Sulfate assimilation pathway (Saccharomyces cerevisiae)

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In yeast, the sulfate assimilation pathway consists of sulfates uptake via Sul1p and Sul2p, activation to adenylate compounds, reduction to sulfide, and subsequent incorporation into carbon chains. The importance of activating sulfate to adenylyl sulfate (APS) and phosphoadenylyl sulfate (PAPS) stems from the fact that physiological reducing agents (e.g. NADPH, NADH) are not large enough in E0' to reduce sulfate directly. Adenylation lowers sulfate's electropotential, enabling cells to circumvent what would otherwise be an endergonic reaction. After its conversion to PAPS, sulfate is reduced to sulfite by PAPS reductase (Met16p) and again to sulfide by sulfite reductase (Met5p and Met10p). The subsequent incorporation of sulfide into a carbon chain varies by organism; in S. cerevisiae, sulfide can react with O-acetylhomoserine via Met17p to form homocysteine, the major intermediate in the biosynthesis and interconversion of methionine and cysteine. Source: https://pathway.yeastgenome.org/