LAUSR

Abstract Humic acid and protein are not only common organic matters in surface water, but also the main components of the extracellular polymer of microorganisms, which can influence on the biotransformation and biodegradation of antibiotics by enzymes. Therefore, the influence of fulvic acid and protein on the biotransformation of sulfamethazine (SMR) by horseradish peroxidase (HRP) was explored. HPLC-MS analysis show that 79.7% of SMR (20 mg/L) was degraded by sulfa bond breaking, hydroxyl substitution and pyrimidine ring opening, while the removal rate was suppressed significantly in the presence of fulvic acid or bovine serum albumin (BSA, model protein). This inhibition is related to the concentration of fulvic acid or BSA, but not to the exposure time. The inhibition of fulvic acid on SMR degradation by HRP is mainly achieved by binding to the HRP-SMR intermediate to prevent the HRP-SMR intermediate from further generating HRP and products, which is called anticompetitive inhibition. HRP activity is not affected by the addition of BSA with time. The inhibition of BSA on SMR degradation by HRP may be achieved by competing with HRP for binding to SMR, which is confirmed with the analysis of molecular docking.