ADENOSINETRIPHOSPHATASES , TRANSPHOSPHORYLASES 



445 



Table 2-26 

 Inhibition of ATPases by Analogs 



shown by Green and Mommaerts (1954). Addition of Ca++ decreases ADP 

 binding at pH 6.4 and increases it at pH 9, whereas Mg++ has no effect at 

 the lower pH but decreases affinity at the higher pH. The K^ for ADP is 

 around 0.13 roM in the absence of Ca++ and Mg++, but around 0.5 mM 

 at pH 6.4 and 40 mM Ca++. Kielley and Kielley (1953) had shown with 

 liver mitochondrial ATPase that ADP does not alter the optimal Mg++ 

 concentration for ATPase activity, indicating that the inhibition is not by 

 the binding of Mg"*""*", but Nanninga (1958) reported that part of the inhi- 

 bition of myosin ATPase by ADP is due to chelation of Ca++. In the pre- 

 sence of excess Ca++ this chelation can be neglected and the true K^ for 

 the enzyme- ADP complex is found to be 4.6 mM at pH 7. 



An interesting phosphonic analog of ATP, adenylmethylenediphospho- 

 nate: 







II II II 



Adenine-ribose— O— P— O— P— CH— P— O- 



O- O- OH 



