The Crystal Structure of Horse Myoglobin 



J. C. KENDREW 



The preliminary results of an x-ray study of horse myoglobin are 

 described. They suggest the hypothesis that the myoglobin molecule 

 is similar in shape and dimensions to one of the four layers in the 

 molecule of horse haemoglobin. That is to say, it consists of a disk 

 about 9 A thick and with other dimensions not greater than 57 A ; 

 the most probable values are 51-5 by 37 A. In the crystal there are 

 two of these disks in each unit cell, parallel to one another and 

 separated by a layer of liquid of crystallization 6-6 A thick. The 

 molecule consists of a single layer of parallel polypeptide chains 

 about 9-5 A apart, having a repeat distance of about 5 A along 

 the chains ; this arrangement is similar to that found in haemoglobin 

 and apparently to that in c/.-keratin. The haem group, which is about 

 1 5 A across, lies perpendicular to the plane of the disk and approxi- 

 mately perpendicular to the polypeptide chains. The arrangement 

 is illustrated diagrammatically in Figure 4. 



The preceding paper describes the results of a prolonged and intensive 

 study of horse haemoglobin. The present one gives an account of a 

 much more superficial examination of horse metmyoglobin* ; more 

 detailed study has so far been prevented by technical difficulties, which 

 are mentioned below and which will take time to overcome. For- 

 tunately the structure of the myoglobin crystals is simpler than that 

 of haemoglobin crystals, and with the aid of the interpretations already 

 put upon the haemoglobin data it is possible to make a number of 

 interesting deductions even from the preliminary results. For further 

 details of the x-ray crystallographic principles used in this research, 

 the reader is referred to the succeeding paper (p. 161). 



I have felt for some time that myoglobin would be a subject of par- 

 ticular interest for x-ray study. There are several reasons for this view. 

 First, its molecular weight is only about 17,000^ 2 which is a very low 

 value by protein standards, so that its structure may be expected to be 

 correspondingly simple. Second, there is a close relationship, in function 

 and in properties, between myoglobin and haemoglobin, and we may 

 anticipate a similarly close relationship between the structures of the 

 two proteins. Third, myoglobin is being closely studied from the 

 chemical point of view by other workers (see the contributions by 

 G. R. Tristram, p. 109, and by R. R. Porter and F. Sanger, p. 121). 



* In the interests of brevity, the term ' myoglobin ' will be used throughout this paper 

 to denote horse metmyoglobin, and ' haemoglobin ' to denote horse methaemoglobin. 



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