Involved in L-serine biosynthesis via the phosphorylated pathway, a three-step pathway converting the glycolytic intermediate 3-phospho-D-glycerate into L-serine. Catalyzes the second step, that is the pyridoxal 5'-phosphate-dependent transamination of 3-phosphohydroxypyruvate and L-glutamate to O-phosphoserine (OPS) and alpha-ketoglutarate.