CCLRC Daresbury Laboratory, Warrington, WA4 4AD , UK
Human Ceruloplasmin (hCP) is a copper-containing glycoprotein with a molecular weight of 132 kDa. hCP belongs to a family of multinuclear "blue" oxidases along with ascorbate oxidase and laccase and it is comprised of a single chain of 1046 amino acids with a carbohydrate content of 7% to 8%. The precise functions of hCP have not been defined, but it has been associated with ferroxidase, amine oxidase, antioxidant activities and copper transport [1]. Sequence analysis of the protein indicates a domain structure involving internal triplication and an extraodinary homology with A-type domains of blood clotting factor VIII [2].
The trigonal crystals of hCP diffract to at least 3.0 Å using the synchrotron source at Daresbury Laboratory. The structure has been solved using conventional MIR methods in conjunction with solvent flattening and histogram matching. Refinement has been undertaken using both simulated annealing and restrained least-squares methods to yield an conventional R-factor of 21.5% for all data in the resolution range 12.0-3.0 Å [3].
The structure reveals that the molecule is comprised of six cupredoxin-type domains with large loop insertions. There are six integral copper atoms, three of which form a trinuclear cluster sited at the interface of domain 1 and 6 and mononuclear sites in domains 2, 4 and 6. The trinuclear centre and the mononuclear copper in domain 6 form a cluster essentially the same as that found in ascorbate oxidase (AO)[4], strongly suggesting an oxidase role for hCP in the plasma. The stucture comparison of hCP with AO and nitrite reductase [5] leads to a concept that these multicopper proteins have evolved from a two domain ancestral protein which had type I coppers in both domains.
[1] Laurie, S.N. & Mohammed, E.S. Coord.Chem.Rev (1980), 33:273-313
[2] Church, W.R. et al. Proc.Natl.Acad.Sci USA (1984) , 81:6935-6937
[3] Zaitseva ,I. et al. J.Biol.Inorg.Chem.(1996), 1:15-23
[4] Messerschmidt, A. et al. J.Mol.Biol (1989), 224:179-205
[5] Godden, J.W. et al. Science (1991), 253:438-442