The Protein Kinase Domain
المؤلف:
Hoffman, R., Benz, E. J., Silberstein, L. E., Heslop, H., Weitz, J., & Salama, M. E.
المصدر:
Hematology : Basic Principles and Practice
الجزء والصفحة:
8th E , P75-76
2025-07-28
629
Protein kinases catalyze the transfer of a phosphate group from adenosine triphosphate (ATP) to specific sites on target proteins. More than 500 protein kinases have been identified in the human genome; approximately 90 of these are tyrosine kinases, the remainder specifically phosphorylate serine or threonine residues. Both serine/ threonine and tyrosine kinases share a conserved bilobed protein fold, composed of a smaller N-terminal subdomain (N-lobe) and larger C-terminal subdomain (C-lobe).[1] The active site cleft, including the site for binding the substrate ATP, is found at the interface between the N- and C-lobes. The phosphate-coordinating “P-loop” is a portion of the β-sheet in the N-lobe that coordinates the triphosphate moiety of ATP. The activity of protein kinases is often regulated by phosphorylation on a loop in the C-lobe termed the activation loop or A-loop. In the absence of phosphorylation, the A-loop may play an inhibitory role, sometimes blocking binding of ATP in the active site, or it may be disordered altogether. Upon autophosphorylation, or phosphorylation in trans by an upstream activating kinase, the activation loop rearranges to adopt a characteristic hairpin con formation that creates the site for docking of the polypeptide segment that will become phosphorylated. Activation loop phosphorylation may also induce other structural rearrangements required for catalytic activation, in particular a reorientation of a helix within the N-lobe (known as the C-helix) that brings a glutamic acid residue into proper position within the active site (Fig. 1A).
Fig1. (A) A kinase domain in complex with an adenosine triphosphate (ATP) analog and peptide substrate (PDB entry 1IR3). The phosphate-binding loop is highlighted purple, the activation loop is red, the substrate peptide is yellow, and the ATP analog is shown in gray. (B) The autoinhibited structure of Abelson tyrosine kinase (c-ABL) in complex with the kinase inhibitor PD166326 (PDB entry 1OPK). The Src homology-3 (SH3), SH2, and kinase domains are shown in yellow, green, and blue, respectively. The SH2–kinase domain linker and the SH3-SH2 connector are shown in red. The myristate is shown in orange spheres in the C-lobe of the kinase. PDB, Protein Data Bank.
Deregulated tyrosine kinases are the cause of a number of hematologic malignancies. Two general classes of tyrosine kinases can be defined: receptor and non-receptor tyrosine kinases. Receptor tyrosine kinases are transmembrane proteins with an extracellular ligand–binding domain, a single transmembrane domain, and the cytoplasmic tyrosine kinase domain. They are typically activated by dimerization upon binding of ligands to their extracellular region, which induces autophosphorylation and activation of their catalytic domains inside the cell.[2] Chromosomal translocations that underlie a number of human leukemias fuse a tyrosine kinase domain to an oligomerization domain from an otherwise unrelated protein, often the dimerization domain of a transcription factor, to generate a constitutively dimeric, and therefore constitutively active kinase. Examples of such oncogenic translocations include the fusion of the dimerization domain of an ETS-family transcription factor ETV6 (also called Tel) to a Jak-family tyrosine kinase in the leukemogenic ETV6-Jak2 fusion,[3] and the fusion of the oligomerization domain of nucleophosmin with the tyrosine kinase domain of ALK in the NPM-ALK fusion in anaplastic large cell lymphoma.[4]
Perhaps the best characterized kinase translocation is the BCR ABL fusion protein produced by the (9:22) chromosomal translocation in chronic myelogenous leukemia. Treatment of this disease with imatinib, a specific inhibitor of ABL, has established a paradigm for targeted therapy in cancer.[5] ABL is a non–receptor tyrosine kinase which contains Src-homology 3 and 2 (SH3 and SH2) domains in addition to its tyrosine kinase domain. In addition, the normal ABL protein is myristoylated at its N-terminus. In the normal protein, the N-terminal region including the myristoyl-group and adjacent sequences and the SH3 and SH2 domains assemble with the kinase domain to lock it in an inactive conformation (Fig. 1B).[6] These interactions are released to activate the kinase when the phosphotyrosine-binding SH2 domain and proline motif-binding SH3 domains bind their cognate ligands in a target protein.[7] The myristoyl group may also be released from its docking site in the C-lobe of the kinase upon activation to promote mem brane localization of the protein. Thus in its normal state, the various domains of ABL comprise an exquisite signaling switch that is regulated by appropriate binding interactions; in the absence of the proper targeting interactions, the kinase is maintained in an inactive state by the intramolecular associations of its domains. In the oncogenic BCR-ABL fusion protein, this regulatory control is lost because the N-terminal regulatory region including the myristoylation site is truncated and replaced with unrelated sequences from the BCR protein. Interestingly, the vacant myristate pocket in BCR-ABL is the target of recently developed allosteric inhibitors of BCR-ABL, which may synergize with ATP-site inhibitors.[8]
High-resolution structural information has facilitated development of selective inhibitors for many tyrosine kinases in addition to BCR-ABL. Prominent examples include ruxolitinib, a Jak1/Jak2 inhibitor that is used to treat myelofibrosis driven by JAK2V617F, and ibrutinib, a Bruton tyrosine kinase (BTK) inhibitor that is used in the treatment of chronic lymphocytic leukemia, mantle cell lymphoma, and Waldenström macroglobulinemia.[9,10].
References
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[5] Druker BJ. Translation of the Philadelphia chromosome into therapy for CML. Blood. 2008;112:4808–4817.
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[7] Nagar B, Hantschel O, Seeliger M, et al. Organization of the SH3-SH2 unit in active and inactive forms of the c-Abl tyrosine kinase. Mol Cell. 2006;21:787–798.
[8] Wylie AA, Schoepfer J, Jahnke W, et al. The allosteric inhibitor ABL001 enables dual targeting of BCR-ABL1. Nature. 2017;543:733–737. https:// doi.org/10.1038/nature21702.
[9] Hobbs GS, Rozelle S, Mullally A. The development and use of Janus kinase 2 inhibitors for the treatment of myeloproliferative neoplasms. Hematol Oncol Clin North Am. 2017;31:613–626. https://doi.org/10.1016/j. hoc.2017.04.002.
[10] Wen T, Wang J, Shi Y, Qian H, Liu P. Inhibitors targeting Bruton’s tyrosine kinase in cancers: drug development advances. Leukemia. 2020;35:312–332. https://doi.org/10.1038/s41375-020-01072-6.
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