130 THE BIOCHEMISTRY OF B VITAMINS 



concentrations, and this value is a quantitative expression of the capabili- 

 ties of the component pair as an oxidizing or a reducing agent. 



In each of the individual reactions of a biological oxidation there are 

 two metabolite systems: each system consists of a substance capable of 

 existing in an oxidized and reduced state ; the oxidized and reduced forms 

 of a particular system must be in a state of dynamic equilibrium ; and the 

 two systems must be connected by suitable means before chemical changes 

 and energy production take place. When this connection between the two 

 systems is established a reduction will take place in one system and an 

 equivalent oxidation in the other. Which system will be reduced and 

 which oxidized and how much energy will be liberated by the reaction are 

 determined by the relative potentials of the two metabolite systems com- 

 posing the reaction. In this case, the potential is called the redox poten- 

 tial; it is a function of the components of the system, their relative 

 concentrations, the temperature, and, usually, the pH at which the reac- 

 tion is carried out. This potential is a quantitative expression of the 

 tendency of the metabolite pair to undergo oxidation or reduction. The 

 lower the potential (based on the positive and negative notation found 

 in all biochemical literature) , the greater is the tendency of a system to 

 accept hydrogen atoms and to exist predominantly in the reduced form. 



In many common galvanic cells no catalyst other than water is needed 

 to establish the equilibrium between the oxidized and reduced states of 

 the material composing the half-cell. There are instances, however, in 

 which a catalyst must be introduced to establish this equilibrium. For 

 example, a platinum electrode must be used to establish the equilibrium 

 between molecular hydrogen and its oxidized state, the hydrogen ion. 

 Several systems which often form component parts of biological oxida- 

 tions (for example, the sulfide-disulfide system, the ascorbic-dehydro- 

 ascorbic acid system, or certain ferrous-ferric complex systems) require 

 no catalyst (at least, no enzyme) to establish the equilibrium between 

 their oxidized and reduced states. When two such systems are coupled, a 

 nonenzymatic oxidation-reduction reaction occurs. Usually, however, the 

 situation is more analogous to the hydrogen-hydrogen ion half-cell, and 

 a catalyst (enzyme) must be introduced to establish the equilibrium 

 between the oxidized and reduced form of the organic metabolite. The 

 classical example is the succinic-fumaric acid system. These two acids 

 cannot be reversibly interconverted by any chemical means yet known, 

 and they cannot function by themselves as a hydrogen donor or an 

 acceptor. Yet, in the presence of an appropriate enzyme, these two acids 

 rapidly reach a state of equilibrium, and this system can act as either an 

 oxidizing or reducing agent depending upon the potential of the system 

 with which it becomes linked. 



