Metal ion catalyzed hydrolysis of ethyl pnitrophenylphosphate
Journal of Inorganic Biochemistry
15N isotope effects in the nitro group and 18O isotope effects in the phenolic oxygen have been measured for the hydrolysis of ethyl p-nitrophenyl phosphate catalyzed by several metal ions. Co(III)–cyclen at pH 7, 50 °C, gave an 15N isotope effect of 0.12% and an 18O one of 2.23%, showing that P–O cleavage is rate limiting and the bond is ∼50% broken in the transition state. The active catalyst is a dimer and the substrate is presumably coordinated to the open site of one Co(III), and is attacked by hydroxide coordinated to the other Co(III). Co(III)–tacn under the same conditions shows a similar 15N isotope effect (0.13%), but a smaller 18O one (0.8%). Zn(II)–cyclen at pH 8.5, 80 °C, gave an 15N isotope effect of 0.05% and an 18O one of 0.95%, suggesting an earlier transition state. The catalyst in this case is monomeric, and thus the substrate is coordinated to one position and attacked by a cis-coordinated hydroxide. Eu(III) at pH 6.5, 50 °C, shows a very large 15N isotope effect of 0.34% and a 1.6% 18O isotope effect. The large 15N isotope effect argues for a late transition state or Eu(III) interaction with the nitro group, and was also seen in Eu(III)-catalyzed hydrolysis of p-nitrophenyl phosphate [Bioorg. Chem. 28 (2000) 283].
J. Rawlings, W. W. Cleland and A. C. Hengge “Metal ion catalyzed hydrolysis of ethyl pnitrophenyl phosphate.” Journal of Inorganic Biochemistry, 2002, 93, (1-2), 61-65.