THE BIOCHEMISTRY OF PLANT VIRUSES 45 



— NH— CH— CO— NH— CH— CO— NH— CH— CO— NH— CH— COO— 



I i I I 



R R Iv R 



It will be noted from this that, in general, one end of a polypeptide chain 

 has a free amino group, while the other has a carboxyl group, so that the 

 molecule has both acidic and basic properties, irrespective of the nature of 

 the R-groups, and that the latter are to one side of the chain proper. Thus 

 the peptide linkages are primarily structural, while it is largely the nature 

 and arrangement of the side chains which give the peptide chains and the 

 proteins their distinctive properties. 



The proteins themselves consist of vast polypeptide chains, not arranged 

 in a linear array as depicted above, but probably intricately coiled and folded 

 and, in the case of the virus proteins, assembled into compact lumps con- 

 taining perhaps as many as 200 or more amino acid residues. Each virus, 

 of course, is made up from many of such lumps or sub units, the total number of 

 amino acid residues in a virus like tobacco mosaic virus being of the order 

 of half a million. 



The present view of the way in which the polypeptide chains are folded is 

 that proposed by Pauling and Corey (1952), and known as the a-helix. This 

 structure has one or two features of special interest. The amino acid residues 

 are arranged in a tight helix with 3.7 residues per turn, resulting in a fairly 

 compact central region, consisting mainly of the polypeptide backbone, while 

 the side chains are arranged on the outside of the helix, and come off at an 

 angle to the main axis. This has the effect of making the relative positions 

 of the ends of the side chains quite different from the relative positions of 

 their a-carbon atoms. This makes the determination of the surface structure 

 of a polypeptide a matter of extreme dijBiculty, even if the amino acid 

 sequence is perfectly well known, and as it is almost certainly the surface 

 structure with its intricate arrangement of various hydrophyhc, hydrophobic, 

 and reactive groupings which decides the biological activity of the protein, 

 it will be realized that the problems which lie ahead are extremely complex. 



A structural feature, which may explain the presence of the one imino acid 

 (proline) in polypeptide chains, is that its structure causes an interruption 

 in the sequence of peptide linkages, and so allows a kinkmg or folding to 

 take place between a-helix sections. Cross linkages can be made between 

 adjacent helices by the — CHg— S — S — CHg — bridges of cystine, thus stabi- 

 lizing the final structure, and several other cross links are possible. 



B. The Determination of the Composition of Proteins 



Before the advent of clrromatographic methods for the detection and 

 estimation of amino acids, the problems involved in the analysis of viruses 



