86 - The Cell 



H— N— C— C— OH + H 



I I II 

 H H O 



-N — C—C- 



I ■ I II 

 H H O 



OH +H -N-C-C- 



I I II 

 H H O 



peptide bonds - = -_-_- - 



dehydration 

 synthesis 



hydrolysis 



OH + H — N— C— COOH 



I I 

 H H 



R, 



R, 



?• 



H-N-C-C-N — C— C— N— C— Ci-N— C-C— OH + 3H 2 



I I II I I II I I II I I II 

 HHOH HOHHOHHO 



Fig. 4-15. Construction of a tetrapeptide chain from four separate amino acids. Note that the formation of 

 each peptide bond involves the splitting off of a water molecule. Consequently the synthesis is a dehydration 

 synthesis. In a protein molecule the peptide chain is much longer, often consisting of more than a thousand 

 amino acid units. 



change in that cell. In the case of sickle cell 

 anemia, the reel protein pigment, hemo- 

 globin, present in the erythrocytes (p. 289), 

 differs from the normal by just one amino 

 acid in the peptide chain. Nevertheless, the 

 sickle-shaped red cells of an individual suffer- 

 ing from this rare hereditary defect are 

 grossly abnormal as to both form and func- 

 tion (p. 507). 



The problem of cell replication hinges, in 

 large measure, upon the mechanisms of pro- 

 tein replication. How does the cell transmit 

 the code that determines precisely the proper 

 amino acids and how the}' become bonded 

 together in precisely the right order and ar- 

 rangement, when protein synthesis takes 

 place? This problem involves an analysis of 

 the DNA components of the nucleus and of 



W 



NH, 



NH 2 



NH, 



II I I I I 



Gly— lieu— Val— Glu-Glu— Cyst— Cyst— Ala— Ser— Val— Cyst -Ser — Leu— Tyr— Glu— Leu-Glu— Asp— Tyr— Cyst— Asp 



Phe-Val-Asp— Glu— His-Leu-Cyst-Gly-Ser-His- Leu- Val- Glu-Ala- Leu-Tyr-Leu- Val-Cyst-Gly 



(b> 



I 

 Glu 



I 

 Arg 



I 

 Ala- Lys— Pro— Thr-Tyr- phe — Phe— Gly 



Fig. 4-16. Sequence and arrangement of amino acids in a molecule of insulin (beef), a protein hormone. 

 Horse insulin and pig insulin resemble beef insulin, but there are slight differences in the amino acid sequence. 

 The amino acid structure of a protein molecule may be ascertained by hydrolyzing (digesting) the protein with 

 different enzymes (see Table 5-1, digestive enzymes) and identifying the liberated amino acids by paper 

 chromatography (Fig. 9-5), and by other means. (Amino acids designated: Ala, alanine; Arg, arginine; Asp, 

 aspartic acid; Cyst, cysteine; Glu, glutamic acid; Gly, glycine; His, histidine; lieu, isoleucine; Leu, leuchine; Lys, 

 lysine; Phe, phenylalanine; Pro, proline; Ser, serine; Thr, threonine; Tyr, tyrosine; Val, valine.) 



