Phosphorylation

Phosphorylation refers to the attachment of a phosphate group to inorganic molecules, especially proteins.

The Ph. usually changes the configuration of the molecule because the phosphate group has a polar charge that interacts with the charges of the molecule and causes shifts in the structure. Thus there are then two different molecules of the (almost) same kind.

The phosphorylated molecule can now dock to certain receptors and trigger further actions due to its new form. One also speaks of the active (phosporylated) and the inactive form of the molecule.

https://en.wikipedia.org/wiki/Substrate-level_phosphorylation