Effect of Cd2+ on tyrosinase: Integration of inhibition kinetics with computational simulation

Abstract

Cadmium ions (Cd²⁺) are a widespread and easily absorbed toxin to both humans and animals that can be spread via food, water, and air pollution. Tyrosinase (EC 1.14.18.1) is a multifunctional copper-containing enzyme that is ubiquitously expressed in animals and plays a critical role in melanin production. We evaluated the toxic effects of Cd²⁺ on tyrosinase activity and conformation by measuring kinetics and computationally simulating the interactions. We found Cd²⁺ to be a slope-hyperbolic noncompetitive-inhibition reversible inhibitor of tyrosinase, with an IC₅₀ of 2.92±0.16mM and K_i of 0.23±0.02mM. Spectrofluorimetric measurements of intrinsic and ANS-binding fluorescence showed that Cd²⁺ did not induce significant changes to tyrosinase overall or to its regional active site conformations. Cd²⁺ showed its inactivation effect not by modulating apparent structural changes to tyrosinase, but by occupying binding sites. To gain further insight into the Cd²⁺/tyrosinase interaction, we performed computational docking and molecular dynamics simulations. The results consistently indicated that Cd²⁺ can interact with several residues near the tyrosinase active site, primarily HIS85 and ASN260. Our study provides insight into the mechanism of the toxic effects Cd²⁺ has on tyrosinase, which could affect the normal pigmentation pathway in animals.

Keywords: Cd(2+); Molecular dynamics; Tyrosinase inhibition.