Transsulfuration in Saccharomyces cerevisiae is not dependent on heme: purification and characterization of recombinant yeast cystathionine beta-synthase

Cystathionine P-synthase [CBS; L-serine hydro-lyase (adding homocysteine), EC 4.2.1.22] catalyzes the first committed step of transsulfuration in both yeast and humans. It has been established previously that; human CBS is a hemeprotein but although the heme group appears to be essential for CBS act ivity, the exact function of the heme group is unknown. CBS activity is abs ent in heme deficient strains of Saccharomyces cerevisiae grown without hem e supplementation. CBS activity can be restored by supplementing these stra ins with heme, implying that there is a heme requirement for yeast CBS. We subcloned, overexpressed and purified yeast CBS. The yeast enzyme shows abs olute pyridoxal 5'-phosphate (PLP) dependence for activity but we could fin d no evidence for the presence of a heme group. Given the degree-of sequenc e and mechanistic similarity between yeast and human CBS, this result indic ates that heme is unlikely to play a direct catalytic role in the human CBS reaction mechanism. Further characterization revealed that, in contrast to human CBS, S-adenosylmethionine (AdoMet) does not activate yeast CBS. Yeas t CBS was found to be coordinately regulated with proliferation in S. cerev isiae. This finding is the most likely explanation of the observed apparent heme dependence of transsulfuration in vivo. (C) 2000 Elsevier Science S.A . All rights reserved.