The apparently symmetrical hexagonal bilayer hemoglobin from Lumbricus terrestris has a large dipole moment

Abstract

The giant ∼3.6 MDa hexagonal bilayer hemoglobin (HBL Hb) from Lumbricus terrestris consists of 12 213-kDa dodecamers of four globin chains ([b+a+c] 3[d] 3) tethered to a central scaffold of ∼36 non-globin, linker subunits L1–L4 (24–32 kDa). Three-dimensional reconstructions obtained by electron cryomicroscopy showed it to have a D 6 point-group symmetry, with the two layers rotated ∼16° relative to each other. Measurement of the dielectric constants of the Hb and the dodecamer over the frequency range 5–100 kHz indicated relaxation frequencies occurring at 20–40 and 300 kHz, respectively, substantially lower than the 700–800 kHz in HbA. The dipole moments calculated using Oncley's equation were 17 300±2300 D and 1400 D for the Hb and dodecamer, respectively. The approximately threefold higher dipole moment of the dodecamer relative to HbA is consistent with an asymmetric shape in solution suggested by small-angle X-ray scattering. Although a two-term Debye equation and a prolate ellipsoid of revolution model provided a good fit to the experimental dielectric dispersion of the dodecamer, a three-term Debye equation based on an oblate ellipsoid of revolution model was required to fit the asymmetric dielectric dispersion curve of the Hb: the required additional term may represent either an induced dipole moment or a substructure which rotates independently of the main permanent dipole component of the Hb. The D 6 point-group symmetry implies that the dipole moments of the dodecamers cancel out. Thus, in addition to a possible contribution from fluctuations of the proton distribution, the large dipole moment of the Hb may be due to an asymmetric distribution of the heterogeneous linker subunits.