The transesterification of the 2-(4-nitrophenyl phosphate) ester of propylene glycol (1) was promoted by La3+, Nd3+, Eu3+, Gd3+, Tb3+, Lu3+, Pb2+, Zn2+, Cu2+, Ni2+, Co2+, Mn2+, Mg2+, and Ca2+ at 37 OC in 0.01 M Hepes buffer (N-(2-hydroxyethy1)piperazine-N'-ethanesulfonic acid), pH 6.85. The ability of metal ions to promote transesterification of 1, as measured by apparent-second-order rate constants, followed the orderThe transesterification of 1 by La3+, Pb2+, Zn2+, Cu2+, Ni2+, Co2+, or Mn2+ exhibited saturation kinetics, consistent with the formation of a reactive metal ion complex with 1. Metal ion association constants ( K ) for formation of the metal-1 complex and a first-order catalytic rate constant (kea,) for decomposition of the metal-I complex were determined for metal ions that showed saturation kinetics. K varied by only a factor of 5, whereas k,,, varied by a factor of 29. Transesterification of 1 bound to Pb2+, La3+, Cu2+, Zn2+, Co2+, Mn2+, or Ni2+ occurred 2.1 X lo4, 1.4 X IO4, 4.6 x io3, 3.6 X IO3, 9.2 X IO2, 7.7 X IO2, or 7.7 X lo2 times more rapidly, respectively, than the transesterification of free 1. Properties of metal ions that may affect their ability to promote transesterification of phosphate esters are discussed; these include the Lewis acidity of the metal ion, the formation of metal hydroxide complexes, metal ion radius, and stabilization of a hypervalent phosphorus(V) intermediate. These metal ion model studies are put in context with metal ion requirements in self-cleaving RNAs.
IntroductionMany of the enzymes that catalyze nucleophilic displacement reactions a t phosphorus(V) require metal ions for activity.' Investigation of metal ion model systems has helped to elucidate the role of metal ions in t'lese reactions which include ATP hydrolysis,2 phosphate diester3 and phosphate monoester hydrolysis: and phosphoryl t r a n~f e r .~ Metal ion catalyzed displacement reactions at phosphorus(V) are also involved in reactions catalyzed by RNA enzymes and self-splicing and self-cleaving RNAs;~ hydrolysis, intermolecular transesterification, or intramolecular transesterification of phosphate diesters are the types of reactions that are catalyzed by these biomacromolecules. Several studies have focused on metal ion requirements in self-splicing or selfcleaving and it appears that at least in some systems several different divalent metal ions may promote ~leavage.~ The site-specific cleavage of phenylalanine t-RNA, a reaction that resembles that of self-cleaving RNAs, is promoted by several metal
