FRANK M. HUENNEKENS 



reflects a certain maturity in the area. Nor is it to be regarded 

 as evidence that the problems have become static, since each 

 plateau of knowledge to which we have ascended becomes, in 

 turn, a stepping stone to the next higher level. 



The present treatise has been envisaged by its editor as a 

 collection of essays surveying the past, present, and future of 

 some aspect of biochemistry, inter alia, enzyme chemistry. 

 Since other contributions will undoubtedly emphasize other 

 aspects of enzymatic reactions, the present discussion will be 

 concerned exclusively with the role of nucleotides and coenzymes 

 in enzymatic reactions without, however, any attempt to docu- 

 ment extensively the properties of individual members. Rather, 

 attention will be directed toward salient features of our present 

 state of knowledge in this area and will extend to a few specula- 

 tive thoughts toward future directions. 



It is common knowledge that whereas all enzymatic re- 

 actions require a protein component (the enzyme), certain ones 

 require, in addition, a cooperative partner (the coenzyme). 

 The coenzyme, sometimes termed cofactor, prosthetic group, or 

 activator,* is usually a heat-stable, dialyzable, organic molecule 

 or metal ion. To omit at present the metal ions, whose func- 

 tion will be considered later, or certain rare cases where a simple 

 anion (e.g., Cl~ with amylase) serves as a coenzyme, it may be 

 stated that most, but not all, coenzymes contain a nitrogenous 

 base (often a vitamin), a sugar (ribose), and phosphate groups, 

 therefore placing these substances in the class of compounds 

 commonly called nucleotides. Conversely, certain other nucleo- 

 tides, the precursors and breakdown products of nucleic acids, 

 exist in tissues without acting as coenzymes. Thus, the 5'- 

 nucleotides either may serve as building blocks for nucleic acids, 

 and as such participate in the energy-requiring process of pro- 

 tein synthesis, or they may become portions of coenzymes in- 

 volved in the breakdown of energy -yielding metabolites. As 



* Although each of these terms has been used, on occasion, in a special 

 sense, the distinction between them appears to be vanishing in the literature ; 

 hence the term coenzyme will be used synonomously for all. 



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