High kinetic stability of Zn(II) coordinated by the tris(histidine) unit of carbonic anhydrase towards solvolytic dissociation studied by affinity capillary electrophoresis.

High kinetic stability of Zn(II) coordinated by the tris(histidine) unit of carbonic anhydrase towards solvolytic dissociation studied by affinity capillary electrophoresis. J Inorg Biochem. 2016 May 17; Authors: Sato Y, Hoshino H, Iki N Abstract Solvolytic dissociation rate constants (kd) of bovine carbonic anhydrase II (CA) and its metallovariants (M-CAs, M=Co(II), Ni(II), Cu(II), Zn(II), and Cd(II)) were estimated by a ligand substitution reaction, which was monitored by affinity capillary electrophoresis to selectively detect the undissociated CAs in the reaction mixture. Using EDTA as the competing ligand for Zn-CA, the dissociation followed the unimolecular nucleophilic substitution (SN1) mechanism with kd=1.0×10(-7)s(-1) (pH7.4, 25°C). The corresponding solvolysis half-life (t1/2) was 80days, showing the exceptionally high kinetic stability of t Zn-CA, in contrast to the highly labile [Zn(II)(H2O)6](2+), where the water exchange rate (kex) is high. This behavior is attributed to the tetrahedral coordination geometry supported by the tris(histidine) unit (His3) of CA. In the case of Co-CA, it showed a somewhat larger kd value (5.7×10(-7)s(-1), pH7.4, 25°C) even though it shares the same tetrahedral coordination environment with Zn-CA, suggesting that the d(7) electronic configuration of Co(II) in the transition state of the dissociation is stabilized by the ligand field. Among M-CAs, only Ni-CA showed a bimolecular nucleoph...
Source: Journal of Inorganic Biochemistry - Category: Biochemistry Authors: Tags: J Inorg Biochem Source Type: research