Crystal Structure of Horse Myoglobin 



peptide chain would have about the same dimensions.) This is 8-5 A 

 longer than a so the agreement, though reasonable, is not exact, but 

 there are not at present enough data to enable us to discriminate 

 between various arrangements which would account for this discrepancy. 

 At least it seems reasonable to suppose that the molecule consists in 

 its main part of four parallel lengths of polypeptide chain. 



Figure 6. Diagram of a single polypeptide chain 

 folded into four parallel segments. 



Second, there should be an agreement between the molecular dimen- 

 sions postulated above and the asymmetry ratio deduced from properties 

 in solution such as sedimentation rate, viscosity, and dielectric constant. 

 Clearly our model demands a high degree of asymmetry, with an axial 

 ratio of the order of 4:1, though this would be reduced somewhat if 

 we may assume that in solution the molecule is covered by a layer of 

 tightly-bound water, and particularly if this water is adsorbed prefer- 

 entially on the flat faces of the disk (as would happen if the structure 

 follows Astbury's model for a-keratin, so that the polar, hydrophilic, 

 side chains stick up and down, perpendicular to the plane of the disk). 

 Thus the model requires a degree of asymmetry greater than that 

 postulated for most other globular proteins. Unfortunately the 

 properties in solution referred to above do not give an unequivocal 

 answer. The question has been discussed by J. Wyman 6 ' 7 who suggests, 

 on the basis of all the available data, that if the equivalent ellipsoid is 

 an oblate one (as seems likely), its asymmetry ratio is 3-6:1. The 

 sedimentation constant alone would suggest a considerably lower figure 

 but, especially in the absence of data about the hydration of myoglobin, 

 the question must be regarded as still unsettled in spite of the very 

 satisfactory agreement between the compromise figure given by Wyman 

 and the x-ray data. We cannot even exclude the possibility of a reversible 

 change of configuration during the transition from crystal to solution. 

 The x-ray crystallographer would fervently hope that this does not turn 

 out to be the truth, since it would mean that his studies of protein 

 crystals are of less direct interest to the biologist, who is mainly con- 

 cerned with the state of the protein molecule in solution. 



Received September 1948 



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