Three-dimensional structure of a hemichrome hemoglobin from Caudina arenicola

The structure of a monomeric hemichrome form of an invertebrate hemoglobin, Hb-C chain, from Caudina arenicola has been refined to an R value of 0.16 using the data from 5.0 to 2.5 Å resolution (R = 0.21 from 10.0 to 2.5 Å resolution). Hb-C crystallizes in space group P2_l with cell constants a = 45.74, b = 45.23 and c = 40.92 Å and β = 104.4° with two monomers packed in the unit cell (V_m = 2.34 ų Da⁻¹). The phases were determined by the multiple isomorphous replacement method with Hg²⁺ the major derivative. The structure consists of 157 amino acids with N- and C-terminal regions and eight α-helices forming a heme pocket. The unique feature of this structure is the hemichrome form with the proximal and distal histidines coordinated to the heme Fe atom, which is nearly in the plane of the porphyrin ring. A total of 111 solvent molecules were added to the structure using difference density peaks of at least 3σ over background. Interestingly, all the heme groups present in the crystal are nearly coplanar.