Solution of a Fab (26-10)/digoxin complex by generalized molecular replacement.

Area of Science:

• Structural Biology
• Immunology
• Biochemistry

Background:

• Monoclonal antibodies are crucial for targeted therapies.
• Understanding antibody-antigen interactions at a molecular level is essential for drug development.
• The antidigoxin antibody (26-10) is a key tool in managing digoxin toxicity.

Purpose of the Study:

• To elucidate the three-dimensional structure of a Fab fragment from a murine antidigoxin antibody (26-10) in complex with digoxin.
• To determine the structure of a mutant Fab fragment to understand structural contributions to binding.
• To refine the molecular replacement strategy for solving complex protein structures.

Main Methods:

• Utilized molecular replacement, a generalized approach involving rigid-body refinement of individual domains (VH, VL, CH1, CL).
• Applied rigid-body refinement against the correlation coefficient between observed and calculated structure factor intensities.
• Employed simulated annealing refinement for atomic positions post-initial model building.

Main Results:

• Successfully solved the structures of the Fab mutant and the Fab/digoxin complex using the refined molecular replacement strategy.
• Identified the correct orientation of the search model and achieved unambiguous solutions for translation searches.
• Reduced the R factor to the low forties post-rigid-body refinement and to the low twenties after simulated annealing.

Conclusions:

• The generalized molecular replacement strategy proved effective for solving complex antibody fragment structures.
• The determined structures provide valuable atomic-level insights into digoxin binding by the 26-10 antibody.
• These findings can inform the design of improved antibody-based therapeutics and diagnostic tools.