Linaclotide: Molecular Structure

Molecular Structure and Properties#

Linaclotide is a 14-amino-acid synthetic cyclic peptide with a molecular weight of 1,526.8 Da, molecular formula C59H79N15O21S6, and CAS number 851199-59-2. The peptide is constrained by three intramolecular disulfide bonds that create a compact, protease-resistant structure adapted for oral administration and local gut activity.

Amino Acid Sequence#

The primary structure of linaclotide:

Cys-Cys-Glu-Tyr-Cys-Cys-Asn-Pro-Ala-Cys-Thr-Gly-Cys-Tyr

(CCEYCCNPACTGCY in single-letter code)

The three disulfide bonds connect:

• Cys1-Cys6: Links the N-terminal region to mid-sequence
• Cys2-Cys10: Spans the core of the peptide
• Cys5-Cys13: Links mid-sequence to the C-terminal region

This disulfide bonding pattern is homologous to the heat-stable enterotoxin (STa) of enterotoxigenic E. coli and to the endogenous intestinal peptides guanylin and uroguanylin. The cyclic structure is essential for GC-C receptor binding and for conferring resistance to gastrointestinal proteases.

Active Metabolite#

Linaclotide is partially degraded in the intestinal lumen to its principal active metabolite, MM-419447, which retains the Cys1-Cys6 and Cys2-Cys10 disulfide bonds but loses the C-terminal tyrosine (des-Tyr14 linaclotide). This metabolite retains GC-C agonist activity and contributes to the pharmacological effect in the distal gut. The metabolite has a longer half-life in intestinal fluid (~6.3 hours) compared to the parent compound (~3 minutes).

GC-C Receptor Pharmacology#

Linaclotide and its active metabolite bind to the extracellular domain of guanylate cyclase-C (GC-C), a transmembrane receptor expressed on the apical surface of intestinal epithelial cells throughout the GI tract, with highest density in the duodenum and jejunum.

Upon ligand binding, the intracellular catalytic domain of GC-C converts GTP to cGMP, producing two downstream effects:

1. Intracellular cGMP: Activates protein kinase G-II (PKG-II), which phosphorylates and activates the cystic fibrosis transmembrane conductance regulator (CFTR), promoting chloride and bicarbonate secretion into the gut lumen
2. Extracellular cGMP: A fraction is exported to the basolateral surface and enters the submucosal space, where it inhibits nociceptive afferent signaling to reduce visceral pain

Physicochemical Properties#

Formulation: Linaclotide is supplied as oral capsules containing linaclotide-coated beads. The beads are designed to release the peptide in the small intestine.

• Solubility: Soluble in aqueous media at intestinal pH
• Acid stability: Stable in gastric acid, allowing oral administration without enteric coating
• Protease resistance: The three disulfide bonds confer substantial resistance to pepsin and trypsin degradation, though gradual degradation occurs in the distal gut

Pharmacokinetics#

Linaclotide is fundamentally different from most peptide therapeutics in that it is designed for local gut activity with negligible systemic exposure.

Absorption: Linaclotide and its active metabolite are minimally absorbed from the GI tract. Plasma concentrations are below the limit of quantitation following therapeutic oral doses. The negligible systemic absorption minimizes potential for off-target effects.

Distribution: Not applicable -- linaclotide acts locally in the intestinal lumen and does not achieve measurable systemic concentrations.

Metabolism: Linaclotide is degraded to its active metabolite (des-Tyr14) within the intestinal lumen. Further degradation occurs through reduction of disulfide bonds and proteolytic cleavage in the distal intestine.

Elimination: Parent compound and metabolites are excreted in feces. No renal or hepatic elimination is relevant given the absence of systemic exposure.

Structural Comparison with Related Peptides#

• vs. Guanylin: Endogenous 15-amino-acid GC-C ligand with 2 disulfide bonds. Linaclotide has higher GC-C binding affinity and greater protease resistance due to its 3 disulfide bonds.
• vs. Uroguanylin: Endogenous 16-amino-acid GC-C ligand primarily active in the proximal intestine.
• vs. Plecanatide (Trulance): Another synthetic GC-C agonist (16 amino acids) designed as a uroguanylin analog with pH-dependent activation. Linaclotide has pH-independent activity throughout the intestine.

Related Reading#

• Linaclotide overview and research guide
• Peptides similar to Linaclotide
• Linaclotide research evidence