The T₂ structure of polycrystalline cubic human insulin

Area of Science:

• Crystallography
• Structural Biology
• Biochemistry

Background:

• Human insulin exists in various structural states influencing its function.
• Understanding insulin polymorphism is crucial for drug formulation and stability.
• Previous studies focused on insulin co-crystallization, leaving pH-induced structural changes less explored.

Purpose of the Study:

• To characterize the polymorphism of human insulin in response to pH variations.
• To determine the crystal structure of pH-induced insulin polymorphs.
• To elucidate the structural basis of pH-driven phase transitions in human insulin.

Main Methods:

• X-ray powder diffraction was used to identify different insulin polymorphs.
• A specific crystallization protocol was employed for co-crystallization studies.
• Multi-profile stereochemically restrained Rietveld refinement was utilized for structure determination.

Main Results:

• Three distinct human insulin polymorphs were identified: two rhombohedral (R3) and one cubic (I213).
• These polymorphs correspond to specific insulin conformations (T6, T3R3f, and T2) at increasing pH.
• The cubic T2 polymorph structure was resolved at 2.7 Å resolution, representing the first such structure from zinc-containing crystals.

Conclusions:

• Human insulin undergoes significant structural changes with pH variations, leading to distinct polymorphic forms.
• The determined cubic insulin structure provides novel insights into its conformational flexibility.
• The study highlights the importance of pH control in insulin crystallization and structural characterization.