IX. GENETICS AND HUMAN HEMOGLOBIN CHEMISTRY 427 



(Hill et al.y 1960; Baglioni and Ingram, 1961; Swenson et al., 1962) and 

 for Hb-0 (Baglioni and Lehmann, 1962). Three types of Hb-G cariying 

 different peptide alterations have been described and possibly several 

 other types may exist among the several Hb-G's reported in the litera- 

 ture. Two types of Hb-0 characterized by different alterations have 

 been described (Baglioni and Lehmann, 1962). The heterogeneity of 

 hemoglobin variants, which are indistinguishable on the basis of their 

 physicochemical behavior, seems to be a rather general phenomenon. 

 It is quite likely that several other examples of heterogeneity W\\\ be 

 found in the near future. 



All the abnormal hemoglobins examined have shown a single amino 

 acid difference with Hb-A. Any single amino acid substitution can only 

 produce one of four different types of electrophoretically altered pro- 

 teins. If we arbitrarily assign the charge to the normal protein, the 

 substitution of a neutral amino acid by an acidic or a basic amino acid, 

 as well as the substitution of a basic or acidic amino acid by a neutral 

 amino acid, will result in a charge alteration of — 1 or -\-\, respectively. 

 The substitution of a basic amino acid by an acidic or of an acidic by 

 a basic will result in charge alteration of —2 or -|-2, respectively. The 

 charge difference is expressed in arbitrary units and should be multiplied 

 by 2 when referred to hemoglobin, since two altered peptide chains are 

 present in the molecule. 



We know many more than four electrophoretically different abnormal 

 hemoglobins, but the abnormal hemoglobins can be roughly grouped in 

 five classes on the basis of their electrophoretic mobility (see Table II). 

 A small electrophoretic difference exists at pH 8.6 between Hb-OArabia, 

 Hb-C, and Hb-E, in which a lysine substitutes for a glutamic acid. These 

 hemoglobins should have the same electrophoretic charge if the charge 

 difference with Hb-A results only from the type of amino acid substitu- 

 tion. The small charge differences between these hemoglobins are prob- 

 ably due to different contributions of the residues to the ionic charge of 

 the protein. The charge contribution is presumably dependent on the 

 place occupied by a residue in the spatial configuration of the protein. 

 The charge of ionizable groups may be modified by interaction with 

 neighboring residues. 



There seems to be no correlation between the type of amino acid 

 involved in substitutions, apart from its charge, and the change in 

 electrophoretic mobility. The Hb-G's, for instance, present different 

 substitutions: lysine for glutamic in Hb-Gsan .lose (Hill et al., 1960), 

 lysine for asparagine in Hb-Gphnadeiphia (Baglioni and Ingram, 1961), 

 and glutamine for glutamic in Hb-Gnonouiiu (Swenson et al., 1962). 



The electrophoretic behavior of the abnormal hemoglobins is dis- 



