132 BIG MOLECULES 



investigation. The X-ray diffraction pattern of even single crystals was too 

 formidable for analysis until M. F. Perutz, about 1950, began to substitute 

 heavy metals ions such as Hg +2 at particular spots on the molecule and to 

 analyze the effects of these strong X-ray scatterers on the spectrum. With 

 this technique, now known as the "method of isomorphous replacement," 

 it was possible by 1960 to show the surprising result that the protein of the 

 molecule at 6 angstroms' resolution looks like several intertwined worms, 

 with the heme groups attached — not a regular array at all. Studies con- 

 tinue on the amino acid sequence, and on the analysis of the X-ray diffrac- 

 tion pattern, in an effort to get even better resolution of the detailed struc- 

 ture of the hemoglobin molecule. 



Inherently simpler, myoglobin (one Fe +2 ion only) has yielded not only 

 to 6 A analysis (1956) but even to 1.5 A resolution (1958), work for which 

 Kendrew and his team received a Nobel Prize in 1961. The main features of 

 this molecule are depicted in the drawing shown in Figure 6-3. The a-helix 



(flat) heme group 



CH 2 



< „ p 



CH,-/VV\ 



C-N N-C 



HC (Fe) CH 







X C-N N=C 



/ I I \ 



HC-C C C C- 



CH ? X C C C 



/ H | 



CH 3 CH 3 



protein (ferrous ion) 



seqments 



R= -CH 2 CH 2 0H 



I 1 1 



30 A 



Figure 6-3. Molecule of Myoglobin. (Drawn from the Model of Kendrew, 1958.) 



hydrogen-bon(d)ed, forms the framework of the worm-like segments, sudden 

 turns in which are thought to be associated with the proline groups — an 

 amino acid residue of odd structural configuration. The heme group sits ex- 

 posed, with the iron ion ready for oxidation or reduction, or, preferably, 

 simply complexing with 2 picked up from air. 



Although this is the configuration of crystalline myoglobin, the shape of the 



