Answer: Through the help of protein kinases
Explanation: Protein kinase help phosphorelate the amino acid residue on the protein. Phosphorylation changes the structure of a protein and modifies it's functions.
Dephosphorylation which is the opposite, is catalysed by protein phosphatase. Enzymes kinase and phosphatase work independently and in balance to regulate the functions of proteins
Phosphorylation has a role to play in signalling pathways and metabolism. Protein phosphorylation is the major molecular mechanism in which protein function is regulated in response to stimuli outside the cell both inside and outside the nervous system.
Most extracellular signals like light, hormones, neurotransmitters, cytokines, produce most of their physiological functions by regulating the phosphorylation of specific phosphoproteins in their target cells.