected by shrinkage of the crystal. Each hemiglobin 

 molecule seems to be made up of four layers which are spaced 9 A 

 apart; these intramolecular layers are parallel to the larger molecular 



