Competitive Inhibition of <i>Bacillus amyloliquefaciens</i> Pectate Lyase B by <i>Oryza sativa</i> Xylanase Inhibitor Protein Explored through Experimental and Molecular Simulation Studies

of 98.68 nM. Fluorescence spectroscopy indicated static quenching upon rePelB-reOsXIP interaction. Molecular dynamics simulations revealed that the Lα4β5 loop of OsXIP inserts into the PelB catalytic groove, with a calculated binding free energy of -87.6 ± 7.4 kcal/mol. Alanine substitutions of PelB residues K99, S337, and V338 weakened binding and reduced inhibition by reOsXIP. These results provide the first evidence that XIP competitively inhibits a pectate lyase, highlighting a broader defensive role against plant cell wall-degrading enzymes.