Intra-A chain disulphide bond forms first during insulin precursor folding.

Area of Science:

• Biochemistry
• Protein Folding
• Molecular Biology

Background:

• Insulin and insulin-like growth factor I (IGF-I) are crucial hormones with similar structures.
• Understanding protein folding pathways is essential for comprehending biological function and disease.
• Disulphide bonds play a critical role in stabilizing protein tertiary structures.

Purpose of the Study:

• To investigate and compare the folding pathway of insulin precursor with that of IGF-I.
• To elucidate the role of the intra-A chain disulphide bond in insulin precursor folding.
• To determine the rate-limiting step in insulin precursor folding.

Main Methods:

• Utilized intra-A chain disulphide-bond deleted [A6, A11-Ser] proteins, including proinsulin, insulin, and A chain.
• Compared recombination yields of native insulin versus modified insulin under identical conditions.
• Analyzed time courses of oxidation and helix structure formation during folding.

Main Results:

• The intra-A chain disulphide bond forms early in insulin precursor folding, unlike in IGF-I where it forms late.
• Deletion of the intra-A chain disulphide bond significantly reduced insulin recombination yield (approx. 7% vs. 22%).
• Early formation of the intra-A chain disulphide bond accelerates subsequent inter-chain disulphide bond formation and stabilizes helix structure.

Conclusions:

• The intra-A chain disulphide bond is critical for stabilizing insulin A chain folding intermediates.
• This stabilization facilitates correct B chain recognition and pairing, essential for efficient insulin assembly.
• Formation of the intra-A chain disulphide bond is the rate-limiting step in insulin precursor folding.