The biology of mammalian multi-copper ferroxidases

Abstract

The mammalian multicopper ferroxidases (MCFs) ceruloplasmin (CP), hephaestin (HEPH) and zyklopen (ZP) comprise a family of conserved enzymes that are essential for body iron homeostasis. Each of these enzymes contains six biosynthetically incorporated copper atoms which act as intermediate electron acceptors, and the oxidation of iron is associated with the four electron reduction of dioxygen to generate two water molecules. CP occurs in both a secreted and GPI-linked (membrane-bound) form, while HEPH and ZP each contain a single C-terminal transmembrane domain. These enzymes function to ensure the efficient oxidation of iron so that it can be effectively released from tissues via the iron export protein ferroportin and subsequently bound to the iron carrier protein transferrin in the blood. CP is particularly important in facilitating iron release from the liver and central nervous system, HEPH is the major MCF in the small intestine and is critical for dietary iron absorption, and ZP is important for normal hair development. CP and HEPH (and possibly ZP) function in multiple tissues. These proteins also play other (non-iron-related) physiological roles, but many of these are ill-defined. In addition to disrupting iron homeostasis, MCF dysfunction perturbs neurological and immune function, alters cancer susceptibility, and causes hair loss, but, despite their importance, how MCFs co-ordinately maintain body iron homeostasis and perform other functions remains incompletely understood.

Keywords: Ceruloplasmin; Copper; Ferroxidase; Hephaestin; Iron; Zyklopen.

Fig. 1

Nature of the mammalian MCFs. A Recombinant mouse HEPH (without its transmembrane domain), generated as described by Deshpande et al. (2017), showing the intense blue color typical of the MCFs and related blue copper proteins. B General structure of the four forms of mammalian MCFs. From left to right: secreted CP; GPI-CP; and the two MCFs with single transmembrane domains, ZP and HEPH. The ribbon diagrams of each protein were generated using AlphaFold (https://alphafold.ebi.ac.uk/entry/Q9BQS7) (Jumper et al. 2021). Created with BioRender.com.

Fig. 2

Relative contributions of CP, HEPH and ZP to iron export from selected tissues. We now know that the MCFs contribute to iron export from multiple tissues, and often from specific cell types within those tissues. For example, in the brain, the MCFs are important for intra-organ iron trafficking. In this diagram, CP, HEPH and ZP are represented by blue, green and yellow balls respectively. For each tissue, the size of the ball represents the relative contribution of that MCF to iron transport from (or within) that tissue. For example, CP appears to be largely, if not solely, responsible for enhancing iron efflux from the liver, whereas in the duodenum, HEPH provides the majority of the ferroxidase activity, with CP contributing, albeit less efficiently, in some circumstances. In a number of tissues, both CP and HEPH appear to contribute significantly. The contributions of ZP to body iron homeostasis have yet to be firmly established, but its role in hair development is well supported. It is presumed a ferroxidase is required for efficient iron transport by the placenta to the fetus, but the identity of that ferroxidase remains uncertain, and is represented here by a grey ball.