Recent Developments in the X-Ray Study 

 of Haemoglobin 



M. F. PERUTZ 



The paper reviews the x-ray analysis of horse methaemoglobin, 

 especially the results of the recently completed three-dimensional 

 Patterson synthesis. A brief description of the principal features of 

 that synthesis is followed by an account of its interpretation. An 

 arrangement of the polypeptide chains compatible with all the 

 chemical and x-ray data is suggested and the possible positions of 

 the haem groups are discussed. The paper concludes with a brief 

 account of comparative x-ray studies of different haemoglobins, 

 discussing possible correlations between structural data and 

 physico-chemical properties. 



HORSE METHAEMOGLOBIN 



Introduction — Most x-ray crystallographers would agree that the 

 work involved in the analysis of a crystal structure increases as an 

 exponential function of the structure's complexity. Due largely to 

 the absence of any direct method for obtaining the atomic positions 

 from the observed intensities of the diffracted rays, a detailed analysis 

 of an organic compound of comparatively moderate size, such as 

 sucrose or cholesterol, takes two or more man-years to complete. 

 On the face of it, therefore, an attempt to analyze the crystal structure 

 of haemoglobin, or of any crystalline protein for that matter, looks 

 about as promising as a journey to the moon. Indeed, had it not 

 been for an improbably large share of good luck, which confuted all 

 the more sober assessments of my chances, I should not know more 

 about the structure of haemoglobin now than when I started. Fortu- 

 nately, the first crystalline protein which came my way was methaemo- 

 globin of horse which has since proved to have the simplest crystal 

 structure of any protein of comparable molecular weight, with features 

 so favourable that they make a crystallographer's heart leap with joy. 



The crystals are easily grown in the right habit for x-ray analysis 

 and can be kept in salt solutions for long periods. The molecules 

 occupy known positions in the unit cell, each of them being placed 

 on a diad axis of symmetry which requires each molecule to consist 

 of two identical halves. As all the diad axes in the crystal point the 

 same way, the molecules must do the same. I soon found that the 

 crystals consist of layers of haemoglobin molecules alternating with 

 layers of liquid ; it was this factor, combined with certain other useful 



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