Ctr1

Ctr1

We are excited to share with the structure of the Cu+ transporter Ctr1 (Ren 2019).   Feifei Ren and coworkers genetically engineered the Ctr1 transporter from the Atlantic salmon, Salmo salar.  This salmon sCtr1 transporter has 78% sequence identity to our human hCtr1.  TM 1-3 are transmembrane domains.   We have been interested in this Cu+ transporter for a long time.  Because Ctr1 prefers  , our thought is that those taking a dietary copper supplement might prefer Cu(I)NA2 in the +1 over other dietary supplements in the +2  oxidation state.

The structure of Ctr1

-Ta6Br12

Ctr1_1a
Figure 1, From Ren (2019) supplemental figure 1. The green bars are the three trans membrane domains.  The selectivity methionines in TM2 are shaded in red.

The Cu+ binding amino acids are in red.  Note the  positively charged lysines (K, 175-176)  and hydrophobic amino acids tryptophan, tyrosine, and phenylalanine (W, Y, F).  Alanine(A), leucine (L), and valine (V) are also pretty hydrophobic.

Ctr1_1b
Figure 2, from Ren supplemental figure 2. a. These authors shaved off the N-terminus and replaced an intracellular loop with another protein. b. a polyacrylamide gel, not shown  c. Crystal structure of sCtr1 crystal with Ta6Br12 (green and magenta spheres) bound to the fusion protein BRIL d.  How the fusion protein packs in a crystal.

Figure 2 may be more than what the average reader wants to know.  Hopefully it will not distract from what is inside this very interesting channel.  Tantalem bromide, Ta6Br12. and the BRIL, whose structure is known, appear to be there as reference points for solving the structure of the salmon Ctr1 channel..

Ctr1_3
Figure 3, from  Figure 5 Ren (2019) a. Color coded charge on the interior of Ctr1 b. a cartoon version of the entrance, selectivity, filter, and central cavity.

Red in Fig 3a is the code for negative charges.  Blue is positive, just like Cu+.  One can imagine that Cu+ will not “want ” to stay long in the central cavity with a combination of positively charged amino acids and some hydrophobic ones too. Cu+ might be drawn to the gate, and then to the intracellular space.

Exploring on your own

You may enjoy an interactive exploration of this structure at the Protein Databank.  These beautiful (we think) Ctr1 images were created with the NGL viewer .  The feature image was drawn using the hydrophobicity scale.  Water loving amino acids like glutamic and aspartic acid as well as lysine and arginine are red.  Hydrophobic amino acids like tryptophan and phenylalanine are green.

 Size Matters

Size matters is part of why we thoguht Ctr1 Cu+ transporter prefers Cu+  over Cu2+ This is our thinking before Ren (2019)

A monovalent cation like Na+ or Cu+ is surrounded by water. The oxygen of water has a partial negative charge (δ-) because it has a bigger nucleus with more protons and therefore likes electrons more than the single proton nucleus of hydrogen (δ+). Divalent cations like Mg2+ and Cu2+ have a higher charge density and the ability to form a larger hydration shell. This water has to be removed before going through any ion channel.

Ctr1_2
Figure 4 Divalent cations tend to be more hydrated than monovalent cations.

Back in the 1990s I hung out with electrophysiologists who discussed the need of stripping the waters of hydration before they could pass through ion channels.  While divalent cations might be smaller due to fewer valence electrons, they have more waters of hydration.  It would seem that the limited hydration of Cu+  could be stripped by that methionine selectivity filter.

Cu+ selectivity begins outside the cell

Haas and coworkers (2011) measured the Cu+  and Cu2+ binding affinities of the extracellular domains of Ctr1 subunits. These domains help insure that Cu2+ does not enter the channel. A lot of mathematics in this publication will be skipped. Briefly, the authors took the extracellular Ctr1 metal binding domain and replaced histidines with alanines. In the figures that we present, they looked at reduction of Cu2+ to Cu+ .

Ctr1_5
Figure 5  Human Ctr1 assembles as a homotrimer in the plasma membrane with an amino-terminal extracellular domain equipped with His (H) and Met (M) potential metal-binding residues. All extracellular His and Met residues are indicated. The functionally critical TMD2 methionine residues and carboxyl-terminal cysteine and histidine residues are indicated.

