Intramolecular disulfide loop formation in a peptide containing two cysteines

Area of Science:

• Biochemistry
• Protein Chemistry
• Chemical Kinetics

Background:

• The Kunitz soybean trypsin inhibitor (KSTI) contains a specific peptide fragment (residues 115-181) that exists as a soluble random-coil at neutral pH.
• This fragment possesses two cysteine residues separated by eight amino acids, making it suitable for studying disulfide bond formation and exchange.

Purpose of the Study:

• To investigate the kinetics of disulfide exchange reactions involving the KSTI fragment (115-181).
• To determine rate constants for intramolecular loop formation in a random-coil peptide context.
• To establish a reference point for the fast limit of intramolecular disulfide exchange.

Main Methods:

• Utilized cyanogen bromide fragmentation to isolate the KSTI peptide fragment (115-181).
• Performed disulfide exchange reactions between the KSTI fragment and N-acetylcysteine methyl ester (both oxidized and reduced forms).
• Quantified four out of six possible rate constants for these exchange reactions using kinetic assays.

Main Results:

• Determined the rate constant for intramolecular loop formation in the KSTI fragment under specific conditions (0.36 +/- 0.15 s-1 at 23°C in 3 M guanidine hydrochloride).
• This rate constant represents the formation of a small, sterically unhindered loop.
• The findings establish a benchmark for rapid intramolecular disulfide exchange in peptides lacking fixed secondary structure.

Conclusions:

• The study provides crucial kinetic data on disulfide exchange in a model random-coil peptide.
• The determined rate constant serves as a valuable reference for understanding loop formation dynamics in peptides.
• These findings contribute to the broader understanding of protein folding and disulfide bond dynamics.