EFFECT OF FREE AND ATP-BOUND MAGNESIUM AND MANGANESE IONS ON THE ATPASE ACTIVITY OF CHAPERONIN GROEL(14)

Hydrolysis of ATP by the GroEL(14) chaperonin oligomer is activated and modulated by Mg2+ or Mn2+ ions. Mg-ATP and Mn-ATP can serve as substrates of the reaction and bind in a positively cooperative manner to the same catalytic sites on GroEL(14), with similar binding constants in the micromolar range. In addition, millimolar amounts of Mg2+ and Mn2+ ions can further activate the GroEL(14)-ATPase while interacting with low-affinity noncatalytic sites on the chaperonin. The extent of ATPase activation by Mn2+ is half of that by Mg2+ ions. When both Mg2+ and Mn2+ ions are present in the same reaction, Mn2+ behaves as a noncompetitive partial inhibitor of the Mg-dependent ATPase. This inhibition requires the presence of ADP in the catalytic site. The binding affinity of Mn-ADP to the site is significantly higher than that of Mg-ADP. A slower release of Mn-ADP from the catalytic site thus changes the rate-determining step of the GroEL(14)-ATPase cycle. In the cell, the concentrations of Mg2+ and Mn2+ ions are such that both divalent ions may modulate chaperonin activity.