Metal Homeostasis

Iron (Fe)#

The most tightly regulated metal in the body, controlled by the Hepcidin-ferroportin axis.^{[1]Mechanism of Iron Ion Homeostasis in Intestinal Immunity and Gut Microbiota RemodelingHonghong Bao, Yi Wang, Hanlin Xiong et al. · 2024Open reference 1 ↓}

Import: Dietary non-heme iron reduced to iron(II) (Fe2+) by DcytB, transported into enterocytes via DMT1 (SLC11A2). Heme iron imported via HCP1. Storage: Intracellular ferritin sequesters excess iron as a mineralized core; liver is the primary storage organ.

Regulation: Hepcidin (hepatic peptide hormone) binds ferroportin, causing its internalization and degradation, blocking iron export from enterocytes and macrophages. Hepcidin is increased by inflammation (IL-6/STAT3) and iron loading; decreased by iron deficiency, erythropoietic demand, and hypoxia.

Export: Ferroportin (SLC40A1) is the sole known cellular iron exporter; hephaestin/ceruloplasmin oxidize iron(II) to iron(III) for transferrin loading. Sensing: Iron regulatory proteins (IRP1/IRP2) bind iron-responsive elements (IREs) in mRNA, post-transcriptionally controlling ferritin, ferroportin, DMT1, and transferrin receptor expression.

Zinc (Zn)#

Import: ZIP family transporters (SLC39A, 14 members) move zinc (Zn) into cytoplasm from extracellular space or organelles. Export/sequestration: ZnT family (SLC30A, 10 members) move zinc out of cytoplasm into organelles or extracellular space. Regulation: MTF-1 (metal-responsive transcription factor) senses cytoplasmic zinc and activates metallothionein and ZnT1 transcription.

Clinical: ZIP4 mutations cause acrodermatitis enteropathica (severe zinc deficiency); ZnT8 autoantibodies are a marker of type 1 diabetes.

Copper (Cu)#

Import: CTR1 (SLC31A1) is the primary copper importer; requires reduction of copper(II) (Cu2+) to copper+ by STEAP reductases. Intracellular trafficking: Copper chaperones (CCS for SOD1, Cox17 for cytochrome c oxidase, ATOX1 for ATP7A/B) deliver copper to specific targets.

Export: ATP7A (Menkes protein) in intestine and most tissues; ATP7B (Wilson protein) in liver for biliary excretion and ceruloplasmin loading. Clinical: Menkes disease (ATP7A loss) causes systemic copper deficiency; Wilson's disease (ATP7B loss) causes hepatic/neurological copper overload.

Pathogen Metal Homeostasis#

Microbes face a dual challenge: acquiring essential metals from a host that actively withholds them Nutritional Immunity (Metal Sequestration), while defending against metal toxicity weaponized by immune cells.^{[2]Metal Ion Acquisition in Staphylococcus aureus: Overcoming Nutritional ImmunityJames E. Cassat, Eric P. Skaar · 2012Open reference 2 ↓}

Fur (ferric uptake regulator): Master iron (Fe)-sensing transcription factor in most bacteria; represses siderophore genes when iron is sufficient. NikR: Nickel-responsive regulator in H. pylori; controls Urease and nickel transporter expression. Zur: Zinc uptake regulator; controls zinc (Zn) import and ZnuABC transporter expression.

MntR: Manganese-responsive regulator controlling manganese (Mn) import. Siderophore systems: High-affinity iron chelators Siderophores and Metallophores produced under iron limitation; enterobactin, pyoverdine, mycobactin, staphyloferrin. Efflux pumps: CzcCBA (cadmium (Cd)/zinc/cobalt (Co)), CopA (copper (Cu)), and others protect bacteria from metal toxicity.