Magnesium regulates both the nucleotide binding and the enzyme activity of isolated chloroplast coupling factor 1

The inactivation and the activation of the ATPase of isolated CF1 as assayed by the hydrolysis of ATP in 10 s depended on prior binding of ADP-Mg and ATP-Mg. The effects of Mg2+ on the nucleotide binding kinetics were studied by monitoring the time courses of UV spectral changes induced by the interaction between CF1 and ADP or ATP using a rapid-scan spectrophotometer equipped with a stopped-flow cell. The apparent rate constant of ADP binding to the two high-affinity sites on CF1 (designated sites B and C in the previous report [Hisabori, T. & Sakurai, H. (1984) Plant Cell Physiol. 25, 483-493]) was drastically increased by prior binding of Mg2+ to CF1, but not ATP. The inhibitory effect of Mg2+ was attributed to a marked increase in kon for the inhibitory ADP binding at the high-affinity sites induced by the previous binding of Mg2+ to the enzyme. The location of site B is suggested to be on the beta subunit based on the difference spectral change induced by binding of the ADP analog 2',3'-O-(2,4,6-trinitrophenyl)ADP to CF1.