Eukaryotic initiation factor 2α kinase is a nitric oxide‐responsive mercury sensor enzyme: Potent inhibition of catalysis by the mercury cation and reversal by nitric oxide

Abstract

The activity of one of the eukaryotic initiation factor 2a kinases, heme-regulated inhibitor (HRI), is modulated by heme binding. Here, we demonstrate for the first time that Hg 2+ strongly inhibits the function of HRI (IC 50 = 0.6 lM), and nitric oxide fully reverses this inhibition. Other divalent metal cations, such as Fe 2+ , Cu 2+ , Cd 2+ , Zn 2+ and Pb 2+ , also significantly inhibit kinase activity with IC 50 values of 1.9-8.5 lM. Notably, inhibition by cations other than Hg 2+ is not reversed by nitric oxide. Our present data support dual roles of Hg 2+ and nitric oxide in the regulation of protein synthesis during cell emergency states.

Elucidation of the Heme Binding Site of Heme-regulated Eukaryotic Initiation Factor 2α Kinase and the Role of the Regulatory Motif in Heme Sensing by Spectroscopic and Catalytic Studies of Mutant Proteins

“…Spectroscopic evidence on truncated HRI mutant proteins indicates that the nitrogen atom of a His residue in the N-terminal domain and the sulfur atom of a Cys residue in the C-terminal domain are the heme axial ligands for HRI under normal conditions (7,8). Hg 2ϩ strongly suppresses HRI catalysis with an IC 50 value of 0.6 M, which is restored by NO, supporting the theory that the Cys thiolate is involved catalytic regulation (12). An earlier study shows that two His residues are heme axial ligands of the isolated N-terminal domain of HRI (13), distinct from the His and Cys residues proposed for full-length HRI (7,8).…”

Heme-Regulated Inhibitor (HRI) eIF2α Kinase and the Heme-Regulated Turnover of Cytochromes P450 and Other Hepatic Proteins

“…Interaction with HRI Cys409 heme axial-ligand induced a shift in heme coordination from the 6-coordinate low spin to the 5-coordinate high spin HRI species and involved an inactive metalated HRI complex due to covalent interactions between Hg(II) and the Cys-SH. 75 Intriguingly, HRI inhibition by Hg(II), but not other metals, is reversed by NO and apparently involves its interaction with three other reactive HRI Cys-residues as well. 75 …”

“…75 Intriguingly, HRI inhibition by Hg(II), but not other metals, is reversed by NO and apparently involves its interaction with three other reactive HRI Cys-residues as well. 75 …”

“…75 Of these, Hg(II) is the most potent, with an IC 50 of 0.6 µM. 75 Such inhibition is apparently due to Hg(II) interactions with HRI Cys and His residues.…”

“…75 Of these, Hg(II) is the most potent, with an IC 50 of 0.6 µM. 75 Such inhibition is apparently due to Hg(II) interactions with HRI Cys and His residues. Interaction with HRI Cys409 heme axial-ligand induced a shift in heme coordination from the 6-coordinate low spin to the 5-coordinate high spin HRI species and involved an inactive metalated HRI complex due to covalent interactions between Hg(II) and the Cys-SH.…”