Interactions between sulfonamide homologues and glycosyltransferase induced metabolic disorders in rice (Oryza sativa L.)

Sulfadiazine and its derivatives (sulfonamides, SAs) could induce distinct biotoxic, metabolic and physiological abnormalities, potentially due to their subtle structural differences. This study conducted an in-depth investigation on the interactions between SA homologues, i.e. sulfadiazine (SD), sulfamerazine (SD1), and sulfamethazine (SD2), and the key metabolic enzyme (glycosyltransferase, GT) in rice (Oryza sativa L.). Untargeted screening of SA metabolites revealed that GT-catalyzed glycosylation was the primary transformation pathway of SAs in rice. Molecular docking identified that the binding sites of SAs on GT (D0TZD6) were responsible for transferring sugar moiety to synthesize polysaccharides and detoxify SAs. Specifically, amino acids in the GT-binding cavity (e.g., GLY487 and CYS486) formed stable hydrogen bonds with SAs (e.g., the sulfonamide group of SD). Molecular dynamics simulations revealed that SAs induced conformational changes in GT ligand binding domain, which was supported by the significantly decreased GT activity and gene expression level. As evidenced by proteomics and metabolomics, SAs inhibited the transfer and synthesis of sugar but stimulated sugar decomposition in rice leaves, leading to the accumulation of mono- and disaccharides in rice leaves. While the differences in the increased sugar content by SD (24.3%, compared with control), SD1 (11.1%), and SD2 (6.24%) can be attributed to their number of methyl groups (0, 1, 2, respectively), which determined the steric hindrance and hydrogen bonds formation with GT. This study suggested that the disturbances on crop sugar metabolism by homologues contaminants are determined by the interaction between the contaminants and the target enzyme, and are greatly dependent on the steric hindrance effects contributed by their side chains. The results are of importance to identify priority pollutants and ensure crop quality in contaminated fields.