MOLECULAR STRUCTURE IN PROTOPLASM 



173 



pTaius, for example, may aet as reserve 

 supplies. Proteins ol' llie iiiiclens and of 

 some jilandular cells apparently have spe- 

 cifie fuiu'tions; and the formation of kera- 

 tin in the e])idermal cells is obviously for 

 structural puri)oses. These proteins all 

 consist of primary chains with modifica- 

 tions of various kinds. For purposes of 

 simplification we shall ignore for the time 

 being these modifications and consider only 

 the simple chain structure as typical of the 

 protein portion of the particles in proto- 

 plasm. We are accepting then a chain of 

 36,000 molecular weight as a representative 

 chain of the proteins in cytoplasm, one of 

 which when completely hydrolyzed will 



TABLE II 



Diagram of a Protein Chain 



yield about 300 amino acid molecules of 

 perhaps twenty kinds. 



Amiiu) acids have one characteristic in 

 common. This generic feature is a car- 

 boxyl group, COOH, and an amino group, 

 NH2 (with one exception), both attached 

 to the same carbon atom, the alpha carbon. 

 The rest of the molecule is the specific por- 

 tion which distinguishes one amino acid 

 from another. The generic group is shown 

 on the right-hand side in Table I. It is 

 this generic part of the molecules which 

 allows them to be linked together into a 

 chain structure, such as illustrated in 

 Table II. 



In the formation of the chain the alka- 

 line group, NHo, of one amino acid is con- 

 densed with the acid group, COOH, of 

 another, resulting in the splitting off of a 

 Avater molecule and the formation of a pep- 

 tide linkage. This is shown in diagram in 

 Fig. 1. With each condensation a carboxvl 



A 



k. 



HO- 



II i: \i^ :?\ *i 



Fig. 1. Diagram to show formation of a peptide 

 linkage. At A three separate amino acids are 

 shown ; at B the source of the water molecule is 

 indicated by the rectangle ; and at C the completed 

 linkage between the adjacent amino acid residues 

 is shown. 



group is left at one end of the peptide 

 chain and an amino group at the other end. 

 This allows for extension into a long poly- 

 peptide chain, such as illustrated by the 

 reproduction of a three-dimensional model 

 made to scale in Figs. 2 and 3. 



One of the curious things about this 

 chain is that the adjoining amino acids are 

 likely to occur on alternate sides of the 

 peptide linkage, and as a result a zig-zag 

 back-bone is produced which consists of a 

 repetition of the peptide group, carbon- 



