HUGO THEORELL 



It is Still common practice to distinguish between "pros- 

 thetic groups" and "coenzymes." As recently discussed by the 

 author (21), there are certain historical reasons for maintaining 

 this terminology which are based upon some simple preparative 

 differences. Whereas prosthetic groups are so tightly bound to the 

 protein that they remain attached to the protein even if this is 

 denatured, for example, by boiling the coenzymes, before 

 anything was known about their chemical nature, were defined 

 as heat-stable, dialyzable activators remaining in solution after 

 the heat denaturation of the proteins. 



At the present time there is an obvious tendency to another 

 distinction from a chemical point of view, since the hematin 

 group of the hemoproteins is practically always called "pros- 

 thetic group" — but never "coenzyme" — whereas the group of 

 substances always called coenzymes are the well-known phos- 

 phorylated compounds DPN, FMN, FAD,* and so on. 



It would perhaps not be practical or even feasible to give 

 up the name "prosthetic groups" in favor of "coenzymes" for 

 various reasons, the main one being that some hemo-proteins, 

 like hemoglobin or myoglobin, are not usually regarded as 

 enzymes. But from a general point of view there is no need to 

 keep up a distinction that is based merely upon how strong the 

 linkages are between the parts of a conjugated protein, since very 

 great variations in this respect occur within both groups. 



The prosthetic groups, as far as we yet know, always remain 

 attached to their proteins during their functional cycles; the 

 coenzymes may either remain "immobile" during their func- 

 tion, as is the case with FMN in the old yellow ferment, or pass 

 through different parts of the functional cycle in combination 

 with different proteins, as is very probably the case with DPN in 

 the liver alcohol dehydrogenase system ("mobile" coenzyme). 

 The proteins, as well as coenzymes and prosthetic groups, are 

 inactive when separated. This is just the reason why the 

 coupling reactions between the proteins and their cofactors are 



* DPN = diphosphopyridinenucleotide; FMN = flavinmononucleo- 

 dde; FAD — flavindinucleotide. 



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