3-Dimensional Structure Of A Hemichrome Hemoglobin From Caudina arenicola
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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 Angstrom resolution (R = 0.21 from 10.0 to 2.5 Angstrom resolution). Hb-C crystallizes in space group P2(1) with cell constants a=45.74, b=45.23 and c=40.92 Angstrom and beta=104.4 degrees with two monomers packed in the unit cell (V-m = 2.34 Angstrom(3) 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 a-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 3 sigma over background. Interestingly, all the heme groups present in the crystal are nearly coplanar.