Wenliang Lei,Juan Liu,Wenyong Zhang,Jing Xu,Tao Bo,Zhiwen Wang,Wei Wang
H 2 S is a crucial endogenous gaseous signaling molecule involved in various metabolic pathways and is associated with the biological response to heavy metal stress. The biomineralization of CdS serves as a critical mechanism for responding to cadmium stress and removing Cd 2+ in different organisms. CdS is also a widely utilized semiconductor material for the photocatalytic degradation of dyes. In this study, we found that cysteine synthetase 1 (TtCsa1) is involved in CdS formation under cadmium stress in Tetrahymena thermophila . TtCsa1 also catalyzed CdS formation in vitro, and the synthesized CdS material exhibited controllable particle size and photoluminescence. Concurrently, cysteine and glutathione functioned as capping agents to regulate CdS particle size. The biosynthetically produced CdS degraded 90 % of methylene blue under UV light. Furthermore, CdS-ZnS nanocomposites were synthesized by adding Zn 2+ into the CdS biosynthetic system, which decreased the size of CdS particles and increased the degradation rate of methylene blue. The results indicate that the CdS biosynthesized by TtCsa1 from Tetrahymena thermophila is effective in the photocatalytic degradation of organic dyes.
Cysteine is implicated in important biological processes. It is synthesized through two different pathways. Cystathionine β-synthase and cystathionine γ-lyase participate in the reverse transsulfuration pathway, while serine acetyltransferase and cysteine synthase function in the de novo pathway. Two evolutionarily related pyridoxal 5′-phosphate-dependent enzymes, cystathionine β-synthase TtCBS1 (TTHERM_00558300) and cysteine synthase TtCSA1 (TTHERM_00239430), were identified from a freshwater protozoan Tetrahymena thermophila . TtCbs1 contained the N-terminal heme binding domain, catalytic domain, and C-terminal regulatory domain, whereas TtCsa1 consisted of two α/β domains. The catalytic core of the two enzymes is similar. TtCBS1 and TtCSA1 showed high expression levels in the vegetative growth stage and decreased during the sexual developmental stage. TtCbs1 and TtCsa1 were localized in the cytoplasm throughout different developmental stages. His-TtCbs1 and His-TtCsa1 were expressed and purified in vitro. TtCbs1 catalyzed the canonical reaction with the highest velocity and possessed serine sulfhydrylase activity. TtCsa1 showed cysteine synthase activity with high K m for O-acetylserine and low K m for sulfide and also had serine sulfhydrylase activity toward serine. Both TtCbs1 and TtCsa1 catalyzed hydrogen sulfide producing. TtCBS1 knockdown and TtCSA1 knockout mutants affected cysteine and glutathione synthesis. TtCbs1 and TtCsa1 are involved in cysteine synthesis through two different pathways in T . thermophila .
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