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.

Importance: The significant upregulation of TtCBS1 in Tetrahymena thermophila under cadmium stress indicates that this enzyme is a key player in the organism's defense mechanism against cadmium toxicity. Cadmium sulfide (CdS) nanoparticles were synthesized using the TtCbs1 single-enzyme system in vitro. Cysteine and glutathione play a critical role in controlling the growth of biosynthetic CdS particle size. Understanding the precise mechanisms by which cysteine and glutathione control particle size could lead to the development of more precise and efficient biomineralization processes. The synthesized CdS quantum dots exhibited significant photocatalytic activity. This work highlights the potential of cystathionine β-synthase from protists in metal detoxification and environmental remediation.