Covalent modification of enzymes
1. Involves adding or removing a group from an enzyme
2. Most common: addition and removal of phosphate group
3. Phosphate group derived from ATP
4. Addition of phosphate (phosphorylation) catalyzed by kinase enzyme
5. Removal of phosphate (dephosphorylation) catalyzed by phosphatase enzyme
6. Phosphate group added to/removed from R group of serine, tyrosine, or threonine amino acid residue