THE STRUCTURE OF GLOBULAR PROTEINS 7 



group, in other words of the tertiary structure of the molecule. The 

 tertiary structure is highly irregular and complex, in sharp contradiction 

 to the simplicity of the secondary structure. More recently [3] the resolu- 

 tion of the analysis has been increased to 2 A (Fig. 2). Although neigh- 

 bouring covalentlv bonded atoms are still not resolved, it is now possible 

 to separate atoms which are hydrogen bonded or in Van der Waals contact, 

 with the result that the atomic arrangement of most of the molecule can 



Fig. 2. Model of myoglobin based on the three-dimensional Fourier syn- 

 thesis at a resolution of 2 A. The model is seen from the same point of view as that 

 of Fig. I. The course of the main-chain is indicated by a white cord: side-chains 

 have been inserted wherever possible. 



be inferred. It turns out that all the straight regions of polypeptide chain 

 are in the a-helical configuration ; in fact the molecule consists of eight seg- 

 ments of a-helix joined by irregular regions of varying length ; the helical 

 segments comprise 75",, of the amino acid residues, in agreement with 

 estimates made on the basis of optical rotation and deuterium exchange 

 studies. The appearance of the haem group corresponds closely with 

 theoretical expectation, and it can be seen that the iron atom is attached 

 to a neighbouring a-helix bv means of a group which is almost certainly 

 the imidazole ring of a histidine residue. 



