3-DIMENSIONAL STRUCTURE OF A HEMICHROME HEMOGLOBIN FROM CAUDINA-ARENICOLA

The structure of a monomeric hemichrome form of an invertebrate hemogl obin, Hb-C chain, from Caudina arenicola has been refined to an R valu e 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 gro up 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 is omorphous replacement method with Hg2+ the major derivative. The struc ture consists of 157 amino acids with N- and C-terminal regions and ei ght a-helices forming a heme pocket. The unique feature of this struct ure is the hemichrome form with the proximal and distal histidines coo rdinated to the heme Fe atom, which is nearly in the plane of the porp hyrin ring. A total of 111 solvent molecules were added to the structu re using difference density peaks of at least 3 sigma over background. Interestingly, all the heme groups present in the crystal are nearly coplanar.