COENZYMES DERIVED FROM B VITAMINS 131 



The specific purposes of the enzymes which mediate biological oxida- 

 tions and reductions are (1) to establish the equilibrium between the 

 two states of a metabolite system and (2) to link this system with another 

 of appropriate potential. When these criteria are met a reaction will then 

 automatically occur. 



The relative electrode potential of a half-cell or the relative redox 

 potential of a metabolite system can be calculated from the standard 

 potential of the half-cell or the metabolite system. The standard potential 

 is that potential, referred to an arbitrary standard (the potential of a 

 |H 2 — H + system), of a half-cell or metabolite system when the oxidized 

 and reduced states are present in equal concentrations (or, are at equal 

 activities) . The actual potential of a system having other concentration 

 ratios differs from the standard potential by a factor which includes the 

 ratio of the oxidized and reduced forms. Hence the actual potential of a 

 system and its tendency to act as an oxidizing or reducing agent can be 

 varied by changing the relative concentration of the two components. 



In a galvanic cell it is possible to reverse the direction of the current 

 and the direction in which the chemical reaction is proceeding by chang- 

 ing the ratio of the oxidized and reduced forms of the components of the 

 half-cells, provided the standard potentials of the two half-cells are not 

 too far apart. If the standard potentials for two metabolite systems 

 composing a reaction are approximately the same value, it is possible to 

 cause the reaction to proceed in either direction by establishing appropri- 

 ate ratios between the oxidized and reduced forms of the two component 

 systems. Such reactions are those in which the net chemical changes have 

 been observed to be reversible. If, however, there is appreciable difference 

 between the standard potentials of the two systems composing the reaction 

 it is impossible to achieve concentration gradients sufficient to reverse the 

 usual course of the reaction. In this case the system having the lower 

 potential will always be observed to be oxidized while the one having the 

 higher potential will be reduced. 



For convenience in studying redox systems, reactions are often set up 

 in which one of the component systems is an artificial one, i.e., is not 

 encountered under natural circumstances. The redox dyes, such as 

 methylene blue, are commonly used for this purpose. Their use often 

 simplifies the process to be studied, enables the investigator to by-pass 

 technical difficulties, and through the change in the color of the dyes at 

 a critical redox potential serves as a convenient indicator in following 

 the progress of the reaction. 



Coenzymes derived from nicotinic acid, riboflavin, and the porphyrins 

 exist in both oxidized and reduced states and, with their appropriate 

 apoenzymes, form redox systems. It is not surprising that the enzyme 



