PROTEIN METABOLISM 489 



do not appear to be as higli as woukl be required, and both peptides 

 and enzymes are actually rather stable when dissolved in water for 

 ordinary periods of time. 



Second, since the peptide bond does resist simple hydrolysis on 

 standing in water, a high energy of activation is indicated. This term 

 refers to the phenomenon of energy needed merely to get a reaction 

 started. Afterwards this energy reappears and is accompanied by the 

 free-energy change characteristic of the reaction. Thus only very 

 energetic molecules can begin to react, and the reaction does not 

 start until energy is supplied to energize the molecules sufficiently. 

 Therefore, a high energy of activation serves as a barrier even to 

 otherwise spontaneous reactions. This factor probably allows one to 

 keep proteins in the presence of water at ordinary temperatures. 



Catalysts, however, lower energies of activation and reduce the mo- 

 lecular energy needed to initiate a reaction. When cells are broken 

 open, such catalysts are found in the form of proteolytic enzymes. 

 Therefore, inside cells the proteins must be protected from such 

 enzymes. Perhaps this third factor in providing cells with proteins 

 is achieved by confinement of the enzymes in some way or by storage 

 in forms inactive in the cells but becoming active when the cells 

 are ruptured. There is some evidence for both ideas. On the other 

 hand, there may be some energy-coupling device in the cells which 

 changes the protein enough to alter the free energy in the direction 

 of stability and thus overcome even the effects of catalysts. Thus 

 these or similar arrangements, perhaps in combination, are believed 

 to ensure the necessary supply of cellular proteins. 



Whatever the mechanism of formation of peptide bonds, it is quite 

 clear that not all conceivable combinations of amino acids exist. 

 Although there are many different proteins in an animal, the number 

 is relatively small compared to the number of possibilities. Therefore, 

 the sequences of the amino acids must be specifically controlled. 

 Moreover, offspring have many proteins apparently identical with 

 those of their parents. Hence reproduction must incorporate this 

 control system into the developing individual. This need for a control 

 mechanism is the third general requirement in protein biosynthesis. 



Diverse studies suggest that the nucleic acids are involved in protein 

 synthesis and help control protein structure. For example, when 

 added substances interfere with the nucleic acids, protein synthesis is 

 blocked. On the other hand, protein synthesis can be prevented under 

 some conditions without immediately stopping the formation of 

 nucleic acids. Thus it seems that the presence of the latter is required 

 for protein synthesis, and some investigators feel that the ribose type 



