Chemistry Reference
In-Depth Information
receptor-mediated endocytosis, apotransferrin retains a high affinity for its receptor at acidic pH values, and is
recycled back to the plasma membrane, where it dissociates from its receptor and goes off into the circulation in
search of further iron. This sequence of events constitutes the transferrin to cell cycle, which ensures iron uptake
by cells that have transferrin receptors.
2.
Copper and zinc
Copper uptake across the gastrointestinal tract is poorly understood
most probably utilising the divalent cation
transporter DMT1. At the cellular level, Cu is imported across the plasma membrane of mammalian cells as Cu
þ
,
by members of the CTR family. The CTR family of proteins have been found in yeast and plants, as we saw, but
also in humans and other mammals. They contain several methionine-rich motifs at their N-terminus, and
conserved cysteine and histidine residues at their C-terminus. Unusually, CTR proteins can mediate the uptake of
platinum anticancer drugs into mammalian cells (see Chapter 22).
The ZIP family are involved in Zn transport into the cytosol, mostly across the plasma membrane. Although
the human genome encodes 14 ZIP-related proteins, ZIP4 appears to mediate Zn uptake. It's involvement in
dietary Zn uptake into intestinal enterocytes is well established, and mutations in ZIP4 have been found in patients
with acrodermatitis enteropathica, a recessive disorder of Zn absorption which results in Zn deficiency. DMT1 is
probably also involved in the transport of dietary zinc across the brush border membrane of the intestine.
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