112 GENERAL BIOCHEMISTRY 



Two or more such chains may be held together with — S — S — bonds 

 of cystine, with each half of the cystine being in a different peptide 

 chain. Salt formation between ionic groups of different chains and 



hydrogen bonds involving \ NH and 0=^C / probably also help 



hold peptide chains together in the protein molecule. 



Under physiological conditions, insulin appears to have four sep- 

 arate peptide chains, two each of two kinds. This particular protein 

 dissociates into equal halves when the environment is altered. More 

 drastic treatment breaks apart the individual peptide chains. Although 

 the available information of this type is rapidly increasing, little is 

 known about the numbers of peptide chains in most proteins. 



Even less is known about the sequences of the amino acids in 

 proteins. Only in the insulins of five species have the arrangements 

 been completely worked out. Insulins from cattle, swine, sheep, horses, 

 and whales have been studied. All are alike, with respect to one chain. 

 All differ in the other type of peptide chain with the differences occur- 

 ring at three adjacent sites along the sequence. The amino acids inter- 

 changed among these positions are alanine, serine, valine, threonine, 

 isoleucine, and glycine. Insulin is the first protein for which the species 

 differences have been chemically located with exactness. 



The problem of the amino acid composition of proteins has been 

 studied for many decades. Yet even the best available results are never 

 in complete agreement. Basically the problem is one of determining the 

 number of amino acids of each kind present in the protein molecule. 

 For proteins exceeding 10^ in molecular weight, some of the amino 

 acids are so abundant that modern assay methods cannot yield the 

 number of these correct to the nearest whole number. With the less 

 common amino acids, the problem of accuracy is not so severe and may 

 not be a factor. However, most determinations require that the amino 

 acid be in free form. Hence the proteins must be hydrolyzed, but un- 

 fortunately some amino acids are not completely stable during hy- 

 drolysis. Even worse, certain amino acids are less stable in the form of 

 peptides than they are in the free state, and controlled degradation 

 studies become inaccurate. Thus losses arc imcertain, and final esti- 

 mates of composition rcflec l variations in methodology. Insulin was 

 perhaps the most intensively analyzed of all proteins, but each labora- 

 tory reported somewhat different compositions. None agreed exactly 

 with the results obtained from the presumably correct studies of the 

 amino acid sequences in the peptide chains. 



Many kinds of protein molecules are believed to contain nothing 

 but amino acids in combined form with perhaps water of hydration 



