BIOCHEMICAL REACTIONS AND THEIR CATALYSTS 109 



ganese, cobalt, and zinc — are the ionic substances most often encountered. 

 Sometimes there exists a certain amount of interchangeability among 

 these metallic ions, but usually it will be found that one particular metal 

 will be much more effective than any of the others, and probably is the 

 one associated with the enzyme system in its natural environment. 



The organic molecules required as cofactors are highly specific, and in 

 almost all cases only one specific compound will function; without it 

 the protein is completely inactive. 



Enzyme chemists are always particularly interested in determining 

 whether a coenzyme is required for a biochemical reaction under in- 

 vestigation. During the purification of an enzyme, attempts are made 

 to see if the enzyme molecule can be separated into inactive components 

 which, upon recombination, will possess the original activity of the 

 enzyme. If such a dissociation can be achieved, the complete enzyme is 

 called the holoenzyme; the inactive protein component, the apoenzyme; 

 and the smaller entities which are split off, the coenzymes or prosthetic 

 groups. The ease with which the coenzymes may be separated from their 

 apoenzymes varies considerably. Dialysis has been the method usually 

 employed for separating these components, since this procedure is less 

 likely to alter irrevocably the protein molecule than any other means of 

 separation now known. Extraction of insoluble cellular material with 

 alkaline or acidic buffers often can effect a resolution of the holoenzyme. 

 If one is interested in obtaining only the coenzyme, heat will often 

 liberate the coenzyme but will leave a denatured apoenzyme. 



A large number of enzymes exist which cannot be dissociated by 

 dialysis or any other method yet tried. It may be that this group of 

 enzymes includes some proteins that contain tightly bound prosthetic 

 groups which, except for their nondissociability, are analogous to the B 

 vitamin coenzymes. Indeed, some of the enzymes in which the B vitamins 

 are implicated have not yet been successfully resolved into their com- 

 ponent parts. This offers some basis for postulating the existence of 

 unidentified "factors" which must be incorporated into enzymes catalyz- 

 ing certain fundamental processes occurring in most cells. From the 

 standpoint of the nutritional requirements of animals such factors could 

 be either essential or nonessential. 



The classification of organic substances which have been definitely 

 shown to be prosthetic groups for important enzyme systems is very 

 simple. They will be found to be one of three types: 



(1) adenylic acid or a phosphorylated derivative, 



(2) a derivative of one of the B vitamins, or 



(3) a metallic complex of a porphyrin. 



Very few substances that have been definitely characterized as true 



