 ATP ( adenosine triphosphate ) has multiple groups with different acid dissociation constants . In neutral solution , ionized ATP exists mostly as ATP4 − , with a small proportion of ATP3 − . As ATP has several negatively charged groups in neutral solution , it can chelate metals with very high affinity . The binding constant for various metal ions are ( given as per mole ) as Mg2 + ( 9554 ) , Na + ( 13 ) , Ca2 + ( 3722 ) , K + ( 8 ) , Sr2 + ( 1381 ) and Li + ( 25 ) . Due to the strength of these interactions , ATP exists in the cell mostly in a complex with Mg2 + . 
