Catalyzes the reversible conversion of 3-phosphohydroxypyruvate to phosphoserine and of 3-hydroxy-2-oxo-4-phosphonooxybutanoate to phosphohydroxythreonine. 2-oxoglutarate + O-phospho-L-serine = 3-phosphooxypyruvate + L-glutamate 2-oxoglutarate + 4-(phosphooxy)-L-threonine = (R)-3-hydroxy-2-oxo-4-phosphooxybutanoate + L-glutamate Binds 1 pyridoxal phosphate per subunit. Amino-acid biosynthesis; L-serine biosynthesis; L-serine from 3-phospho-D-glycerate: step 2/3. Cofactor biosynthesis; pyridoxine 5'-phosphate biosynthesis; pyridoxine 5'-phosphate from D-erythrose 4-phosphate: step 3/5. Homodimer. Expressed in ovary and head. Belongs to the class-V pyridoxal-phosphate-dependent aminotransferase family. SerC subfamily.