Structure of Cystathionine β-Synthase from Toxoplasma gondii, a key enzyme in its H2S production machinery.

Cystathionine {beta}-synthase (CBS), the pivotal enzyme of the reverse transsulfuration pathway, catalyzes the pyridoxal-5-phosphate-dependent condensation of serine with homocysteine to form cystathionine. Additionally, CBS performs alternative reactions that use homocysteine and cysteine as substrates leading to the endogenous biosynthesis of hydrogen sulfide (H2S), an important signal transducer in many physiological and pathological processes. Toxoplasma gondii, the causative agent of toxoplasmosis, encodes a functional CBS (TgCBS) that contrary to human CBS, is not allosterically regulated by S-adenosylmethionine and can use both, Ser and O-acetylserine (OAS) as substrates. TgCBS is also strongly implicated in the production of H2S, and thus involved in redox homeostasis of the parasite. Here, we report its crystal structure, the first CBS from a protozoan described so far. Our data reveals a basal-like fold that unexpectedly differs from the active conformations found in other organisms, but structurally similar to the pathogenic activated mutant D444N of the human enzyme.