MECHANISMS OF GENE ACTION 289 



ties, and thereby the positions of the atoms. Because in hemoglobin the 

 unit cell is some 40 atoms thick, the final two-dimensional picture which 

 emerges from this analysis is too complicated to resolve in terms of a 

 three-dimensional picture. Recently a new method, called isomorphous 

 replacement, has been devised by Perutz at the Cavendish Laboratory 

 in Cambridge, England, to meet this problem. The method compares 

 the X-ray pattern of a crystalline protein with that obtained from the 

 same protein after attachment to a specific site of a heavy atom, like gold 

 or mercury, which shifts the pattern. By comparing diffraction patterns 

 before and after replacement, certain data can be obtained which greatly 

 simplify the analysis. 



With this method, a group of investigators have been studying the 

 a- and /3-chains of hemoglobin and of the related protein myoglobin, 

 which is an oxygen carrier in muscle and consists of just one polypeptide 

 chain. Perutz and co-workers have established the tertiary configura- 

 tions of the a- and (8-chains of hemoglobin, based upon three-dimen- 

 sional Fourier syntheses obtained by X-ray analysis at the 5.5 A level of 

 resolution. This magnificent feat of reconstruction of the hemoglobin 

 molecule may be visualized by means of the models shown in Figure 

 10.8. The similarity in over-all structure of the a- and the /3-chains is 

 apparent from the models, and is particularly surprising since their 

 amino acid compositions differ considerably, and the structure of the two 

 chains is affected by different genetic loci (Table 10.2). 



Kendrew and co-workers have established, by similar methods, the 

 three-dimensional configuration of myoglobin at the 2.2 A level of resolu- 

 tion. The myoglobins from different species have amino acid composi- 

 tions which differ to some extent, sufficiently in sperm-whale and seal 

 myoglobins to determine differences between them in crystal lattice. 

 Nonetheless, the two-dimensional X-ray diffraction patterns of these two 

 proteins are essentially identical. Also, the three-dimensional configura- 

 tions of myoglobin and of the hemoglobin polypeptides are very similar. 

 These are just a few points of special relevance to genetics which have 

 already emerged from this remarkable series of investigations. 



In considering the relation between amino acid composition and ter- 

 tiary structure, one line of interpretation proposes that certain amino 

 acids may be critical for determination of the folded configuration, such 

 as proline for bends of the a-helix, and cysteine for the position of the 



Bottom. The entire hemoglobin molecule consisting of two each of the a- end the 

 /3-choins assembled. Two heme groups can be seen lying in separate pockets on the 

 surface of the molecule. O2 indicates the face of the heme to which oxygen attaches. 

 HS marks the position of one of the sulfhydryl groups. N denotes the N-terminal end 

 of one of the polypeptide chains. 



