How are protein kinases classified?

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