Ceruloplasmin structure

It has also been proposed that because of sequence and possible structural homology with the blood clotting factors V and VIII [6] that ceruloplasmin may also participate in blood clotting and its regulation [5]. 

Various spectroscopic methods have been used to probe the nature of the copper centers in the members of the blue copper oxidase family of proteins (e.g. see ref. 13). Prior to the X-ray determination of the structure of ascorbate oxidase in 1989, similarities in the EPR and UV-vis absorption spectra for the blue multi-copper oxidases including laccase and ceruloplasmin had been observed [14] and a number of general conclusions made for the copper centers in ceruloplasmin as shown in Table 1 [13,15]. It was known that six copper atoms were nondialyzable and not available to chelation directly by dithiocarbamate and these coppers were assumed to be tightly bound and/or buried in the protein. Two of the coppers have absorbance maxima around 610 nm and these were interpreted as blue type I coppers with cysteine and histidine ligands, and responsible for the pronounced color of the protein. However, they are not equivalent and one of them, thought to be involved in enzymatic activity, is reduced and reoxidized at a faster rate than the second (e.g. see ref. 16). There was general concurrence that there are two type HI... 

Figure 1. (a) A schematic representation of the overall organization of the molecule of human ceruloplasmin. Domains 2,4, and 6 contain mononuclear copper centers, while the trinuclear copper cluster is located at the interface of domains 1 and 6. (b) An a-carbon ribbon diagram of the human ceruloplasmin molecule viewed along the pseudo threefold axis highlighting the triplication of the structure. Domains 1, 3, and 5 are depicted by striped motifs, whereas domains 2, 4, and 6 are dark shaded. The copper... 

Figure 6. Structural relationships between ascorbate oxidase, ceruloplasmin, nitrite reductase, and blood clotting factor VIII.

The present volume is the fourth in the series and covers the topics lithium in biology, the structure and function of ceruloplasmin, rhenium complexes in nuclear medicine, the anti-HIV activity of macrocyclic polyamines and their metal complexes, platinum anticancer dmgs, and functional model complexes for dinuclear phosphoesterase enzymes. The production of this volume has been overshadowed by a very sad event—the passing away of the senior editor, Professor Robert W. Hay. It was he who conceived the idea of producing this series and who more than anyone else has been responsible for its continuation. A tribute by one of his many friends, Dr. David Richens, is included in this Volume. 

Table 5.2 contains data about selected copper enzymes from the references noted. It should be understood that enzymes from different sources—that is, azurin from Alcaligenes denitrificans versus Pseudomonas aeruginosa, fungal versus tree laccase, or arthropodan versus molluscan hemocyanin—will differ from each other to various degrees. Azurins have similar tertiary structures—in contrast to arthropodan and molluscan hemocyanins, whose tertiary and quaternary structures show large deviations. Most copper enzymes contain one type of copper center, but laccase, ascorbate oxidase, and ceruloplasmin contain Type I, Type II, and Type III centers. For a more complete and specific listing of copper enzyme properties, see, for instance, the review article by Solomon et al.4... 

Copper oxidases are widely distributed in nature, and enzymes from plants, microbes, and mammals have been characterized (104,105). The blue copper oxidases, which include laccases, ascorbate oxidases, and ceruloplasmin, are of particular interest in alkaloid transformations. The principle differences in specificity of these copper oxidases are due to the protein structures as well as to the distribution and environment of copper(II) ions within the enzymes (106). While an in vivo role in metabolism of alkaloids has not been established for these enzymes, copper oxidases have been used in vitro for various alkaloid transformations. 

The location of the copper with respect to the Greek key fold is interesting when compared to that of the cupredoxins. While the copper in the cupredoxins lies in the interior of the /8 barrel bound by three interior-facing residues of the carboxy-terminal loop in the )8 barrel, and by a histidine in an adjacent strand, the copper in SOD lies on the outside of its jS barrel, bound by one residue from the carhoxy-terminal loop and three from the adjacent strand (cf. Figs. 2c-5c with Fig. 8c.) A structural comparison of plastocyanin and SOD, coupled with sequence alignment of plastocyanin and ceruloplasmin (Ryden, 1988), showed that three of the SOD ligands correspond to putative copper ligands in ceruloplasmin. Why this is so will become more evident after the description of the ascorbate oxidase structure and its relationship to ceruloplasmin. 

With the structure of ascorbate oxidase in hand, a new structurally based alignment of the sequences of ascorbate oxidase, laccase, and ceruloplasmin has been performed (Messerschmidt and Huber, 1990). In brief, while gene triplication for ceruloplasmin is still revelant, its sequence can be further subdivided into two domains per unit of triplicated sequence, or six domains in total. Each of these sequences bears some resemblance to each of the three domains of ascorbate oxidase, as does each of the two domains in laccase. The coppers of the trinuclear site of ceruloplasmin then are predicted to be bound between domains 1 and 6, with a type I site also lying in both domains 6 and 4 (see Huber, 1990). The relative orientation of each of these domains is not predicted by this alignment, but it turns out that the structure of nitrite reductase may shed some light on this (see Section V,C). 

Studies of other sources of ceruloplasmin may eventually prove useful in structure elucidation, but have already clarified some of the copper chemistry. Ceruloplasmin from goose serum has been isolated, purified, and characterized. This ceruloplasmin has less carbohydrate attached, but two forms may be isolated under some conditions. It is clear that these are not products of proteolytic degradation, but perhaps they might have a different carbohydrate attached. The two type I sites have higher extinction coefficients than type I sites in other ceruloplasmins, reflecting a modestly different environment (Hilewicz-Grabska et al, 1988). 

Factor IXa causes a rapid activation of factor X only if Ca2+, phospholipid,553 554 and the accessory factor Villa555 are present. The IXa Villa complex acts on X about 2 x 105 times faster than does IXa alone. This complex cleaves the same bonds in X as does the VIIa Va complex formed in the tissue factor pathway.514 The 2332-residue factor VIII and factor V have similar structures that include three repeats of a domain homologous to the blue copper-containing plasma protein ceruloplasmin (Chapter 16).556-559 Tyrosine 1680 of VIII apparently must be converted to a sulfate ester for full activity.560... 

Ceruloplasmin is an enzyme synthesised in the liver which contains six atoms of copper in its structure. Ceruloplasmin carries 90% of plasma copper the other 10% is carried by albumin. Ceruloplasmin exhibits a copper-dependent oxidase activity, which is associated with possible oxidation of Fe + (ferrous iron) into Fe (ferric iron), therefore assisting in its transport in the plasma in association with transferrin, which can only carry iron in the ferric state. 

---