Identification of Crucial Histidines for Heme Binding in the N-terminal Domain of the Heme-regulated eIF2α Kinase*

The heme-regulated eukaryotic initiation factor-2α (eIF2α) kinase (HRI) regulates the initiation of protein synthesis in reticulocytes. The binding of NO to the N-terminal heme-binding domain (NTD) of HRI positively modulates its kinase activity. By utilizing UV-visible absorption, resonance Raman, EPR and CD spectroscopies, two histidine residues have been identified that are crucial for the binding of heme to the NTD. The UV-visible absorption and resonance Raman spectra of all the histidine to alanine mutants constructed were similar to those of the unmutated NTD. However, the change in the CD spectra of the NTD construct containing mutation of His⁷⁸ to Ala (H78A) indicated loss of the specific binding of heme. The EPR spectrum for the ferric H78A mutant was also substantially perturbed. Thus, His⁷⁸ is one of the axial ligands for the NTD of HRI. Significant changes in the EPR spectrum of the H123A mutant were also observed, and heme readily dissociated from both the H123A and the H78A NTD mutants, suggesting that His¹²³ was also an axial heme ligand. However, the CD spectrum for the Soret region of the H123A mutant indicated that this mutant still bound heme specifically. Thus, while both His⁷⁸ and His¹²³ are crucial for stable heme binding, the effects of their mutations on the structure of the NTD differed. His⁷⁸ appears to play the primary role in the specific binding of heme to the NTD, acting analogously to the “proximal histidine” ligand of globins, while His¹²³ appears to act as the “distal” heme ligand.