LL-37

Overview

LL-37 is the only human cathelicidin — a class of antimicrobial peptides forming part of innate immunity. It is studied both as a direct antimicrobial agent and as an immune signalling molecule, and it is mechanistically unlike every other compound documented here.

We do not stock it. It is included because it demonstrates that a peptide can work without a receptor at all.

Chemical identity

• Sequence: LLGDFFRKSKEKIGKEFKRIVQRIKDFLRNLVPRTES
• CAS number: 154947-66-7
• Molecular formula: C₂₀₅H₃₄₀N₆₀O₅₃
• Molecular weight: approximately 4493 g/mol
• Classification: Cathelicidin antimicrobial peptide
• Length: 37 residues

The name is descriptive rather than coded: LL for the two leucines it begins with, 37 for its length. At 4,493 g/mol it is far beyond the size where a structure diagram remains readable.

The composition matters more than usual here. Eleven residues are lysine or arginine against five acidic ones, giving a net positive charge around +6. That charge is not incidental — it is the mechanism.

Where it comes from

LL-37 is not produced directly. It is released by proteolytic cleavage from a precursor protein, hCAP18, stored in neutrophil granules and expressed in epithelial tissue at the body’s barriers — skin, airway, gut.

That arrangement is typical of innate immunity: the weapon is kept inert until needed, and activated by cleavage at the site of infection. The peptide is one component of a system that also includes defensins and other antimicrobial peptides, most organisms having several.

Humans have exactly one cathelicidin, which is unusual — many mammals carry a family of them — and it means LL-37 carries the whole of that particular defensive function.

How it kills

This is what separates LL-37 from everything else in this library.

Every other compound documented here works by binding something: a receptor, an enzyme, a binding protein. LL-37 works physically.

In solution the peptide is largely unstructured. On contact with a membrane it folds into an amphipathic alpha helix — a helix with its positively charged residues arranged along one face and its hydrophobic residues along the other. That geometry is the whole design. The charged face is drawn to the membrane surface by electrostatic attraction; the hydrophobic face inserts into the lipid interior.

Enough peptide accumulating on a membrane disrupts its integrity, permeabilising the cell. The bacterium dies from having lost the barrier between inside and outside — a structural failure rather than a metabolic one.

Why it spares human cells

An obvious question follows: what stops it destroying host membranes too?

The answer is surface charge. Bacterial membranes present strongly anionic lipids on their outer face — phosphatidylglycerol and cardiolipin in the membrane itself, lipopolysaccharide or teichoic acids in the wall beyond it. Mammalian plasma membranes present mostly zwitterionic lipids outward, with the anionic ones kept on the inner leaflet, and they contain cholesterol, which stiffens the bilayer against insertion.

So a cationic peptide is drawn to bacterial surfaces and comparatively indifferent to ours. Selectivity comes from a physical property of the target rather than from molecular recognition — the peptide is not distinguishing bacteria from human cells so much as being attracted to negative charge and finding it in one place and not the other.

That selectivity is relative rather than absolute, and at sufficient concentration these peptides do affect mammalian cells. It is a margin, not a switch.

The second job

Direct killing is only part of what LL-37 does, and possibly not the most important part.

The peptide is also an immune signal: chemotactic for neutrophils, monocytes and T cells, involved in wound healing and angiogenesis, and able to modulate inflammatory responses to bacterial products. These effects occur at concentrations well below those needed for membrane disruption.

Since tissue concentrations under most conditions sit in that lower range, a reasonable reading is that LL-37’s routine role is as a signalling molecule that recruits and coordinates immune cells, with direct antimicrobial action reserved for the higher concentrations reached at sites of active infection. Its dual character is what most current research addresses.

A note on resistance

Antimicrobial peptides attract interest partly because resistance to them is harder to acquire than resistance to conventional antibiotics.

Most antibiotics inhibit a specific protein, and a point mutation altering that protein can confer resistance in a single step. Resisting a membrane-disrupting peptide requires changing the composition or charge of the membrane itself — a larger, more costly alteration affecting many aspects of the organism’s biology.

Harder is not impossible: bacteria do modify their surface charge in response, and such mechanisms are documented. But the barrier is higher, and that difference is much of why this class remains under investigation.

Handling and storage

LL-37 is more difficult to work with than most peptides here, and for reasons that follow from what it does.

• It sticks to surfaces. A membrane-seeking amphipathic peptide will also adsorb onto plastic and glass, and losses to container walls can be substantial — enough to make a prepared solution weaker than intended. Low-binding tubes and pipette tips are worth using, and preparing in glass silanised or otherwise treated is standard practice in the field.
• No structure to preserve in storage. Unusually, the peptide is unstructured in solution and folds only on contact with membranes, so denaturation is not the concern it is for a folded protein.
• No methionine or cysteine, so oxidation and disulphide integrity are not concerns.
• Lyophilised storage: sealed, refrigerated; freeze for long-term storage.
• Reconstitution: add diluent slowly down the vial wall. Avoid foaming — a surface-active peptide concentrates at air–liquid interfaces.

Purity and analytical verification

Two considerations, and the second is easy to underestimate.

Length. Thirty-seven coupling steps generate a substantial deletion-sequence population, so purity figures here represent real purification work.

Counterion burden. LL-37 carries eleven basic residues, and peptides purified by reversed-phase HPLC are isolated as salts — typically with trifluoroacetate. The arithmetic is worth doing:

• Six trifluoroacetate counterions, matching the net charge, add roughly 684 Da — the peptide is then about 87% of the salt’s mass.
• Counterions at every basic residue would add around 1,254 Da — about 78%.

So a vial labelled 10 mg may contain between roughly 7.8 and 8.7 mg of peptide depending on salt form. As with KPV, the figure to ask for is net peptide content, which is a different measurement from HPLC purity. For a peptide this cationic, the difference between the two is large enough to matter in any quantitative work.

References

• Zanetti M. Cathelicidins, multifunctional peptides of the innate immunity. Journal of Leukocyte Biology, 2004. PMID 12960280
• Dürr UH, Sudheendra US, Ramamoorthy A. LL-37, the only human member of the cathelicidin family of antimicrobial peptides. Biochimica et Biophysica Acta, 2006. PMID 16716248
• Vandamme D, Landuyt B, Luyten W, Schoofs L. A comprehensive summary of LL-37, the factotum human cathelicidin peptide. Cellular Immunology, 2012. PMID 23246832

Summary

LL-37 is the only human cathelicidin antimicrobial peptide, 37 residues, CAS 154947-66-7, molecular formula C₂₀₅H₃₄₀N₆₀O₅₃, molecular weight approximately 4,493 g/mol, released by proteolysis from the precursor hCAP18. It carries a net positive charge of about +6 and folds into an amphipathic helix on contact with membranes, killing bacteria by physically disrupting them rather than by binding any molecular target. Selectivity for bacterial over human cells follows from membrane surface charge rather than from recognition. It also acts as an immune signalling molecule at concentrations below those required for killing, and that role may be the more common one in tissue. We do not stock it.

This page is provided for research reference. Compounds described in this library are for laboratory research use only, are not drugs, foods or cosmetics, and may not be used for any purpose other than in vitro or non-human laboratory research.