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

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.

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..

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 thought 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.

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⁺ .

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…

 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.

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.

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]