ENZYMES 141 



ticipates in the transfer of phosphate groups. As shown in the follow- 

 ing structure, this particular substance is closely related to adenylic 

 acid and to one half of DPN + . This coenzyme functions by transfer 

 of the terminal phosphate to a substrate molecule in the presence of 

 any one of several enzymes. This loss of a phosphate group yields 



0~0 



II I II 

 y. X), CH.,OPOPOPO 



/^,, / N " I IN 



N-^ N Xh H/ -0 OH 



H " --"^ 



N / 



ATP 



adenosine diphosphate (ADP), which under certain conditions can 

 transfer one of its phosphates and tiun into adenylic acid (adenosine 

 monophosphate). The first biochemical step in the metabolism of 

 glticose illustrates the role of ATP. 



hexokinase c * -tat^ 



Glucose + ATP > aflucose-6-phosphate + ADP 



The specific enzyme requires a second cofactor, magnesium ion filling 

 this role in nature. Other metabolic steps add phosphate to the 

 ADP, reforming the ATP utilized above. Thus a coenzyme consumed 

 at one stage and transformed into another substance is ultimately 

 restored by some subsequent reaction. 



The second group of cofactors includes materials, mainly inorganic 

 ions like the magnesium above, called activators. As the name suggests, 

 the activators are required for or markedly accelerate enzyme action. 

 They probably play roles in enzyme-substrate complexes, perhaps 

 serving as links between enzyme and substrate. Activators also par- 

 ticipate in systems involving coenzymes, so they may mediate inter- 

 actions between enzyme, coenzyme, and substrates. Activators are not 

 always completely specific, and it is usually possible to replace the 

 ion functioning in the cell with a chemically related ion that works 

 nearly as well. In fact the selection of the natural activator from 

 among the list of ions that work can be very diffictdt when two or 

 more of the possibilities exist in the cell. Magnesium and manganous 

 ions are natural activators for several enzyme systems. Chloride, 

 potassium, molybdenum, zinc, and calcium ions probably serve in 

 certain cases, and other ions undoubtedly do so in still other systems. 



