Microcin J25 has a threaded sidechain-to-backbone ring structure and not a head-to-tail cyclized backbone.

Area of Science:

• Biochemistry
• Structural Biology
• Antimicrobial Peptides

Background:

• Microcin J25 is a 21-amino acid peptide with potent antibacterial activity against Gram-negative bacteria.
• Previous studies proposed a head-to-tail cyclized structure, contributing to its thermal stability.
• This stability is notable for a peptide lacking disulfide bonds.

Purpose of the Study:

• To elucidate the precise three-dimensional structure of Microcin J25.
• To determine the structural basis for its remarkable thermal stability and potent antibacterial activity.
• To re-evaluate the previously proposed head-to-tail cyclization model.

Main Methods:

• Nuclear Magnetic Resonance (NMR) spectroscopy was employed to deduce the peptide's 3D structure.
• Biophysical techniques were used to assess thermal stability and structural integrity.
• Enzymatic digestion with thermolysin was performed to test structural robustness.

Main Results:

• The study reveals Microcin J25 lacks a head-to-tail cyclic structure.
• A novel side-chain to backbone cyclization (Glu8 to N-terminus) forms an embedded ring threaded by the C-terminal tail, creating a noose-like feature.
• Two aromatic residues flanking the ring prevent slippage, maintaining structural integrity even after thermolysin digestion.

Conclusions:

• The newly determined noose-like structure explains Microcin J25's exceptional stability and potent antibacterial activity.
• This revised structural model reconciles previous NMR and biophysical data.
• The unique structure represents a novel mechanism for peptide stability and function.