Component of serine palmitoyltransferase (SPT), which catalyzes the committed step in the synthesis of sphingolipids, the condensation of serine with palmitoyl CoA to form the long chain base 3-ketosphinganine. H(+) + hexadecanoyl-CoA + L-serine = 3-oxosphinganine + CO2 + CoA Lipid metabolism; sphingolipid metabolism. Forms a heterodimer with sptB. Belongs to the class-II pyridoxal-phosphate-dependent aminotransferase family.