NUCLEOTIDES AND COENZYMES 



systems where anionic pyrophosphate groups are involved 

 argues strongly for the above hypothesis, as does the fact that 

 K^ values for the metal ion are often of the same order of 

 magnitude as the coenzyme. Also, metal ions are often not 

 very specific, as for example, the replacement of Mg + + by Mn + + 

 in many systems, and the finding that aldolases from different 

 sources require Fe + +, Zn + +, or Co + +, whereas muscle aldolase 

 is inhibited by all of these ions. 



No discussion of metal ion effects would be complete with- 

 out reference to the recent elegant work on metal ion-pyridoxal 

 phosphate complexes. Baddiley (2) and Snell (32) have shown 

 that ternary Schiff-base complexes containing a metal, pyri- 

 doxal phosphate, and amino acids can carry out a remarkable 

 series of reactions (transamination, deamination, decarboxyla- 

 tion) in the absence of enzymes. Studies of this sort, where well- 

 defined, simple model systems can duplicate accurately the en- 

 zymatic processes, give us valuable insight into the mechanism of 

 enzyme and coenzyme action. 



The above material has been presented as a survey of cur- 

 rent thinking and trends in the field of coenzymes and nucleo- 

 tides. At the time of this writing there come to mind five areas 

 in which major discoveries are still to be made. These are: 

 oxidative phosphorylation at the coenzyme level, protein synthesis, 

 biosynthesis of large molecules, hormonal control of enzymes, and 

 genetic control of enzyme synthesis. To this may be added the 

 sixth, and more vast, area : the relation of enzyme chemistry to 

 problems of medicine. Present knowledge of coenzymes, and 

 their function in many metabolic pathways, gives us assurance 

 that such nucleotides of biological importance undoubtedly 

 play a key role as well in these unconquered frontiers. 



References 



1. Axelrod, A. E., V. R. Potter, and C. A. Elvehjem, J. Biol. Chem., 142, 85 

 (1942). 



2. Baddiley, J., Nature, 770,711 (1952). 



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