Workflow for Crystallographic Fragment Screening by Crystal Soaking for Protein Targets: A Case Study on Thioredoxin Glutathione Reductase From Schistosoma mansoni.

Among the biophysical techniques used in fragment-based drug discovery (FBDD) campaigns, crystallography is the most sensitive, allowing for the identification of low-affinity ligands and the characterization of protein-ligand complexes at atomic resolution. Although powerful, the proper application of this technique depends on obtaining crystals capable of diffracting X-rays at high resolution. Additionally, in crystallographic compound screening, the crystals must be resistant to multiple organic solvents used in chemical libraries, such as DMSO. In this protocol, we describe recombinant protein production suitable for crystallization and procedures for X-ray crystallographic screening of a library of 768 fragments. As a case study, we used the Schistosoma mansoni thioredoxin glutathione reductase (SmTGR), a redox enzyme with a key role in controlling oxidative stress in parasites of the Schistosoma genus, which causes schistosomiasis. As a validated pharmacological target, SmTGR is used in the development of new schistosomicidal drugs. The experimental pipeline includes SmTGR expression, purification, and crystallization, crystal soaking, diffraction data collection, and refinement. The 768 fragments from the Diamond-SGC Poised Library (DSPL) were individually soaked onto the crystals, and diffraction data were collected and processed at the I04-1 beamline of the Diamond Light Source synchrotron. Diffraction data were subsequently analyzed using PanDDA to identify fragment-binding events and to enable reliable detection of low-occupancy ligands within the protein crystal structures. In addition to the core experimental steps, this protocol incorporates systematic approaches to overcome limitations frequently encountered in crystallographic screening campaigns, including assessment of crystal solvent tolerance, acceleration of crystal mounting through the use of auxiliary devices, acoustic dispensing-based soaking of hundreds of fragments for low material consumption and high throughput, automated data collection, and efficient data analysis pipeline for the detection of weakly bound ligand. This protocol can be broadly applied to screen diverse compound sets against multiple targets amenable to crystallization. Key features • Obtaining SmTGR through expression in ExpiSf9 cells with proper yield and purity for crystallization assays. • Systematic testing of buffer solutions to determine crystallization conditions, assessment of crystal tolerance to DMSO, and crystallographic data collection. • Integration of crystallization, acoustic dispensing, shifter-aided crystal mounting, data collection, and analysis powered by an in-house software pipeline. • This protocol builds upon the method developed by [1] and extends its application to other soluble proteins.