Serine acetyltransferase (SAT): SAT catalyzes the CoA-dependent acetylation of the side chain hydroxyl group of L-serine to form O-acetylserine, as the first step of a two-step biosynthetic pathway in bacteria and plants leading to the formation of L-cysteine. This reaction represents a key metabolic point of regulation for the cysteine biosynthetic pathway due to its feedback inhibition by cysteine. The enzyme is a 175 kDa homohexamer, composed of a dimer of homotrimers. Each subunit contains an N-terminal alpha helical region and a C-terminal left-handed beta-helix (LbH) subdomain with 5 turns, each containing a hexapeptide repeat motif characteristic of the acyltransferase superfamily of enzymes. The trimer interface mainly involves the C-terminal LbH subdomain while the dimer (of trimers) interface is mediated by the N-terminal alpha helical subdomain.
Comment:Based on the structures of serine acetyltransferase bound to CoA (1SST) and substrate (1T3D).
Comment:Serine acetyltransferase is hexameric, composed of a dimer of homotrimers. The active sites are located at the interface between two LbH subunits in the homotrimer. The trimer structure contains three active sites and each subunit contributes to two active sites via residues located on two surfaces.