BIOCHEMICAL REACTIONS AND THEIR CATALYSTS 103 



of metabolic processes, but it also enables the investigator who is studying 

 an isolated system to have some idea of the concentration conditions that 

 he must use to demonstrate the activity of an enzyme preparation. 



Exact statements of the energy relationships and equilibrium con- 

 ditions involve the use of thermodynamic concepts and equations. How- 

 ever, without these it can be stated as an approximation that when the 

 amount of free energy liberated by a reaction is comparatively small, 

 then the equilibrium concentrations of the reactants and products can 

 usually be obtained, such that it is possible for an organism to make use 

 of either the forward or the reverse reaction. On the other hand, if an 

 enzymatic reaction liberates a large amount of energy, it would probably 

 be difficult to establish the concentrations needed to reverse the reaction. 

 If this reverse process is needed for synthetic purposes, some indirect 

 route in which high-energy units are utilized will have to be employed 

 (pp. 218 and 235). 



Types of Biochemical Reactions. Next to be considered are the funda- 

 mental chemical reactions required for the synthesis and interconversion 

 of constituents essential for cell structure and function. These processes 

 involve such diverse reactions as the formation and hydrolysis of peptide, 

 ester, and glucosidic linkages; the oxidation of alcohols, aldehydes, and 

 amines; the reduction of acids and aldehydes; the hydrogenation and 

 dehydrogenation of hydrocarbon chains; the formation and cleavage of 

 carbon-to-carbon bonds; and the synthesis and degradation of hetero- 

 cyclic compounds. The demands presented by the variety and complexity 

 of these reactions upon the synthetic abilities of the cell are equalled by 

 the drastic limitations put upon the conditions under which the reactions 

 must be carried out. All transformations must take place within a very 

 narrow temperature range, with extremely low concentrations of reactants, 

 at an approximately neutral pH and a temperate redox potential. The 

 powerful but caustic agents which are indispensable to the synthetic 

 organic chemist must be wholly avoided. Even the simplest types of 

 biochemical reactions would be impossible under such conditions, if it 

 were not for the remarkable catalytic abilities of the enzymes. 



It is interesting to speculate on the number of enzyme systems re- 

 quired to account adequately for all these reactions which must be taking 

 place within cells. The number must indeed be large. If, however, one 

 tabulates according to type the numerous enzymatic reactions which 

 have been demonstrated and postulated for the normal metabolism of 

 carbohydrates and fats, he will find that the reactions can be classified 

 chemically into a surprisingly small number of groups. By suitable com- 

 binations of these relatively few types of reactions it is possible to carry 



