Tyrosine phosphorylation is the addition of a phosphate (PO43-) group to the amino acid tyrosine on the protein. This transfer of phosphate group from ATP to the amino acid tyrosine on the protein is made possible through the enzyme tyrosine kinase. Tyrosine phosphorylation is considered to be one of the key steps in signal transduction and regulation of enzymatic activity. In tyrosine phosphorylation are important two classes of tyrosine kinase, receptor tyrosine kinase and nonreceptor tyrosine kinase. Receptor tyrosine kinases are type I transmembrane proteins possessing an N-terminal extracellular domain, which can bind activating ligands, a single transmembrane domain, and a C-terminal cytoplasmic domain that includes the catalytic domain. Nonreceptor tyrosine kinases lack a transmembrane domain, most are soluble intracellular proteins, but a subset associate with membranes via a membrane-targeting posttranslational modification, such as an N-terminal myristoyl group, and can act as the catalytic subunit for receptors that lack their own catalytic domain.

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Wiki: Tyrosine Phosphorylation

    Protein phosphorylation is a post-translational modification of proteins in which a serine, a threonine or a tyrosine residue is phosphorylated by a protein kinase by the addition of a covalently bound phosphate group. Regulation of proteins by phosphorylation is one of the most common modes of regulation of protein function, and is often termed "phosphoregulation". In almost all cases of phosphoregulation, the protein switches between a phosphorylated and an unphosphorylated form, and one of these two is an active form, while the other one is an inactive form. A serine/threonine protein kinase is a kinase enzyme that phosphorylates the OH group of serine or threonine (which have similar sidechains). At least 125 of the 500+ human protein kinases are serine/threonine kinases (STK). Serine/Threonine Kinase receptors play a role in the regulation of cell proliferation, programmed cell death (apoptosis), cell differentiation, and embryonic development.

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Wiki: Threonine Phosphorylation

    Protein phosphorylation is a post-translational modification of proteins in which a serine, a threonine or a tyrosine residue is phosphorylated by a protein kinase by the addition of a covalently bound phosphate group. Regulation of proteins by phosphorylation is one of the most common modes of regulation of protein function, and is often termed "phosphoregulation". In almost all cases of phosphoregulation, the protein switches between a phosphorylated and an unphosphorylated form, and one of these two is an active form, while the other one is an inactive form. A serine/threonine protein kinase is a kinase enzyme that phosphorylates the OH group of serine or threonine (which have similar sidechains). At least 125 of the 500+ human protein kinases are serine/threonine kinases (STK). Serine/Threonine Kinase receptors play a role in the regulation of cell proliferation, programmed cell death (apoptosis), cell differentiation, and embryonic development.

Reference
Wiki: Serine Phosphorylation

    Protein phosphorylation is a post-translational modification of proteins in which a serine, a threonine or a tyrosine residue is phosphorylated by a protein kinase by the addition of a covalently bound phosphate group. Regulation of proteins by phosphorylation is one of the most common modes of regulation of protein function, and is often termed "phosphoregulation". In almost all cases of phosphoregulation, the protein switches between a phosphorylated and an unphosphorylated form, and one of these two is an active form, while the other one is an inactive form.

Reference
Wiki: Phosphorylation

    Dephosphorylation is the removal of a phosphate (PO43-) group from an organic compound by hydrolysis. It is a reversible post-translational modification that is coupled to the addition of phosphate groups, or phosphorylation. A highly regulated process, dephosphorylation activates and deactivates enzymes by cleaving phosphoric esters and anhydrides. A notable occurrence of dephosphorylation is the conversion of ATP to ADP and inorganic phosphate. Dephosphorylation employs a type of hydrolytic enzyme, or hydrolase, which cleave ester bonds. The prominent hydrolase subclass used in dephosphorylation is phosphatase. Phosphatase removes phosphate groups by hydrolysing phosphoric acid monoesters into a phosphate ion and a molecule with a free hydroxyl (-OH) group. The reversible phosphorylation-dephosphorylation reaction occurs in every physiological process, making proper function of protein phosphases necessary for organism viability. Because protein dephosphorylation is a key process involved in cell signalling, protein phosphatases are implicated in conditions such as cardiac disease, diabetes, and Alzheimer's disease.

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Wiki: Dephosphorylation