M. F. PERUTZ 



to the conclusion that the fundamental stereo-chemical configuration 

 of the polypeptide chains in haemoglobin and myoglobin, and in the 

 large group of proteins of the a-keratin type, are the same. 



The position of the haem groups is one of the most vital structural 

 problems in the x-ray analysis of haemoglobin, vital not only for the 

 understanding of the reaction mechanism between haemoglobin and 

 oxygen, but also because it affects the much wider problem of chemical 

 interaction between prosthetic groups across the framework of protein 

 molecules. The scattering contribution of the haems relative to that 

 of the globin is very small, and at the present stage they cannot be 

 located by direct x-ray analysis. On the other hand, we now have a 

 variety of crystallographic and other data which serve at least to 

 narrow down their possible positions. I have described in earlier 

 papers (Perutz 5 and /) how the orientation of the haem groups can 

 be deduced from the pleochroic absorption of haemoglobin crystals. 

 These observations showed that the four haem groups in the haemo- 

 globin molecule must be approximately parallel to each other, with 

 their flat sides normal to the X axis. In the light of the evidence just 

 described this would mean that their flat sides would also be approxi- 

 mately normal to the length of the polypeptide chains. 



There is also certain evidence in favour of the haem groups lying 

 on the surface of the haemoglobin molecules 3 . It was mentioned 

 before that the haemoglobin molecules are rigid and impenetrable to 

 liquid. On the other hand, reactions between the haem groups and 

 diffusing ions take place inside the haemoglobin crystals, just as though 

 the haemoglobin molecules were in solution, and without causing any 

 change in unit cell dimensions. For instance, crystals of acid methae- 

 moglobin suspended in ammonium sulphate solutions can be trans- 

 formed into alkaline methaernoglobin by adding diammonium phosphate 

 to the suspension medium ; on addition of sodium azide, crystals of 

 acid methaernoglobin are transformed into the addition compound 

 azide methaernoglobin. Such reactions can be performed in a drop 

 of liquid on a slide and watched with a microspectroscope. Now if 

 it is true that the haemoglobin molecules are impenetrable to liquid 

 and if reaction between the haem group and the liquid of crystallization 

 can take place nevertheless, the haem groups must obviously be located 

 on the surface of the globin portion of the molecule. 



As regards their positions on the surface of the globin molecule the 

 following considerations are relevant. (7) A diad axis of symmetry 

 passes through the centre of the molecule, which means that a rotation 

 by 180° about that axis must bring one half of the molecule into 

 congruence with the other. (2) J. Wyman, Jr. (see p. 95) shows that 

 when the haemoglobin molecules split into halves in urea solution, 



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