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-re placement method. The search model was a concatenated model of three h emoglobin structures superimposed on the backbone of monomeric, hemich rome hemoglobin C-chain (Hb-C) from the same organism. Hb-D crystalliz es in space group P4(1)2(1)2 with cell constants a = b = 77.0 and c = 61.5 Angstrom with one subunit in the asymmetric unit. The dimer twofo ld axis corresponds to a crystallographic twofold along one of the bod y diagonals of the unit cell. Rotation and translation searches as wel l 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 Angstrom reso lution (R = 0.26 for 10.0 to 2.9 Angstrom resolution). The homodimeric structure of Caudina Hb-D features close heme-heme contacts with an F e-Fe distance of 19.0 Angstrom. The subunit-subunit interface involves both the E and F helices from each subunit with the E helices oriente d antiparallel at 50 degrees with respect to one another, similar to t he quaternary structure observed for the homodimeric hemoglobin from S capharca inaequivalvis.