CRYSTALLOGRAPHIC ANALYSIS OF OXYGENATED AND DEOXYGENATED STATES OF ARTHROPOD HEMOCYANIN SHOWS UNUSUAL DIFFERENCES

The X-ray structure of an oxygenated hemocyanin molecule, subunit II o f Limulus polyphemus hemocyanin, was determined at 2.4 Angstrom resolu tion and refined to a crystallographic R-factor of 17.1%. The 73-kDa s ubunit crystallizes with the symmetry of the space group R32 with one subunit per asymmetric unit forming hexamers with 32 point group symme try. Molecular oxygen is bound to a dinuclear copper center in the pro tein's second domain, symmetrically between and equidistant from the t wo copper atoms. The copper-copper distance in oxygenated Limulus hemo cyanin is 3.6 +/- 0.2 Angstrom, which is surprisingly 1 Angstrom less than that seen previously in deoxygenated Limulus polyphemus subunit I I hemocyanin (Hazes et al., Protein Sci. 2:597, 1993). Away from the o xygen binding sites, the tertiary and quaternary structures of oxygena ted and deoxygenated Limulus subunit II hemocyanins are quite similar. A major difference in tertiary structures is seen, however, when the Limulus structures are compared with deoxygenated Panulirus interruptu s hemocyanin (Volbeda, A., Hol, W.G.J. J. Mol. Biol. 209:249, 1989) wh ere the position of domain 1 is rotated by 8 degrees with respect to d omains 2 and 3. We postulate this rotation plays an important role in cooperativity and regulation of oxygen affinity in all arthropod hemoc yanins. (C) 1994 Wiley-Liss, Inc.