These authors ignored the methionine motif that previous studies showed bound  Cu+ with high affinity.   They were more concerned with the histidines that are not present in the yeast version of Ctr1.  Cu2+ binding to histidine is pH dependent.  Yeast acidify their environment.  Besides, we humans have more Ctr1 in the neutral pH of our small intestine, Figure 8.  Haas and coworkers were also interested in an ATCUN motif, or “amino terminal Cu and Ni binder” that is present in human albumin, an abundant blood protein.  We’ve covered some recent experiments on the ATCUN motif.  Some think that Cu2+  bound to this motif may contribute to Alzheimer’s Disease.  We’ve addressed this potential impact of Cu2+ bound to albumin.

When Cu2+  binds…

Ctr1_6
 Figure 6, adapted from Hass (2011) Absorbance spectra of Ctr1 model peptides (~500 μM) with 1 equivalent of Cu2+ at pH 7.4 in HEPES buffer. Peptides with His in the third position from the amino-terminus display absorbance at 525 nm characteristic of a typical ATCUN Cu2+ binding site. Substitution of all His-to-Ala, individual substitution of the His at position 3 (H3A) or acetyl capping of the N-terminus (Ac-Ctr1-14) results in a different type of binding site and a shift in λmax compared to the wild-type peptide (Ctr1-14). In HA all three His are replaced by Ala. The peptide labeled Ctr1-14(MNle) refers to the sequence where all 4 Met residues are replaced by Nle while all His residues remain wild type, whereas MNleH56A contains all Met-to-Nle replacements in addition to replacement of both His 5 and 6 with Ala. Finally, Ac-Ctr1-14 contains the native sequence but lacks a free amino-terminal amine due its acetyl cap.  This cap is to prevent binding to Cu2+

 Absorbance of oxidized Cu2+peptide complexes are known. let’s reduce them

Differences in the ability of ascorbate to reduce Cu2+ complexes of the various peptides were measured in the presence of HEPES buffer at pH 7.4. The experiments were conducted under aerobic conditions where availability of oxygen would allow cycling of copper oxidation states (i.e. after being reduced by ascorbate, the Cu+ could be re-oxidized by O2 from the air). However, if the peptides are able to stabilize Cu+ then the re-oxidation to Cu2+ will be slowed and Cu+ complex should accumulate. In fact, this seems to be the case with some of the peptides and clear differences can be noted depending on the presence of the fifth and sixth position His.  It is assumed that whole human serum albumin (HSA) protein was used.  This protein has a Cu2+   binding site at His27.

Ctr1_7
Ascorbate-dependent reduction of Cu2+ in complex with Ctr1-14 model peptides containing the ATCUN site. Solutions of 500 μM peptide- Cu2+ complex and 1 mM ascorbate in 50 mM HEPES at pH 7.4 monitored for 1 hour with UV-Vis at 525 nm, the characteristic absorbance band due to the Cu2+ complex.

Reduction of Cu2+  on Ctr1 is a very slow process

Note the scale on this graph.  It takes an hour at room O2  and accorbate to reduce the Cu2+ complex absorbance by 90%.  Ctr1 expression is good in the duodenum, Figure 8.

Ctr1_8
Figure 8 Issues with absorbing via Ctr1 A. expression of Ctr1, from Protein Atlas. B. pH of the GI tract and transit times of food.

So say we take a CuCl2 supplement

Does the the Cu2+  just bind to the N-terminus of Ctr1 of our duodenum and wait for some ascorbate or another reducing agent to float by?  Will it take longer if the   O2 content of our duodenum is less than room O2 ? Many have speculated on a   Cu2+ reductase next to Ctr1.  So far, no one has found it.

The Mitosynergy store sells   Cu(I)NA2 powder as well as an encapsulated form in  Vegan, stomach safe capsules.  Some customers report greater results from the stomach protected capsules than the powder.

References

Haas KL, Putterman AB, White DR, Thiele DJ, Franz KJ.(2011) Model peptides provide new insights into the role of histidine residues as potential ligands in human cellular copper acquisition via Ctr1. J Am Chem Soc. 33(12):4427-37. [PMC free article]

Ren F, Logeman BL, Zhang X, Liu Y, Thiele DJ, Yuan P. (2019) X-ray structures of the high-affinity copper transporter Ctr1. Nat Commun. 2019 Mar 27;10(1):1386. [PMC free article]

Published by BL

I like to write educational websites

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