NUCLEOTIDES AND COENZYMES 



In contrast to reactions of type (a), where no coenzyme is re- 

 quired, type (c) reactions often require a coenzyme. This is 

 because the enzyme lacks the proper chemical groupings to act 

 as "middleman" carrying, at one stage, the group being trans- 

 ferred. Hence, it n:iust call in a special assistant, the coenzyme, 

 which is bound to the enzyme and which intervenes as shown in 

 equations (1) and (2) where the symbol X might well be replaced 

 by X — E. It is unlikely that in this case we can reduce the prob- 

 lem to a series of binary complexes, for this would involve the 

 enzyme, at one stage, holding the transferring group, thereby 

 apparently obviating the need for the coenzyme. 



Reactions of type (b) are more complex for they involve the 

 creation of a new bond, usually with the elimination of water 

 (e.g., esters, anhydrides, peptides, glycosides). The energy 

 requirement for this sort of reaction is met by the addition ot 

 still another substrate to the reaction, usually ATP, which ac- 

 tivates one of the condensing partners. The sequence may be 

 written : 



BOH + ATP > B - AMP + PP (18) 



followed by a transfer to the actual coenzyme 



B - AMP + CoA > B - CoA + AMP (19) 



and, finally, transfer of the activated group from the coenzyme 

 to the acceptor : 



B - CoA + AH > A - B + CoA - H (20) 



Obviously, there are many possible variations of tlie above 

 schemes, but these illustrations may be considered as representa- 

 tive. 



In light of the above discussion one may return to the ques- 

 tion of what is so special about the structure of each of the co- 

 enzymes in Table I, that makes it unique in being able to 

 transfer particular groups. Each of the coenzymes shown has 

 itself a functional group which seems of suitable structure to 

 carry reversibly the group being transferred. To press further 

 along these lines would undoubtedly be desirable, but at present 



509 



