Structural stability in the 4-zinc human insulin hexamer

Area of Science:

• Protein crystallography
• Structural biology
• Biochemistry

Background:

• Human insulin differs from porcine insulin at the B-chain's COOH terminus.
• Semisynthetic and biosynthetic human insulins are subjects of recent X-ray studies.
• 4-zinc human insulin is a clinically used slow-acting preparation.

Purpose of the Study:

• To determine and refine the crystal structure of 4-zinc human insulin.
• To elucidate the structural basis for its slow-acting properties.

Main Methods:

• X-ray diffraction studies were performed on 4-zinc human insulin.
• The crystal structure was refined using 1.85-Å resolution data.
• Crystallographic data were analyzed to determine unit cell parameters and space group.

Main Results:

• The crystal structure of 4-zinc human insulin was refined to a residual of 0.173.
• A conformational change in the B-chain's first eight residues (extended to alpha-helical) was observed.
• A unique 8-Å tunnel formed by alpha-helices shields zinc and chloride ions within the hexamer.

Conclusions:

• The alpha-helical conformation and shielded zinc ions explain the decreased hexamer dissociation rate.
• This structural shielding mechanism accounts for the slow-acting nature of 4-zinc human insulin.
• The structure is nearly isomorphous with 4-zinc porcine insulin, with key differences in zinc coordination and B-chain conformation.