Molecular dynamics simulations and kinetic measurements provide insights into the structural requirements of substrate size-dependent specificity of oligogalacturonide oxidase 1 (OGOX1)

Oligogalacturonides (OGs) are pectin fragments released from the breakdown of the homogalacturonan during pathogenesis that act as Damage-Associated Molecular Patterns. OG-oxidase 1 (OGOX1) is an Arabidopsis berberine bridge enzyme-like (BBE-l) oligosaccharide oxidase that oxidizes OGs, impairing their elicitor activity and concomitantly releasing H2O2. The OG-oxidizing activity of OGOX1 is markedly pH-dependent, with optimum pH around 10, and is higher towards OGs with a degree of polymerization higher than two. Here, the molecular determinants of OGOX1 responsible for the binding of OGs with different lengths have been investigated through molecular dynamics simulations and enzyme kinetics studies. OGOX1 was simulated in complex with OGs with different degree of polymerization such as di-, tri-, tetra- and penta-galacturonide, in water solution at alkaline pH. Our simulations revealed that, among the four OGOX1/OG combinations, the penta-galacturonide (OG5) showed the best conformation in the active site to be efficiently oxidized by OGOX1. The optimal conformation can be stabilized by salt-bridges formed between the carboxyl groups of OG5 and five positively charged amino acids of OGOX1, highly conserved in all OGOX paralogs. Our results suggest that these interactions limit the mobility of OG5 as well as longer OGs, contributing to maintain the terminal monomer of OGs in the optimal orientation in order to be oxidized by the enzyme. In accordance with these results, the enzyme efficiency (Kcat/KM) of OGOX1 on OG5 (40.04) was found to be significantly higher than that on OG4 (13.05) and OG3 (0.6).