Kinetic and structural characterization of manganese(II)-loaded methionyl aminopeptidases

Manganese(II) activation of the methionyl aminopeptidases from Escherichia coli (EcMetAP-I) and the hyperthermophilic archaeon Pyrococcus furiosus (PfMetAP-II) was investigated. Maximum catalytic activity for both enzymes was obtained with 1 equiv of Mn(II), and the dissociation constants (K(d)) for the first metal binding site were found to be 6 +/- 0.5 and 1 +/- 0.5 microM for EcMetAP-I and PfMetAP-II, respectively. These K(d) values were verified by isothermal titration calorimetry (ITC) and found to be 3.0 +/- 0.2 and 1.4 +/- 0.2 microM for EcMetAP-I and PfMetAP-II, respectively. The hydrolysis of MGMM was measured in triplicate between 25 and 85 degrees C at eight substrate concentrations ranging from 2 to 20 mM for PfMetAP-II. Both specific activity and K(m) values increased with increasing temperature. An Arrhenius plot was constructed from the kcat values and was found to be linear over the temperature range 25-85 degrees C. The activation energy for the Mn(II)-loaded PfMetAP-II hydrolysis of MGMM was found to be 25.7 kJ/mol while the remaining thermodynamic parameters calculated at 25 degrees C are DeltaG+ = 50.1 kJ/mol, DeltaH+ = 23.2 kJ/mol, and DeltaS++ = -90.2 J x mol(-1) x K(-1).