The enzyme protein serine/threonine phosphatase is a form of phosphoprotein phosphatase that acts upon phosphorylated serine/threonine residues: [a protein]-serine/threonine phosphate + H2O = [a protein]-serine/threonine + phosphate Serine and threonine phosphates are stable under physiological conditions, so a phosphatase enzyme has to remove the phosphate to reverse the regulation signal. Wikipedia