Structural studies of a crystalline insulin analog complex with protamine by atomic force microscopy

Area of Science:

• Biomolecular Crystallography
• Surface Science
• Materials Science

Background:

• Crystallographic studies of insulin-protamine complexes like NPH insulin are difficult due to high solvent content and crystal disorder.
• Understanding the structure of these complexes is crucial for developing effective diabetes therapies.

Purpose of the Study:

• To investigate the utility of in situ tapping mode atomic force microscopy (TMAFM) for characterizing crystalline insulin-protamine complexes.
• To compare the structure of NPL insulin-protamine complexes with NPH insulin.

Main Methods:

• In situ tapping mode atomic force microscopy (TMAFM) was performed at 6°C on NPL insulin-protamine crystals.
• Height and phase imaging were used to obtain lattice images of the crystal plane.
• Results were compared with low-resolution X-ray diffraction data of NPH insulin.

Main Results:

• TMAFM revealed micron-sized islands and 44-Å tall steps on the NPL crystals.
• Lattice images were consistent with the arrangement of insulin-protamine complexes on the P4(1)2(1)2 (110) crystal plane.
• Tip-sample adhesion varied between complex interstices and individual complexes, indicating structural differences.

Conclusions:

• Low-temperature TMAFM height and phase imaging are effective for structural characterization of biomolecular complexes.
• NPH and NPL insulins are likely isostructural.
• TMAFM provides valuable insights into the arrangement and interactions of insulin-protamine complexes.