Two ADP binding sites have been demonstrated on the reconstitutively active β-subunit, that was removed from the Rhodospirillum rubrum membrane-bound ATP synthase. One is a high affinity site (Kd = 0.7 μM) that does not require MgCl2 and is unaffected by it. The second is a low affinity binding site (Kd = 80 μM) that is absolutely dependent on MgCl2. For stable binding of ADP to this site, MgCl2 must be present not only during the binding step but also during the elution-centrifugation step used to separate the β-subunit bound [3H]ADP from the free ligand. When MgCl2 is removed together with the free ligand [3H]ADP dissociates very rapidly from this second binding site.
- Rhodospirillum rubrum FF-ATPsynthase β-Subunit ADP-binding site MgCl dependence Binding affinity