Unusual flexibility of distal and proximal histidine residues in the haem pocket of Drosophila melanogaster haemoglobin

Abstract

Several pH-dependent low-spin ferric haem forms are identified in a frozen solution of the ferric ¹²¹Cys→Ser mutant of Drosophila melanogaster haemoglobin (DmHb1*) using electron paramagnetic resonance (EPR) techniques. Different forms with EPR parameters typical of bis-histidine coordinated haem iron centers were observed. Strong pH-dependent changes in the EPR signatures were observed related to changes in the haem pocket. The pulsed EPR data indicate that both the distal and proximal histidine exhibit a large libration around the Fe-N(His) axis. The resonance Raman spectra of the CO-ligated ferrous form of Drosophila melanogaster haemoglobin are typical of an open conformation, with little stabilization of the CO ligand by the surrounding amino-acid residues. The EPR data of the cyanide-ligated ferric DmHb1* indicates a close similarity with cyanide-ligated ferric myoglobin. The structural characteristics of DmHb1* are found to clearly differ from those of other bis-histidine-coordinated globins.

The heme pocket of the globin domain of the globin-coupled sensor of Geobacter sulfurreducens — An EPR study

“…1). The rather narrow line widths, in comparison to the EPR spectra of other heme proteins, [13][14][15][16][17][18] indicate a relatively small g strain. g strain is induced by the occurrence of a range of (slightly different) molecular conformations in the heme-binding site.…”

“…Given the sensitivity of the principal g values to ligand rotations [21,23], it is safe to say that the other conformers observed in the crystal structure [7] are not present in the frozen solutions of ferric GsGCS 162 . It is not uncommon that different spectroscopic methods find different heme pocket conformers for the same globin [14,40]. Even contradictions between different X-ray diffraction studies of the same globin are reported [28,29].…”

“…Since the largest nuclear quadrupole value (in absolute values) of the pyrrole nitrogens lies in the heme plane perpendicular to the Fe-N pyrrole bond and the largest principal g value is along the heme normal, the orientation of the g-tensor frame versus the molecular frame can be determined [13,14,33]. Fig.…”

An N-Myristoylated Globin with a Redox-Sensing Function That Regulates the Defecation Cycle in Caenorhabditis elegans

“…Although the g max value of ferric human neuroglobin (NGB) and human cytoglobin (CYGB) is also significantly larger than 3 [63], the EPR features are still more typical of a normal rhombic or type-II EPR signal. One of the three ferric heme forms found for hemoglobin 1 of Drosophila melanogaster ( Dm Hb1) is characterized by a HALS-type EPR spectrum with even larger g max (3.50), but little is known about the specific ligation of the heme in this form or about the function of this form [64]. In the case of GLB-26, a bis-histidine coordination of the heme group (binding of E7His and F8His) is assumed [29].…”

“…For bis-imidazole-coordinated ferric porphyrins, large g max signals are indicative of dihedral angles between the imidazole planes larger than ∼60° [65]. Detailed information on the orientation of the imidazole planes relative to each other and to the heme can be obtained from a pulsed EPR analysis [64], [66]. However, since only the low-field EPR signal is clearly resolved in the case of GLB-26 (Fig.…”