Development of an immobilized Mycobacterium tuberculosis purine nucleoside phosphorylase platform for ligand fishing and inhibition assays

Purine nucleoside phosphorylase (PNP) from Mycobacterium tuberculosis (MtPNP) plays a crucial role in purine metabolism, making it an attractive target for developing new tuberculosis treatments. In this study, we developed a ligand screening platform using MtPNP covalently immobilized on magnetic particles (MtPNP-MPs). The immobilization process achieved a high enzyme loading and preserved the enzyme catalytic activity, enabling its use in both activity and affinity-based screening assays. The activity of MtPNP-MPs was monitored by quantifying hypoxanthine released from inosine phosphorolysis, and kinetic studies revealed Michaelis-Menten behavior for inosine and inorganic phosphate substrates, with K_M values comparable to those of free MtPNP. A proof-of-concept inhibitor study using the transition state analog DI4G demonstrated the platform capability for recognizing and characterizing inhibitors, yielding an IC₅₀ value of 91.4 nM and a competitive inhibition mechanism with a K_i of 69.2 nM. Furthermore, the MtPNP-MPs exhibited high stability, retaining over 80 % of their activity after six months of storage and more than 90 % after five consecutive reaction cycles, highlighting their potential for reuse in high-throughput assays. We optimized key parameters for ligand fishing assay, including the amount of MtPNP-MPs, incubation time, and elution conditions. While higher organic solvent concentrations and longer elution times improved ligand isolation, these conditions also reduced enzyme activity. This trade-off between ligand isolation yield and enzyme reusability suggests that elution conditions should be tailored based on the ligand binding strength. Overall, this study establishes the MtPNP-MPs platform as a versatile tool for ligand identification and inhibitor characterization, with promising applications in the screening of complex libraries, such as natural products, for bioactive compounds.