Displays phosphatase activity for serine/threonine residues. Has apparently no phosphoglycerate mutase activity. May be regulator of mitochondrial dynamics (By similarity). May be a central mediator for programmed necrosis (By similarity). H2O + O-phospho-L-seryl-[protein] = L-seryl-[protein] + phosphate H2O + O-phospho-L-threonyl-[protein] = L-threonyl-[protein] + phosphate The N-terminal 35 amino acids, including the potential transmembrane alpha-helix, function as a non-cleaved mitochondrial targeting sequence that targets the protein to the cytosolic side of the outer mitochondrial membrane. Phosphorylated by the RIPK1/RIPK3 complex under necrotic conditions. This phosphorylation increases PGAM5 phosphatase activity (By similarity). Belongs to the phosphoglycerate mutase family. BPG-dependent PGAM subfamily.