Crystal Structure of Horse Myoglobin 



Myoglobin, then, appears to consist of an approximately circular 

 disk whose diameter (deduced from the dimensions of the wet unit cell) 

 is not more than about 57 A, and whose thickness is about 9 A. Each 

 unit cell contains two of these disks arranged parallel to one another 

 and perpendicular to the b axis, with their centre lines 15-4 A apart in 

 the wet condition. The molecules are separated by about 6-6 A of 

 liquid of crystallization. (There is no direct evidence from the myo- 

 globin crystal about the thickness of the molecule — the figure given 

 is by analogy with a single layer of haemoglobin — though the value 

 of b in the dry cell sets an upper limit of 14 A.) 



Each molecule consists of a single layer of parallel polypeptide 

 chains, about 9-5 A apart and having repeats of about 5 A along the 

 chains. As in haemoglobin, we may note a close analogy to the structure 

 of a-keratin, in which the chains are separated by a distance of 9-3 A 

 and which gives a ' backbone spacing ' of 5-1 A. 



The haem group lies perpendicular to the plane of the disk and also 

 approximately perpendicular to the polypeptide chains. Since a haem 

 group is about 15 A across and we have postulated a disk thickness of 

 only 9 A, the haem must stick out on one or both sides of the disk. 



Figure 5. Diagrammatic sketch 



of a possible arrangement of 



myoglobin molecules in the unit 



cell. 



Figure 5 is an attempt to illustrate this scheme diagrammatically ; 

 it is in fact identical with Figure 3b with the addition of polypeptide 

 chains and haem groups. In drawing such a scheme, however, it is 

 difficult to avoid being more concrete than the evidence warrants, and 

 the following provisos must be made : 



1 The actual shape of the disk is only very approximate. 



2 The two shaded rectangles show only the orientations of the haem 

 groups and not their actual positions, though the positions 

 illustrated may perhaps be regarded as more plausible than any 



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