Disulfide-Bond Cyclization of Peptides and ICK Structures

Principles Introduction

Disulfide bond cyclization between cysteines links two Cys residues via an S–S bond to form a closed loop. Inhibitor cystine knot (ICK) peptides are a class of such cyclic peptides characterized by three disulfide bonds forming a "cystine knot". Specifically, two of the disulfides (and their intervening backbone segments) form a ring, and the third disulfide threads through this ring to create an interlocked topology. The ICK family includes numerous peptides and their cyclized homologues (cyclotides), which exhibit diverse bioactivities and serve as scaffolds for designing novel drugs or biopesticides.

Stability Mechanism

The schematic above illustrates the cyclotide structure (Kalata B1) with three disulfide bonds (yellow) forming a cystine knot. This interlocked topology creates a tightly-packed hydrophobic core, rendering ICK peptides extraordinarily resistant to heat, extreme pH, and proteolytic degradation. For example, the spider-venom toxin Hv1a (an ICK peptide) remains largely intact at 75 °C, at pH 1, and in the presence of strong proteases; its stability drops dramatically when the disulfides are reduced. In other words, the ICK fold significantly enhances the thermal, chemical, and proteolytic stability of these peptides.

Applications

ICK peptides have promising applications in therapeutics and agriculture. Many legume-derived ICK peptides (e.g. PA1b-family members and soybean leginsulin variants like aglycin, vglycin, iglycin) demonstrate glucose-lowering effects: oral administration improves insulin signaling and β-cell function, resulting in lower blood glucose. In agriculture, ICK peptides can serve as potent bioinsecticides; for instance, PA1b is highly toxic to grain weevils and mosquitoes by oral ingestion. Cyclotides and other ICK peptides also exhibit broad antimicrobial and anticancer activities, suggesting potential as novel anti-infective and anticancer agents.

Representative ICK Peptides

PA1b (Pea Albumin 1 Subunit b): A 37-aa peptide with three disulfides (ICK fold) from pea seeds. It is an orally active insecticidal toxin, highly potent against grain weevils and mosquitoes by binding insect gut V-ATPase.

Aglycin: A 37-aa, 6-Cys peptide (three disulfides) from soybean. Also known as a "leginsulin," Aglycin mimics insulin activity. Oral Aglycin lowers blood glucose in diabetic models by enhancing insulin signaling, and it resists digestion by gastric and intestinal proteases.

Vglycin: An ICK peptide highly similar to aglycin, derived from soybean/pea. It exhibits similar insulin-like effects, improving insulin receptor signaling and glucose tolerance.

Iglycin, Dglycin: Other PA1b-like peptides from soybean in the leginsulin family. They also show insulin-mimetic bioactivities, regulating glucose metabolism and protecting β-cell.

α-Astratide aM1: A 6-Cys cystine-rich peptide from the medicinal plant Astragalus membranaceus, with high sequence identity to PA1b. aM1 is insecticidal (cytotoxic to insect Sf9 cells) and markedly decreases insulin secretion in mouse pancreatic β-cell, indicating its role in glucose homeostasis.

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References

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