phosphate group

Meaning: A phosphate group is a phosphorus atom bonded to four oxygens, written –OPO₃²⁻ when attached to a molecule at physiological pH. It is usually joined to an –OH group (on serine, threonine, tyrosine, a sugar or a nucleoside) by a phosphoester bond. Phosphoanhydride bonds link the phosphates of ATP, and phosphodiester bonds form the backbone of DNA and RNA. Because the group is negatively charged, adding one changes a protein's charge, folding and activity. Kinases add phosphate, often from ATP; phosphatases remove it by hydrolysis.

On the MCAT: A passage describes a kinase or phosphatase acting on an enzyme or receptor, and you predict whether activity rises or falls. Questions also ask why ATP hydrolysis is favorable (charge repulsion among adjacent phosphates and resonance stabilization of the products), why phosphorylating glucose traps it in the cell (the charged product cannot easily cross the membrane), and how phosphate buffers near pH 7 (H₂PO₄⁻ ⇌ HPO₄²⁻ + H⁺, pKa about 7.2).

Common confusion: Phosphorylation does not automatically activate a protein. It activates some (glycogen phosphorylase) and inhibits others (glycogen synthase), so read the passage. Also, breaking a bond costs energy; ATP hydrolysis releases energy because its products are more stable than the reactants.