THE STRUCTURE OF HUMAN CERULOPLASMIN AT 3.0Å RESOLUTION: THE BEGINNING OF THE END OF AN ENIGMA. Peter Lindley, Irina Zaitseva, Vjacheslav Zaitsev, Adam Ralph, Graeme Card, CCLRC Daresbury Laboratory, Warrington WA4 4AD, UK

Ceruloplasmin is a member of the multi-copper oxidase family of proteins which includes laccase and ascorbate oxidase. Sequence homology also suggests that it is structurally related to blood clotting factors V and VIII. The protein comprises a single polypeptide chain of 1046 amino acid residues and upto four glycan chains (7 - 8% by weight) giving an overall molecular weight of some 132 kDa

The X-ray structure of human ceruloplasmin has been elucidated at a resolution of 3.0 Å [1]. The structure reveals that the molecule is comprised of six plastocyanin-type domains arranged in a triangular array. There are six integral copper atoms, three of which form a trinuclear cluster sited at the interface of domains 1 and 6, and three mononuclear sites in domains 2, 4 and 6. The mononuclear copper in domain 6 and the trinuclear cluster form a four-copper oxidase centre almost identical to that found in ascorbate oxidase [2] and strongly suggesting an oxidase role for ceruloplasmin. Each of the mononuclear coppers is coordinated to a cysteine and two histidine residues and those in domains 4 and 6 also coordinate to a methionine residue to give typical "blue" type I copper centres. For the copper in domain 2 the methionine residue is substituted by a leucine and this copper is probably in the reduced state. The trinuclear cluster contains a pair of type III spin-paired coppers and a type II copper. The cluster is bound to the protein by eight histidine residues, four each from domains 1 and 6.

Ceruloplasmin has long been known as "the enigmatic blue plasma protein" since its precise functions have not been defined. The X-ray structure confirms that probability that the protein is multi-functional and suggests strong evidence for ferroxidase and anti-oxidant activity. The putative role of ceruloplasmin in iron metabolism and other fucntional aspects will be discussed.

[1] Zaitseva, I., Zaitsev, V., Card, G., Moshkov, K., Bax, B., Ralph, A. & Lindley, P. J. Biol. Inorg. Chem., 1, (1996), 1-9.

[2] Messerschmidt, A., Ladenstein, R., Huber, R., Bolognesi, M., Avigliano, L., Petruzelli, R., Rossi, A. & Finazzi-Agró, A. J. Mol. Biol., 224, (1992), 179-205.