Rapid experimental SAD phasing and hot-spot identification with halogenated fragments

Area of Science:

• Structural Biology
• Biophysics
• Drug Discovery

Background:

• Identifying ligand binding sites ('hot spots') is crucial for drug discovery and understanding protein function.
• Determining protein structures using X-ray crystallography can be challenging, particularly phase determination.

Purpose of the Study:

• To identify novel ligands capable of binding to multiple 'hot spots' in HIV-1 reverse transcriptase (RT).
• To explore the utility of identified ligands for facilitating X-ray crystallographic phase determination.

Main Methods:

• X-ray crystallographic fragment screening was employed to identify potential ligands.
• Single-wavelength anomalous dispersion (SAD) phasing was utilized with halogenated pyrazoles for structure determination.

Main Results:

• 4-bromopyrazole was identified as a 'magic bullet' ligand binding to numerous hot spots in HIV-1 RT.
• The anomalous signal from 4-bromopyrazole and 4-iodopyrazole enabled the SAD-based structure determination of three proteins: HIV-1 RT, influenza A endonuclease, and proteinase K.
• These compounds revealed 'hidden' binding pockets in unliganded protein structures, aiding in silico screening.

Conclusions:

• 4-bromopyrazole is a valuable tool for both ligand hot-spot identification and X-ray crystallographic phase determination.
• The use of inexpensive, readily available compounds like 4-bromopyrazole can streamline structural biology workflows.
• This approach offers a rapid method for solving novel protein structures, overcoming common bottlenecks.