GHK-Cu Copper Binding: The Coordination Chemistry of a Tripeptide

Glycyl-histidyl-lysine, usually shortened to GHK, is only three amino acids long. It still grips a copper(II) ion tightly. This article is for research use only and looks at that grip as chemistry: which atoms hold the metal, how the complex changes with pH, and what the published binding constants actually mean. If you want the wider structural picture first, our earlier piece on GHK-Cu copper peptide structure covers the peptide itself. Here we stay with the metal site.

The ligand: what GHK brings to the metal

In short, GHK offers a handful of nitrogen and oxygen atoms positioned so they can wrap around one copper ion. According to the reference entry on the complex, the free peptide is C₁₄H₂₄N₆O₄ with a molar mass of 340.38 g/mol, and the copper complex is C₁₄H₂₂CuN₆O₄.

Picture a short chain with four acid-base handles. A potentiometric and spectroscopic study lists four exchangeable protons, in decreasing order of pKa: the lysine side-chain amine, the N-terminal amine, the histidine imidazole, and the C-terminal carboxylate. Which of them hold a proton at a given pH decides what the peptide can offer a metal ion. pH is never a background detail here.

The history is brief. The factor was first isolated from human plasma albumin in 1973 by Pickart and Thaler, and in 1977 Schlesinger, Pickart and Thaler showed that the active peptide was glycyl-L-histidyl-L-lysine, as summarised in the same reference entry. In 1980 the idea that the peptide might help copper move into cells was first proposed. Everything below concerns the chemistry of the metal site in solution and in crystals, not any biological outcome.

The 3N core: how Cu(II) is held

The central answer is that three nitrogen atoms from a single peptide clamp the copper. Several sources agree on which three. The review by the GHK-Cu developer and the reference entry both name the histidine imidazole nitrogen, the glycine alpha-amino nitrogen, and the deprotonated amide nitrogen of the Gly-His peptide bond.

Three nitrogen donors

The 2020 ternary-complex paper describes this as a tridentate nitrogenous chelate: the N-terminal amine, the amide nitrogen and the imidazole nitrogen of the same peptide, a so-called 3N species. The amide nitrogen is unusual. Normally a peptide-bond nitrogen keeps its proton. Copper(II) is acidic enough to pull it off, and then the nitrogen becomes a strong donor. That is why the Gly-His link, with the imidazole nearby to help position the metal, matters so much.

The fourth planar site

Copper(II) generally prefers four donors in a flat square. With three taken by the peptide, one planar position is left open. The same study notes that this fourth site can be filled by solvent water, by an imidazole nitrogen from a second GHK (giving Cu(GHK)₂), or by the imidazole of cis-urocanic acid. So the 3N core is the stable part. The fourth position is where exchange and ternary complexes happen.

What each residue contributes

Substitution studies summarised in the reference entry found that glycine plays the major role in copper binding, while lysine interacts with copper mainly at alkaline pH. That fits the 3N picture: glycine supplies the amino group and the first peptide link, and the lysine side chain sits further away and only becomes involved once the solution is basic enough to deprotonate it.

The ligand: what GHK brings to the metal

The 3N core: how Cu(II) is held

Three nitrogen donors

The fourth planar site

What each residue contributes

Species and pH dependence

The takeaway here is that "GHK-Cu" is not one fixed species; it is a family that shifts with pH. Potentiometry in the ternary-complex study (25 °C, argon atmosphere, pH about 2.7 to 11.5) identified Cu(II) aquo ion, CuH(GHK), Cu(GHK), CuH_-1(GHK), CuH_-2(GHK) and CuH₂(GHK)₂.

CuH(GHK) and Cu(GHK) are both 3N complexes. They differ only in whether the C-terminal carboxylate carries a proton. At the alkaline end, two further deprotonations are reported for Cu(GHK): one at pKa 9.61, assigned to an equatorial water molecule, and one at pKa 10.77, assigned to the lysine epsilon-amino group. These numbers explain the lysine behaviour described above: it only joins in under quite basic conditions.

A second peptide can also attach, but weakly. The same work reports a conditional constant of 237 ± 5 M^-1 at pH 7.4 for Cu(GHK) + GHK giving Cu(GHK)₂ by spectroscopy, and 265 ± 46 M^-1 by isothermal titration calorimetry. The authors judge an earlier potentiometric value of roughly 500 M^-1 unreliable for binding this weak. At high GHK excess a 3+1N "auto-ternary" species, Cu(GHK)(H₂GHK), appears.

Stability constants: reading the numbers

In plain terms, a stability constant tells you how strongly one partner holds the other, and the headline figure for this complex is a log K of 16.44. The developer's review gives log₁₀ = 16.44 for GHK-Cu, against 8.68 for the copper complex of the two-residue Gly-His peptide, and 16.2 for the high-affinity copper site of plasma albumin. A systematic review repeats the value as "approximately log K = 16.44 at physiological pH" without giving a primary source for that passage.

