There are multiple ways in which protein kinases can be classified.
Classification system 1
Protein kinases are classified into five major groups based on the type of amino acid residues that are phosphorylated by the enzyme. The five types of protein kinases include:
Serine/threonine-specific protein kinase (STPK):A large family of protein kinases, STPKs can be further divided into two categories – classical STPKs, which are present in all eukaryotic cells and atypical STPKs, which are found only in certain types of cells. STPKs phosphorylate serine and threonine residues on proteins.
Tyrosine- specific protein kinases (TKs):Tyrosine-specific protein kinases can be further divided into two categories – receptor-associated TKs, which are present in the cytoplasm and bind to proteins called receptors and non-receptor TKs, which are present in the nucleus and bind to DNA. Both types of TKs phosphorylate tyrosine residues.
Dual specificity protein kinases (DSPKs):DSPKs can be further categorized as type I DSPKs, type II DSPKs, and atypical DSPKs. All categories of DSPKs phosphorylate both serine and tyrosine residues on proteins.
Aspartic acid/glutamic acid-specific protein kinases:This category of protein kinases phosphorylate aspartate/ glutamate. The phosphorous receptor is the acyl group of the protein.
Histidine-specific protein kinases:This category of protein kinases phosphorylates histidine in a substrate protein.
Classification system 2
Protein kinases can also be classified according to their function. Under this classification system, there are four main categories of protein kinases:
Signaling protein kinases, which play a pivotal role in signal transduction
Metabolic protein kinases, which are responsible for regulating metabolism
Housekeeping protein kinases, which perform essential cellular functions