COENZYMES DERIVED FROM B VITAMINS 129 



nisms of oxidations, particularly those types occurring in biological 

 systems, have been published recently. 12 - 13 Since the changes in the struc- 

 ture of organic molecules can be much more easily indicated in terms of 

 hydrogen atoms, this convention of depicting "oxidations" will be used 

 in preference to the more exact one in which the electronic changes are 

 described. 



The enzymatic oxidation of many organic compounds involves no 

 changes other than the removal of two hydrogen atoms. The catalysts 

 for this type of reaction are found to contain, as cofactors, derivatives 

 of either nicotinic acid, riboflavin, or a porphyrin. There are several types 

 of oxidative processes which are not simple dehydrogenations. Oxidative 

 decarboxylations and transaminations are illustrative of the more com- 

 plex types of oxidative reactions. In these cases it will be found that 

 vitamins other than the ones just mentioned function in the catalytic 

 processes; these reactions will be considered elsewhere. The dehydrogena- 

 tions considered below (catalyzed by enzymes containing nicotinic acid, 

 riboflavin, and porphyrins) are those in which a double bond of one of 

 the following types is created by the removal of two hydrogen atoms and 

 an existing double bond in another molecule is reduced by the addition 

 of these two atoms: 



C=0 C=N— C=C 0=0 



An interpretation of the oxidation and reduction of inorganic substances 

 can often be most easily made by considering the process in terms of a 

 galvanic cell. Likewise, the presentation of certain facts pertaining to 

 biological processes in which there are oxidations and reductions occurring 

 can be most easily made by drawing an analogy between the biological 

 systems and a galvanic cell. 



The galvanic cell is composed of two half-cells; each half-cell contains 

 both an oxidized and a reduced form of some substance capable of existing 

 in the two states (for example, a metal and its ion) ; these oxidized and 

 reduced components of a half-cell must be in a state of dynamic equilib- 

 rium; and the two half-cells must be connected by suitable conductors 

 before chemical changes and energy production take place. When the 

 circuit between the two half-cells is closed, a reduction will take place at 

 one electrode, and a compensating oxidation will occur at the other. 

 Which part (oxidation or reduction) of the overall reaction will occur at 

 a specific electrode and how much energy will be released by the reaction 

 are determined by the relative potentials of the two half-cells composing 

 the cell. At a given temperature and pH this electrode potential of a half 

 cell is a function of the components of the half-cell and their relative 