That gap is large, roughly eight orders of magnitude, and the reference entry points out that the three-nitrogen set alone does not obviously explain it. One proposal, drawn from the crystal work, is that an extra oxygen from a lysine carboxylate of a neighbouring complex also reaches the copper. Whether that contact matters the same way in dilute solution is less settled. That is why the geometry section below separates crystals from solutions.

The 2020 study reports a different kind of number: a conditional constant for Cu + GHK ⇌ Cu(GHK) of 4.17 × 10¹² M^-1 at pH 7.4 and 4.79 × 10¹⁰ M^-1 at pH 6.5, from potentiometry at 1 µM. "Conditional" means the value applies to those exact conditions, with protons competing for the ligand. It's the main reason constants from different papers can't be lined up without checking pH, buffer, temperature and method. The authors also note that frozen-solution EPR gave constants about two orders of magnitude higher than their room-temperature work, a reminder that even sample handling shifts the answer.

Geometry: planar, pyramidal or bridged?

The short version is that solution data suggest a flat four-coordinate site, while crystals and simulations often add a fifth, more distant contact. The ternary-complex paper reports no structure of its own; the planar geometry there is inferred from UV-vis, circular dichroism and EPR, which is why it speaks of an "equatorial plane".

Solid-state structures tell a more varied story. A computational study, Theoretical study of copper binding to GHK peptide, summarises the picture this way: the Perkins crystal structure has three different peptides linking one copper through 3N2O coordination, while the binuclear structure of Hureau and colleagues has a distorted square pyramid with 3N1O in the equatorial positions and an apical oxygen at about 2.5 Å, compared with about 2.0 Å for the equatorial ligands. In the simulations, the four equatorial bonds in a 3N1O arrangement remain stable, while the fifth, apical contact from the C-terminal carboxylate is more fluxional.

Taken together, a fair reading is that the 3N1O plane is the dependable core, and the apical contact is soft: it appears readily in crystals, where neighbouring molecules are packed close, and comes and goes in water. Anyone comparing a crystal geometry with a solution measurement should keep that difference in mind.

How researchers measure the complex

Each technique answers a different question, so a full picture takes several. The reference entry lists X-ray crystallography, EPR, X-ray absorption spectroscopy, NMR and titration as the main methods used on this complex, and the 2020 paper adds potentiometry, UV-vis and CD titrations with global spectral fitting, and ITC in HEPES buffer at pH 7.4.

Potentiometry follows protons as pH changes and yields the species list and their pKa values.
UV-vis and CD track the copper d-d bands; a shift of about 46 nm (cis-urocanic acid) or 39 nm (trans) signalled a new fourth ligand in the ternary work. For background on the technique, see our note on UV/Vis absorbance.
EPR reports on the Cu(II) centre directly and can show nitrogen coupling.
ITC measures heat released on binding, useful for weak steps such as the second peptide.
Crystallography and NMR give atom-level structures; for the latter, our overview of NMR in solution explains the principle, though paramagnetic copper makes the experiment harder.

Material used for such work also needs checking for stray metals, which is where ICP-MS elemental analysis comes in. Trace copper or other ions in a buffer or peptide lot can bias a binding experiment.

Conclusion

GHK binds Cu(II) through a robust three-nitrogen core, with a flexible fourth planar site and, in crystals, an extra apical contact. Its log K of 16.44 is large, but every constant is tied to the conditions under which it was measured. For anyone designing an in-vitro experiment, the practical lessons are to state the pH and buffer, to remember that species shift across the pH range, and to treat crystal geometries as one view among several. As always, these materials are for research use only.

Frequently Asked Questions

Which atoms hold copper in GHK-Cu?

Three nitrogens from one peptide: the histidine imidazole nitrogen, the glycine alpha-amino nitrogen, and the deprotonated amide nitrogen of the Gly-His bond. A fourth planar site is filled by water or another ligand.

Why do reported binding constants differ between studies?

Values depend on the method, pH, buffer and competing ligands. Potentiometry, spectroscopy and calorimetry each report conditional constants under their own conditions, so numbers are compared only with those conditions stated.

Is the solid-state structure the same as the structure in solution?

Not necessarily. Crystals show neighbouring peptides supplying oxygen donors (a 3N2O arrangement in one structure, a distorted square pyramid in another), while solution geometry is inferred from UV-vis, CD and EPR.

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The ligand: what GHK brings to the metal

The 3N core: how Cu(II) is held

Three nitrogen donors

The fourth planar site

What each residue contributes

The ligand: what GHK brings to the metal

The 3N core: how Cu(II) is held

Three nitrogen donors

The fourth planar site

What each residue contributes

Species and pH dependence

Stability constants: reading the numbers

Geometry: planar, pyramidal or bridged?

How researchers measure the complex

Conclusion

Frequently Asked Questions

Which atoms hold copper in GHK-Cu?

Why do reported binding constants differ between studies?

Is the solid-state structure the same as the structure in solution?

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