X-ray structure determination of a dimeric hemoglobin from Caudina arenicola.
The X-ray structure of a dimeric, cyanomet-liganded hemoglobin D-chain (Hb-D) from Caudina arenicola has been determined by the molecular-replacement method. The search model was a concatenated model of three hemoglobin structures superimposed on the backbone of monomeric, hemichrome hemoglobin C-chain (Hb-C) from the same organism. Hb-D crystallizes in space group P4(1)2(1)2 with cell constants a = b = 77.0 and c =61.5 A with one subunit in the asymmetric unit. The dimer twofold axis corresponds to a crystallographic twofold along one of the body diagonals of the unit cell. Rotation and translation searches as well as model refinement were carried out in X-PLOR with the final model having an R value of 0.19 using the data from 5.0 to 2.9 A resolution (R = 0.26 for 10.0 to 2.9 A resolution). The homodimeric structure of Caudina Hb-D features close heme-heme contacts with an Fe-Fe distance of 19.0 A. The subunit-subunit interface involves both the E and F helices from each subunit with the E helices oriented antiparallel at 50 degrees with respect to one another, similar to the quaternary structure observed for the homodimeric hemoglobin from Scapharca inaequivalvis.
Department of Chemistry and Biochemistry, University of Texas at Austin, 78712, USA.
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Mitchell
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Ernst
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Hackert