Multi-copper oxidases (MCOs) are a small group of enzymes that oxidize their substrate with the concomitant reduction of dioxygen to two water molecules. Generally, multi-copper oxidases are promiscuous with regards to their reducing substrates and are capable of performing various functions in different species. To date, three multi-copper oxidases have been detected in humans—ceruloplasmin, hephaestin and zyklopen. Each of these enzymes has a high specificity towards iron with the resulting ferroxidase activity being associated with ferroportin, the only known iron exporter protein in humans. Ferroportin exports iron as Fe2+, but transferrin, the major iron transporter protein of blood, can bind only Fe3+ effectively. Iron oxidation in enterocytes is mediated mainly by hephaestin thus allowing dietary iron to enter the bloodstream. Zyklopen is involved in iron efflux from placental trophoblasts during iron transfer from mother to fetus. Release of iron from the liver relies on ferroportin and the ferroxidase activity of ceruloplasmin which is found in blood in a soluble form. Ceruloplasmin, hephaestin and zyklopen show distinctive expression patterns and have unique mechanisms for regulating their expression. These features of human multi-copper ferroxidases can serve as a basis for the precise control of iron efflux in different tissues. In this manuscript, we review the biochemical and biological properties of the three human MCOs and discuss their potential roles in human iron homeostasis.

The YFF1 is a new universal molecular mechanic force field designed for drug discovery purposes. The electrostatic part of YFF1 has already been parameterized to reproduce ab initio calculated dipole and quadrupole moments. Now we report a parameterization of the van der Waals interactions (vdW) for the same atom types that were previously defined. The 6-12 Lennard-Jones potential terms were parameterized against homodimerization energies calculated at the MP2/6-31 G level of theory. The Boys-Bernardi counterpoise correction was employed to account for the basis-set superposition error. As a source of structural information we used about 2,400 neutral compounds from the ZINC2007 database. About 6,600 homodimeric configurations were generated from this dataset. A special "closure" procedure was designed to accelerate the parameters fitting. As a result, dimerization energies of small organic compounds are reproduced with an average unsigned error of 1.1 kcal mol(-1). Although the primary goal of this work was to parameterize nonbonded interactions, bonded parameters were also derived, by fitting to PM6 semiempirically optimized geometries of approximately 20,000 compounds.

Hephaestin is a multicopper ferroxidase expressed mainly in the mammalian small intestine. The ferroxidase activity of hephaestin is thought to play an important role during iron export from intestinal enterocytes and the subsequent iron loading of the blood protein transferrin, which delivers iron to the tissues. Structurally, the ectodomain of hephaestin is predicted to resemble ceruloplasmin, the soluble ferroxidase of blood. In this study, the human hephaestin ectodomain was expressed in baby hamster kidney cells and purified to electrophoretic homogeneity. Ion exchange chromatography of purified recombinant human hephaestin (rhHp) resulted in the isolation of hephaestin fractions with distinct catalytic and spectroscopic properties. The fraction of rhHp with the highest enzymatic activity also showed an enhanced molar absorptivity at 600 nm, characteristic of type 1 copper sites. Kinetic analysis revealed that rhHp possesses both high-affinity and low-affinity binding sites for ferrous iron. To investigate the role of particular residues in iron specificity of hephaestin, mutations of putative iron ligands were introduced into rhHp using site-directed mutagenesis. Kinetic analysis of ferroxidation rates of wild-type rhHp and mutants demonstrated the important roles of hephaestin residues E960 and H965 in the observed ferroxidase activity.

Corticosteroid-binding globulin (CBG) was isolated from chicken serum and identified by mass spectrometry and genomic analysis. This revealed that the organization and synteny of avian and mammalian SerpinA6 genes are conserved. Recombinant zebra finch CBG steroid-binding properties reflect those of the natural protein in plasma and confirm its identity. Zebra finch and rat CBG crystal structures in complex with cortisol resemble each other, but their primary structures share only ∼40% identity, and their steroid-binding site topographies differ in several unexpected ways. Remarkably, a tryptophan that anchors ligands in mammalian CBG steroid-binding sites is replaced by an asparagine. Phylogenetic comparisons show that reptilian CBG orthologs share this unexpected property. Glycosylation of this asparagine in zebra finch CBG does not influence its steroid-binding affinity, but we present evidence that it may participate in protein folding and steroid-binding site formation. Substitutions of amino acids within zebra finch CBG that are conserved only in birds reveal how they contribute to their distinct steroid-binding properties, including their high (nanomolar) affinities for glucocorticoids, progesterone, and androgens. As in mammals, a protease secreted by Pseudomonas aeruginosa cleaves CBG in zebra finch plasma within its reactive center loop and disrupts steroid binding, suggesting an evolutionarily conserved property of CBGs. Measurements of CBG mRNA in zebra finch tissues indicate that liver is the main site of plasma CBG production, and anti-zebra finch CBG antibodies cross-react with CBGs in other birds, extending opportunities to study how CBG regulates the actions of glucocorticoids and sex steroids in these species.

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