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 a large number of catalytically inactive chitinase-like lectins (chitolectins) including YKL-39, YKL-40 (HCGP39), YM1, oviductin, and AMCase (acidic mammalian chitinase), as well as catalytically active chitotriosidases. The conserved domain is an eight-stranded alpha/beta barrel fold belonging to the family 18 glycosyl hydrolases. The fold has a pronounced active-site cleft at the C-terminal end of the beta-barrel. The chitolectins lack a key active site glutamate (the proton donor required for hydrolytic activity) but retain highly conserved residues involved in oligosaccharide binding. Chitotriosidase is a chitinolytic enzyme expressed in maturing macrophages, which suggests that it plays a part in antimicrobial defense. Chitotriosidase hydrolyzes chitotriose, as well as colloidal chitin to yield chitobiose and is therefore considered an exochitinase. Chitotriosidase occurs in two major forms, the large form being converted to the small form by either RNA or post-translational processing. Although the small form, containing the chitinase domain alone, is sufficient for the chitinolytic activity, the additional C-terminal chitin-binding domain of the large form plays a role in processing colloidal chitin. The chitotriosidase gene is nonessential in humans, as about 35% of the population are heterozygous and 6% homozygous for an inactivated form of the gene. HCGP39 is a 39-kDa human cartilage glycoprotein thought to play a role in connective tissue remodeling and defense against pathogens.

Glycosyl hydrolases family 18.

Glyco_18 domain.

The class V plant chitinases have a glycosyl hydrolase family 18 (GH18) domain, but lack the chitin-binding domain present in other GH18 enzymes. The GH18 domain of the class V chitinases has endochitinase activity in some cases and no catalytic activity in others. Included in this family is a lectin found in black locust (Robinia pseudoacacia) bark, which binds chitin but lacks chitinase activity. Also included is a chitinase-related receptor-like kinase (CHRK1) from tobacco (Nicotiana tabacum), with an N-terminal GH18 domain and a C-terminal kinase domain, which is thought to be part of a plant signaling pathway. The GH18 domain of CHRK1 is expressed extracellularly where it binds chitin but lacks chitinase activity.