The Chemical 



the RNA components ol the cytoplasm, as 

 will be considered in Chapters 7 and 27. 



Three-Dimensional Configuration of 

 Protein Molecules: Primary, Secondary, 

 and Tertiary Structure. The primary struc- 

 ture of a protein molecule is constituted by 

 one or more peptide chains. Within each 

 chain the amino acid units are held together 

 by peptide linkages; the peptide chains, if 



-S-S-Kl) 



Fig. 4-17. Helical configuration of a protein mole- 

 cule. Three types of bonds determine the primary, sec- 

 ondary, and tertiary aspects of protein structure-. (1) 

 strong bonds (peptide and disulfide), which determine 

 primary structure; (2) weaker hydrogen bonds, which 

 determine secondary structure; (3) very weak forces 

 (e.g., electrostatic), which effect the tertiary structure. 

 Each broad part, along the length of the spiral, repre- 

 sents one amino acid. 



more than one is present, are united by one 

 or more disulfide ( — S — S — ) bridges, as in 

 the insulin molecule (Fig. 4-16). These chem- 

 ical bonds, namely peptide and disulfide, 

 are relatively stable. Collectively they de- 

 termine what is called the primary structure 

 of a protein molecule (Fig. 4-17). 



Peptide chains are not straight, however. 

 Rather, they display a coiled, or helical struc- 

 ture (Fig. 4-17). Usually each complete coil 

 of the helix is constituted by about four 

 amino acids, and the distance pre-empted by 

 one coil, along the central axis of the helix, 



and Physical Structure of Protoplasm - 87 



is about 1.5 Angstroms. However, the tight- 

 ness of coiling and consequently the total 

 length of a protein molecule may vary con- 

 siderably, depending upon temperature, pH, 

 and other factors. An unfolded, or extended, 

 protein molecule may be several times longer 

 than the same molecule in the folded, or 

 tightly coiled, form. 



The forces and factors that influence the 

 helical structure of a protein molecule, espe- 

 cially the degree of its folding and unfolding, 

 collectively determine what is called the 

 secondary and tertiary structure of the pro- 

 tein (Fig. 4-17). 



Hydrogen bonds are particularly impor- 

 tant in relation to secondary structure. Hy- 

 drogen bonds are relatively weak linkages 

 that result from the tendency of hydrogen to 

 share elections with two neighboring atoms, 

 especially O and N, when these atoms are 

 situated very close together (Fig. 4-17). A 

 fully folded protein molecule appears to 

 display one hydrogen bond between every 

 third amino acid in the coiled peptide chain. 

 Collectively, the many hydrogen bonds, al- 

 though weak individually, exert consider- 

 able force in stabilizing the helical struc- 

 ture. Still weaker forces, such as electrostatic 

 attractions between oppositely ionized molec- 

 ular foci (Fig. 4-17), are regarded as the de- 

 terminants of tertiary structure in proteins 

 and other macromolecular configurations. 



The three-dimensional configuration of 

 proteins and other macromolecular com- 

 ponents of the protoplasm has great influ- 

 ence upon their physiological activity. For 

 example, enzymes are mainly of protein com- 

 position (Chap. 5) and each displays a con- 

 figuration that must fit the molecular form 

 of the substrate molecules that are being ac- 

 tivated (p. 105). Moreover, the folding of 

 elongate, partially unfolded protein mole- 

 cules may explain the property of contrac- 

 tility, which is exhibited by protoplasm gen- 

 erally (Fig. 4-23). However, in the completely 

 unfolded state, such as is induced by abnor- 

 mally high temperature, exceptionally high 

 or low pH conditions, or other drastic 



