Cystathionine β-synthase TtCbs1 from Tetrahymena thermophila catalyzes the synthesis of CdS quantum dots for methyl orange decolorization

Cystathionine β-synthase (CBS) is a pivotal enzyme in the transsulfuration metabolic pathway, catalyzing the synthesis of H2S and cystathionine. These metabolites mediate stress responses and maintain cellular redox homeostasis. In Tetrahymena thermophila, TtCBS1 expression was upregulated under cadmium stress, with cysteine further enhancing its transcription. The recombinant His-TtCbs1 protein was expressed in Escherichia coli and purified by affinity chromatography. The His-TtCbs1 catalyzed the synthesis of monodisperse cadmium sulfide quantum dots (CdS QDs) in cysteine and Cd2+ solutions. The QDs exhibited an average diameter of 3.51 nm, with size increasing over reaction time. Fourier-transform infrared spectroscopy revealed characteristic amide I (1,640 cm⁻¹) and amide II (1,520 cm⁻¹) peaks, confirming interactions of CdS and the protein. Cysteine and glutathione stabilized the CdS QDs in the solution. Under UV light irradiation, the CdS QDs decolorized 91% of methyl orange. This study shows that CBS from protists mitigates cadmium toxicity by synthesizing CdS QDs. The biosynthesized CdS QDs also function as effective biocatalysts for the decolorization of organic dyes.