Interactions of metal ions with mu-monothiopyrophosphate.
mu-Monothiopyrophosphate (MTP) binds monovalent and divalent metal ions with dissociation constants (Kd) similar to those for pyrophosphate (PPi). The values of Kd for metal-MTP complexes are the following, as measured kinetically in the hydrolysis of MTP (microM): Mg2+, 32 +/- 4; Mn2+, 5.4 +/- 1.4; and Co2+, 27 +/- 15. The thermodynamically measured (EPR) values for Mg2+ and Co2+ are 28 +/- 13 microns and 11 +/- 4 microM, respectively; and the Kd for the complex MnPPi is 3.4 +/- 0.5 microM. The metal-MTP complexes undergo hydrolysis at rates modestly faster or slower than the rate at which MTP itself reacts. The complexes MgMTP2-, CoMTP2-, and MnMTP2- undergo hydrolytic cleavage with release of thiophosphate with observed first-order rate constants of 1.6 x 10(-2) min-1, 2.3 x 10(-2) min-1, and 0.6 x 10(-2) min-1, respectively, at 35 degrees C, compared with 1.1 x 10(-2) min-1 for MTP4- under the same conditions. Alkali metal cations also stimulate or retard the hydrolysis of MTP. At 25 degrees C and pH 12.2, the observed rate constant for tetramethylammonium MTP4- is 2.1 x 10(-3) min-1, and the estimated rate constants (min-1) for saturating alkali metals under the same conditions are as follows: Li+, 0.25 x 10(-3); Na+, 3.9 x 10(-3), K+, 6.7 x 10(-3); and Cs+, 6.7 x 10(-3). Divalent metal ions markedly retard the hydrolysis of MTP at pH 7 and 8 because complexation shifts the pH rate profile more than 2 pH units toward the acid side.(ABSTRACT TRUNCATED AT 250 WORDS)
Lightcap
E S
ES
Institute for Enzyme Research, Graduate School, University of Wisconsin-Madison 53705.
Halkides
C J
CJ
Frey
P A
PA
D002413:Cations, Divalent / Q000737:chemistry; D002414:Cations, Monovalent / Q000737:chemistry; D011756:Diphosphates / Q000737:chemistry*; D004578:Electron Spin Resonance Spectroscopy; D006863:Hydrogen-Ion Concentration; D006868:Hydrolysis; D008670:Metals / Q000737:chemistry*; D013329:Structure-Activity Relationship; D013816:Thermodynamics
D002413:Cations, Divalent; D002414:Cations, Monovalent; D011756:Diphosphates; D008670:Metals; C046010:monothiopyrophosphoric acid