UNISITE ATP HYDROLYSIS BY SOLUBLE RHODOSPIRILLUM-RUBRUM F1-ATPASE IS ACCELERATED BY CA2+

At saturating concentrations of ATP, soluble F-1 from the Rhodospirill um rubrum (RF1) exhibits a higher rate of hydrolysis with Ca2+ than wi th Mg2+. The mechanisms involved in the expression of a higher catalyt ic activity with Ca2+ were explored by measuring the ATPase activity o f RF1 at substiochiometric concentrations of ATP (unisite conditions). At a ratio of 0.25 [gamma-P-32]ATP per RF1, the enzyme exhibited a 50 times higher hydrolytic rate with Ca2+ than with Mg2+. The rate of [g amma-P-32]ATP binding to RF was in the same range with the two divalen t metal ions. Centrifugation-filtration of RF1 exposed to substoichiom etric [gamma-P-32]ATP concentrations and Mg2+ through Sephadex columns yielded an enzyme that contained [gamma-P-32]ATP and [P-32]phosphate in a stoichiometry that was close to one. In the presence of Ca2+, the eluted enzyme did not contain [gamma-P-32]ATP nor [P-32]phosphate. Th is indicated that the rate of product release was faster with Ca2+ tha n with Mg2+. It was also observed that the ratio of multisite to unisi te hydrolysis rates was of similar magnitude with both divalent cation s. This suggests that they do not affect differently the cooperative m echanisms that may exist between catalytic sites. In consequence, the higher ATPase activity of RF1 in presence of Ca2+ strongly suggests th at the retention time of products is decreased in the presence of this cation. (C) 1998 Elsevier Science B.V.