phosphatase

Meaning: A phosphatase catalyzes hydrolysis of a phosphate ester, releasing inorganic phosphate (Pi) and leaving a free hydroxyl group on the substrate. Kinases add phosphate to Ser, Thr, or Tyr side chains, and phosphatases remove it, so phosphorylation acts as a reversible on/off switch. Added phosphate brings negative charge, which can change a protein's shape and activity. Some enzymes are activated by phosphorylation (glycogen phosphorylase) and others are inhibited by it (glycogen synthase), so phosphatases switch each in the opposite direction. Sugar phosphatases also matter in metabolism: fructose-1,6-bisphosphatase and glucose-6-phosphatase bypass irreversible glycolysis steps in gluconeogenesis.

Equation: R–O–PO₃²⁻ + H₂O → R–OH + HPO₄²⁻, where R is a protein or sugar, H₂O is water, and HPO₄²⁻ is inorganic phosphate (Pi). These are chemical species, so no SI unit applies; amounts are in mol.

On the MCAT: A passage gives a signaling or metabolic pathway and adds a kinase or phosphatase inhibitor, or a mutation. You predict whether the target ends up more or less phosphorylated and whether its activity rises or falls. Gluconeogenesis questions use the phosphatase steps.

Common confusion: A phosphatase is not a phosphorylase. A phosphatase uses water to remove phosphate. A phosphorylase uses Pi to cleave a bond, as glycogen phosphorylase does to make glucose-1-phosphate. Also, removing phosphate does not always inactivate; the effect depends on the protein.