Effects of cadmium on the cuttlefish Sepia pharaonis' arginine kinase: unfolding kinetics integrated with computational simulations

Abstract

Arginine kinase is closely associated with adaptation to environmental stresses such as high salinity and heavy metal ion levels in marine invertebrates. In this study, the effects of Cd2+ on the cuttlefish Sepia pharaonis’ arginine kinase (SPAK) were investigated. SPAK was isolated from the muscles of S. pharaonis and upon further purification, showed a single band on SDS-PAGE. Cd2+ effectively inactivated SPAK, and the double-reciprocal kinetics indicated that Cd2+ induced non-competitive inhibition of arginine and ATP. Spectrofluorometry results showed that Cd2+ induced tertiary structure changes in SPAK with the exposure of hydrophobic surfaces that directly induced SPAK aggregation. The addition of osmolytes, glycine, and proline successfully blocked SPAK aggregation and restored the conformation and activity of SPAK. Molecular dynamics simulations involving SPAK and Cd2+ showed that Cd2+ partly blocks the entrance of ATP to the active site, and this result is consistent with the experimental results showing Cd2+-induced inactivation of SPAK. These results demonstrate the effect of Cd2+ on SPAK enzymatic function and unfolding, including aggregation and the protective effects of osmolytes on SPAK folding. This study provides concrete evidence of the toxicity of Cd2+ in the context of the metabolic enzyme SPAK, and it illustrates the toxic effects of heavy metals and detoxification mechanisms in cuttlefish