128 



PROTEINS 



more such data before the amino acid content of the diet can be cal- 

 culated with any assurance of being correct. It is still not certain 

 that the ratio of an individual amino acid to the total nitrogen in a food 

 remains constant from sample to sample. In a standard product such 

 as milk the variation is probably small, but in a food such as white 

 potatoes large fluctuations are to be expected. 



In recent work Brand has obtained a yield of 116.3 per cent for /?-lacto- 

 giobulin and a recovery of 100 per cent of the nitrogen. Since all the 

 nitrogen, and also all the surfur, in the protein was accounted for in 

 the percentages of amino acids, it was possible to calculate the number 

 of molecules (actually residues equivalent to the molecular weights minus 

 the molecular weight of water) of each amino acid in the protein. For 

 example, the number of alanine residues is calculated as follows: 



The alanine content of (3-lactoglobulin is 6.2 per cent (Table 5-4) and the molecular 

 weight is 41,500. 



6.2% alanine (m.w. 89) =4.95% alanine residue (m.w. 71). 

 .0495 X 41,500 = 2054 g. alanine residues per gram molecular weight of^-lactoglobulin. 

 2054 -^ 71 =: 28.9, or in round numbers 29 residues. 



The calculations gave a total for all the amino acids of 370 residues. 

 Adding together the molecular weights of all the residues, a total of 

 42,020 was obtained, which is in good agreement with that obtained by 

 ultracentrifuge measurements (41,500). From these data Brand pro- 

 posed an amino acid formula for y8-lactoglobulin as follows: 



Gly8Ala29Val2iLeu5oIleu27Proi5Phe9CySH4(CyS— )8Met9Try4Arg7His4Lys33 



Asp36Glu24 ( GluNHs ) saSeraoThroiTyrg ( HoO) 4 



In this formula the amino acids are shown by the first three letters 

 of the name, and the number of residues by the figure following the 

 abbreviation. (CyS — )8 means that 8 half molecules or 4 whole mole- 

 cules of cystine are present. Glutamic acid residues with the second 

 carboxyl group neutralized as an amide, i.e., glutamine, are represented 

 as (GluNH2)32- The four molecules of water come from the H and 

 OH of terminal NH^ — and COOH groups, respectively, in polypeptide 

 chains. ^-Lactoglobulin is believed to contain 4 polypeptide chains per 

 molecule. 



In a similar manner the number of amino acid residues present can 

 be calculated for the proteins listed in Table 5-4, for which the data are 

 sufficiently complete. 



Linkage of amino acids to form protein 



As indicated below in the formulas, the amino acids contain at least 

 one amino group and one carboxyl group. The amino group gives basic 



