The structure and mechanism of serine acetyltransferase from Escherichia coli

Serine acetyltransferase (SAT) catalyzes the first step of cysteine synthesis in microorganisms and higher plants. Here we present the 2.2 Angstrom crystal structure of SAT from Escherichia coli, which is a dimer of trimers, in complex with cysteine. The SAT monomer consists of an amino-terminal alpha-helical domain and a carboxyl- terminal left-handed beta-helix. We identify His(158) and Asp(143) as essential residues that form a catalytic triad with the substrate for acetyl transfer. This structure shows the mechanism by which cysteine inhibits SAT activity and thus controls its own synthesis. Cysteine is found to bind at the serine substrate site and not the acetyl-CoA site that had been reported previously. On the basis of the geometry around the cysteine binding site, we are able to suggest a mechanism for the O-acetylation of serine by SAT. We also compare the structure of SAT with other left-handed beta-helical structures.