J. C. KENDREW 



others, if we accept a close similarity between the myoglobin 

 molecule and a single layer of haemoglobin, since in the latter 

 there is evidence that the haem groups are on the outer surface 

 of the molecule. 



3 The detailed structure of the polypeptide chains is unknown, as is 

 the relation between them (but see below). 



4 The two molecules in the cell may be ' staggered ' as shown in 

 Figure 3d, instead of being directly over one another — in fact 

 other evidence (not mentioned here) makes a staggered arrange- 

 ment in the wet crystal rather more likely. Also the two molecules 

 may not be strictly parallel, though they must be nearly so. 



Throughout I have stressed that there seems to be a close relation- 

 ship between the myoglobin molecule and a single layer in horse haemo- 

 globin. This relationship includes the overall dimensions of the disk, 

 and also the arrangement of haem groups and polypeptide chains — 

 and in the latter respect there is also a close similarity to a-keratin. 

 Furthermore, the b projection of myoglobin when superimposed on 

 the zero-level b Patterson section of haemoglobin (so that rod corre- 

 sponds with rod) shows some surprising coincidences of peaks which, 

 while they cannot as yet be interpreted, can hardly be accidental. 



Finally I wish to indicate how the crystallographic data may be 

 correlated with two other and quite distinct lines of evidence. 



First, R. R. Porter and F. Sanger have shown by the end-group 

 method that the horse myoglobin molecule contains only a single free 

 a-amino group ( 5 ; also see this volume, p. 122). If we provisionally 

 exclude the possibility of closed polypeptide rings, this means that the 

 molecule consists of a single polypeptide chain. To accommodate it 

 within our model, the chain must be folded in a zigzag somewhat as 

 in Figure 6. We may go so far as to calculate the dimensions of such a 

 model, assuming that the myoglobin molecule contains in all about 

 146 amino-acid residues (see p. Ill), and using the a-keratin model 

 with 3 amino-acid residues per unit backbone repeat of 5-1 A and 

 chains 9-3 A apart. This gives a total chain length of 248 A. In the 

 dry crystal it may be supposed that adjoining molecules are in fairly 

 close contact with one another so that the a and c dimensions should 

 approximate to those of the molecule ; the values found are 51-5 and 

 37-0 A (see Table I). The c dimension is roughly perpendicular to the 

 chains, and would exactly accommodate four of them (4 x 9-3 A 

 = 37-2 A). Figure 6 shows an arrangement of the chain in 4 lengths 

 (assuming all the loops of equal length, a scheme which gives the most 

 compact molecule) ; it is easy to calculate that a molecule of 146 

 amino-acids arranged in this way would have an overall length of 

 about 60 A. (Similar arrangements containing closed loops of poly- 



158 



