Design, Synthesis, and Bioassay for the Thiadiazole-Bridged Thioacetamide Compound as Cy-FBP/SBPase Inhibitors Based...

Cyanobacterial fructose-1,6-/sedoheptulose-1,7-bisphosphatase (Cy-FBP/SBPase) was an important regulatory enzyme in cyanobacterial photosynthesis and was a potential target enzyme for screening to obtain novel inhibitors against cyanobacterial blooms. In this study, we developed a novel pharmacophore screening model based on the catalytic mechanism and substrate structure of Cy-FBP/SBPase and screened 26 S series compounds with different structures and pharmacophore characteristics from the Specs database by computer-assisted drug screening. These compounds exhibited moderate inhibitory activity against Cy-FBP/SBPase, with 9 compounds inhibiting >50% at 100 μM. Among them, compound S5 showed excellent inhibitory activity against both Cy-FBP/SBPase and Synechocystis sp. PCC6803 (IC₅₀ = 6.7 ± 0.7 μM and EC₅₀ = 7.7 ± 1.4 μM). The binding mode of compound S5 to Cy-FBP/SBPase was predicted using the molecular docking theory and validated by sentinel mutation and enzyme activity analysis. Physiochemical, gene transcription level, and metabolomic analyses showed that compound S5 significantly reduced the quantum yield of photosystem II and the maximum electron transfer rate, downregulated transcript levels of related genes encoding the Calvin cycle and photosystem, reduced the photosynthetic efficiency of cyanobacteria, thus inhibited metabolic pathways, such as the Calvin cycle and tricarboxylic acid cycle, and eventually achieved an efficient algicide. In addition, compound S5 had a high safety profile for human-derived cells and zebrafish. In summary, the novel pharmacophore screening model obtained from the current work provides an effective solution to the cyanobacterial bloom problem.