Tyrosine kinase, catalytic domain. Phosphotransferases. Tyrosine-specific kinase subfamily.

Protein tyrosine kinase.

Protein kinase domain.

Catalytic domain of Protein Kinases. Protein Kinases (PKs), catalytic (c) domain. PKs catalyze the transfer of the gamma-phosphoryl group from ATP to serine/threonine or tyrosine residues on protein substrates. The PK family is part of a larger superfamily that includes the catalytic domains of RIO kinases, aminoglycoside phosphotransferase, choline kinase, phosphoinositide 3-kinase (PI3K), and actin-fragmin kinase. PKs make up a large family of serine/threonine kinases, protein tyrosine kinases (PTKs), and dual-specificity PKs that phosphorylate both serine/threonine and tyrosine residues of target proteins. Majority of protein phosphorylation, about 95%, occurs on serine residues while only 1% occurs on tyrosine residues. Protein phosphorylation is a mechanism by which a wide variety of cellular proteins, such as enzymes and membrane channels, are reversibly regulated in response to certain stimuli. PKs often function as components of signal transduction pathways in which one kinase activates a second kinase, which in turn, may act on other kinases; this sequential action transmits a signal from the cell surface to target proteins, which results in cellular responses. The PK family is one of the largest known protein families with more than 100 homologous yeast enzymes and 550 human proteins. A fraction of PK family members are pseudokinases that lack crucial residues for catalytic activity. The mutiplicity of kinases allows for specific regulation according to substrate, tissue distribution, and cellular localization. PKs regulate many cellular processes including proliferation, division, differentiation, motility, survival, metabolism, cell-cycle progression, cytoskeletal rearrangement, immunity, and neuronal functions. Many kinases are implicated in the development of various human diseases including different types of cancer.

This conserved domain family includes xylanase inhibitor Xip-I, and the class III plant chitinases such as hevamine, concanavalin B, and PPL2, all of which have a glycosyl hydrolase family 18 (GH18) domain. Hevamine is a class III endochitinase that hydrolyzes the linear polysaccharide chains of chitin and peptidoglycan and is important for defense against pathogenic bacteria and fungi. PPL2 (Parkia platycephala lectin 2) is a class III chitinase from Parkia platycephala seeds that hydrolyzes beta(1-4) glycosidic bonds linking 2-acetoamido-2-deoxy-beta-D-glucopyranose units in chitin.