Co-operating ATP sites in the multiple drug resistance transporter Mdr1

The ATPase activity of the multiple drug resistance transporter Mdr1 (P-glycoprotein, gp170) depended on the concentration of ATP with both positive and negative co-operativity both in the absence and in the presence of verapamil. Four co-operating binding sites for ATP were required to adequately model the experimental findings. The activation energy for the ATPase activity increased from approximate to 385 kJ mol(-1) at 10 mu M ATP to 512 kJ.mol(-1) at 1600 mu M, while changes in verapamil concentration had little effect. This indicates that the reaction mechanism of ATP hydrolysis depends on ATP concentration and is further evidence for co-operation of ATP binding sites. Free ATP in higher concentration was inhibitory; however, this inhibition could be reduced by complexing the ATP with Mo2+. Free Mg2+ had little effect on Mdr1 apart from complexing ATP.