Kinetic studies of the demetallization and inactivation of concanavalin A

The demetallization of various metallo derivatives of Concanavalin A (i.e., MnMnPL, CoMnPL, CaCaPL, CoCaPL and MnCaPL, where PL represents protein in a locked conformation) has been examined by three separate procedures. These include the treatment of the protein with the metal ion chelators, EDTA and terpyridine, and subjecting the protein to low pH (i.e., pH 1.2). In all three procedure and for all five species examined, the immediate product of protein demetallization was the PL conformation previously described by Brown, R.D., III, Brewer, C.F. and Koenig, S.H. (Biochemistry (1977) 16, 3883-3896). The rates of dissociation of the metals from the different protein species, as measured spectrophotometrically using terpyridine, were found to be identical to the rates (k1) of loss of protein sugar binding affinity in the presence of EDTA as measured by assays with the fluorescent sugar, 4-methylumbelliferyl alpha-D-mannoside. The kinetic and thermodynamic data associated with the inactivation of the protein species have allowed the different metallo derivatives to be classed into two general categories. Class I forms include MnMnPL, CoMnPL and CaCaPL and possess an average k1 (25 degrees C) value of 3.88 X 10(-2) s-1 and an average Ea of 14.2 kcal X mol-1. Class II forms CoCaPL and MnCaPL have average values for k1 (25 degrees C) and Ea of 3.67 X 10(-5) s-1 and 21.6 kcal X mol-1, respectively.