phosphorylated

Meaning: A molecule is phosphorylated when a phosphate group is covalently attached to it. In proteins the phosphate usually goes onto the hydroxyl of serine, threonine or tyrosine. A kinase transfers it from ATP, and a phosphatase removes it by hydrolysis, so the change is reversible and fast. The added group carries negative charge, which can alter folding, electrostatic interactions and the active site. Small molecules are also phosphorylated: glucose becomes glucose-6-phosphate, which is charged and so stays trapped in the cell.

Equation: Protein–OH + ATP → Protein–O–PO₃²⁻ + ADP (catalyzed by a kinase). Protein–O–PO₃²⁻ + H₂O → Protein–OH + Pi (catalyzed by a phosphatase). ATP is adenosine triphosphate, ADP is adenosine diphosphate, Pi is inorganic phosphate. This is a reaction scheme, so no SI units apply.

On the MCAT: Passages show a kinase cascade or a regulated enzyme. You predict whether phosphorylation raises or lowers activity, or what a mutant does. Replacing serine with alanine blocks phosphorylation, while aspartate often mimics it. A gel may show a band shift.

Common confusion: Phosphorylated does not mean activated. Glycogen phosphorylase is activated by phosphorylation, but glycogen synthase is inhibited. Kinases add phosphate, and phosphatases remove it. Oxidative phosphorylation, which makes ATP from ADP, is a different usage.