Multicopper oxidases hephaestin

Based on present sequence data, known or likely ferroxidase enzymes can be identihed in several eukaryotes. These enzymes are listed in Table 11. All are multicopper oxidases, by sequence homology at least. In mammals, they include ceruloplasmin and, most likely, hephaestin (Hp), although only mouse Hp (mHp) has been characterized at this time (Vulpe et al., 1999). The alignments in Fig. 5A show that mHp is essentially... 

A second member of the ceruloplasmin family multicopper oxidases with six BCB domains was recently identified as the causative agent of sex-linked anemia (sla) in mice (Vulpe et al., 1993). It was named hephaes-tin and shown to be expressed mostly in the small intestine and the colon, where it is presumably involved in gastrointestinal iron uptake. Hephaes-tin displays a high level of sequence identity to ceruloplasmin and differs from it only by an additional C-terminal transmembrane domain, which anchors the protein to the cell membrane. A 582-nucleotide in-frame deletion in the mRNA for hephaestin sla mice has been identified compared to normal animals. The mice with such a mutation are unable to release iron from enterocytes (intestinal epithelial cells) into the circulation, which results in severe anemia. The GPI-anchored form of ceruloplasmin could potentially also mediate similar cellular iron efflux in the central nervous system. There is a transferrin-independent iron uptake system that requires Fe(III) to be reduced to Fe(II) at the cell surface for uptake to occur (DeSilva et al., 1996). Ceruloplasmin would oxidize Fe and prevent its uptake by this mechanism. Briefly, the role of ceruloplasmin is most likely to prevent excessive intracellular iron accumulation by tightly controlling iron efflux and inhibiting its uptake. 

AO = Ascorbate oxidase (h)Cp = (human) Ceruloplasmin CT = Charge transfer Hp = Hephaestin GPl = Glycosyl-phosphatidylinositol Lac = Laccase MCO = Multicopper oxidase T1(2,3)D = Type 1 depleted (and/or type 2 or type 3) Tf = Transferrin. 

Once inside the mucosal cell, iron then has to be transported across the membrane to serum transferrin. This appears to take place via the Iregl transporter protein (also known as ferroportin 1 or MTPl). Iregl is a transmembrane protein located at the basolateral membrane of the cell that has been shown to be involved in iron uptake. Oxidation of Iregl-bound ferrous iron and its release to transferrin is likely to be enhanced by the membrane-bound multicopper ferroxidase hephaestin. This protein is 50% identical to ceruloplasmin, a soluble protein identified as having a possible role in iron loading of transferrin see Copper Proteins Oxidases). Mutation of hephaestin in mice leads to a build up of iron in duodenal cells and overall iron deficiency in the body. ... 

The second class consists of multidomain blue copper proteins composed of exclusively two or more BCB domains and includes nitrite reductase (Section IV, E), multicopper blue oxidases such as laccase, ascorbate oxidase, ceruloplasmin, and hephaestin (Section VII), and some sequences found in extreme halophilic archaea (see Section V, E). 

Multicopper blue oxidases are synthesized as a single polypeptide chain, which is composed of three BCB domains in the case of laccases (LC) and ascorbate oxidases (AO) and six such domains in ceruloplasmin (CP) and hephaestin (HP). Structurally they are arranged in a triangular manner. These enzymes, along with heme-copper oxidases (cytochrome c oxidases and quinol-oxidases) and a cyanide-resistant alternative oxidase found in mitochondria of plants and fungi, are the only known enzymes capable of catalyzing four-electron reduction of dioxygen to water. In the... 

An important family of multicopper enzymes couple the reduction of O2 to H2O with substrate oxidation. They include ascorbate oxidase, ceruloplasmin, Fet3, hephaestin, and laccase, and contain at least four copper ions. The four Cu ions are distributed between one type 1 blue copper site, one type 2 site, and one type 3 copper site. The blue Type 1 site is usually located some 12—13 A distant from a trinuclear site which has the two Type 3 coppers, linked by a bridging oxygen and one Type 2 copper. We illustrate this class of oxidases with laccase which catalyses the four-electron reduction of O2 to water, coupled with the oxidation of small organic... 

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