430 CORRADO BAGLIONI 



appear to \)v iinoh-i'd iiu)iv than oner by an amino acid rei^lacemcnt: 

 this is observed for residues 6, 63, and 121 of the /? chain (see Table II 

 for references). This does not necessarily imply that mutations occur 

 more frequently in given regions of the hemoglobin genes {hot spots). 

 ^lutations inohahly occur in any position along the genes, with the 

 consequent amino acid substitutions at various positions. However, 

 abnormal hemoglobins cariying amino acid substitutions are probably 

 synthesized at a normal rate only if the altered molecules are able to 

 fold in the proper way. Therefore, we would not expect to observe substi- 

 tutions involving certain amino acids because the corresponding muta- 

 tions are particularly rare, but more likely because these mutations 

 result in no production of hemoglobin. Higher levels of organization, the 

 secondary and tcrtiaiy structures, which are dependent upon the primaiy 

 structure, are in fact necessary for a protein to be stabilized. 



2. The Consequences of Amino Acid Substitutions on the Secondary and 

 Tertiary Structures of Hemoglobin 



The secondary structure refers to the geometrical arrangement of the 

 amino acids of a pejitide chain in helical and non-helical regions; the 

 tertiary structure refers to the final folded configuration of a peptide 

 chain. We know very little about the alterations of the secondary and 

 tertiary structures in abnormal hemoglobins. Watson and Kendrew 

 (1961) have recently suggested that most of the residues involved in the 

 amino acid substitutions of the abnormal hemoglobins stick outward 

 from the three-dimensional configuration of the hemoglobin chains, so 

 that these substitutions would not be expected to affect the tertiar}' 

 structure considerably. 



AVatson and Kendrew (1961) have carefully compared the amino acid 

 sequence of the sperm whale myoglobin with that of the human hemo- 

 globin chains. The tertiary structure of the sperm whale myoglol)in has 

 been resolved by X-ray ciystallography to a very high degree of pre- 

 cision by Kendrew and his collaborators (Kendrew et al., 1961); of the 

 1200 atoms contained in the myoglobin molecule, excluding the hydro- 

 gens, some 75% have been localized without ambiguity in a three- 

 dimensional model of this protein. Perutz et al. (1960) have shown by 

 X-ray crystallograjihy that the tertiary structure of the a and p chains 

 of horse hemoglobin closely resembles that of the spenn whale myoglobin. 

 Watson and Kendrew (1961) also have assumed that the secondary and 

 tertiary structures of the human hemoglobin chains closely resemble 

 that of the spenn whale myoglobin and have ideally aligned the hemo- 

 globin peptide chains along the model of the sperm whale myoglobin. 



By this comparison most of the residues involved in substitutions in 



