Divalent metal binding properties of the methionyl aminopeptidase from Escherichia coli

The metal-binding properties of the methionyl aminopeptidase from Escherichia coli (MetAP) were investigated. Measurements of catalytic activity as a function of added Co(LI) and Fe(II) revealed that maximal enzymatic activity is observed after the addition of only 1 equiv of divalent metal ion. Based on these studies, metal binding constants for the first metal binding event were found to be 0.3 +/- 0.2 mu M and 0.2 +/- 0.2 mu M for Co(II)- and Fe(II)-substituted MetAP, respectively. Binding of excess metal ions (>50 equiv) resulted in the loss of similar to 50% of the catalytic activity. Electronic absorption spectral titration of a 1 mM sample of MetAP with Co(II) provided a binding constant of 2.5 +/- 0.5 mM for the second metal binding site. Furthermore, the electronic absorption spectra of Co(II)-loaded MetAP indicated that both metal ions reside in a pentacoordinate geometry. Consistent with the absorption data, electron paramagnetic resonance (EPR) spectra of [CoCo(MetAP)] also indicated that the Co(II) geometries are not highly constrained, suggesting that each Co(II) ion in MetAP resides in a pentacoordinate geometry. EPR studies on [CoCo(MetAP)] also revealed that at pH 7.5 there is no significant spin-coupling between the two Co(II), ions, though a small proportion (similar to 5%) Of the sample exhibited detectable spin-spin interactions at pH values > 9.6. EPR studies on [Fe(III)_(MetAP)I and [Fe(III)Fe(III)(MetAP)1 also suggested no spin-coupling between the two metal ions. H-1 nuclear magnetic resonance (NMR) spectra of [Co(II)_(MetAP)] in both H2O and D2O buffer indicated that the first metal binding site contains the only active-site histidine residue, His171. Mechanistic implications of the observed binding properties of divalent metal ions to the MetAP from E. coli are discussed.