Invited ReviewA historical overview of protein kinases and their targeted small molecule inhibitors
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Section snippets
The protein kinase enzyme family
Protein kinases play pivotal roles in nearly every aspect of cellular function [1]. They control metabolism, transcription, cell division and movement, programmed cell death, and they participate in the immune response and nervous system function. Protein phosphorylation involves the balanced action of protein kinases and phosphoprotein phosphatases making phosphorylation–dephosphorylation an overall reversible process [1], [2]. Owing to the overall importance of protein phosphorylation,
Phosphorylated proteins
Casein, a milk protein, and phosvitin, an egg yolk protein, are two of the earliest known phosphoproteins [10]. Casein contains about 3% and phosvitin contains about 10% phosphorus by weight. The latter contains one phosphate group for every two amino acid residues thereby making it the most highly phosphorylated protein in nature. Lipmann and Levine identified phosphoserine in phosvitin in 1932 [10]. At the time threonine was unknown. W.C. Rose and two of his graduate students described it as
Protein kinases and their activation by second messengers
Working in the Ben May Laboratory for Cancer Research at the University of Chicago, Williams-Ashman and Kennedy [16] reported that protein phosphorylation was especially active in malignant cells such as Ehrlich ascites tumor cells. Subsequently, Kennedy and Smith isolated radioactive phosphoserine of very high specific activity from the protein fraction of these tumor cells after incubation with [32P]-phosphate [17]. They demonstrated that the phosphate moiety of phosphoserine in the protein
Primary structures of protein kinases
Czernilofsky et al. [66], [67] reported the amino acid sequence of the Schmidt-Rupin strain of v-Src in 1980 based upon its nucleotide sequence and Shoji et al. [68] reported the sequence of the catalytic subunit of bovine PKA in 1981 using Edman degradation of cyanogen bromide and trypsin peptides. Owing to the incomplete nature of the protein sequence data bases at the time, the identification of v-Src as a protein kinase was not made until 1982 [69]. The signatures that enabled this
The regulatory spine
Kornev et al. [81], [82] compared the spatial arrangements of amino acid residues in about two dozen active and inactive protein kinases using a local spatial pattern (LSP) alignment algorithm. They used this information to establish the existence of a regulatory and a catalytic spine within the protein kinase domain. In contrast to protein kinase amino acid signatures such as Y/HRD or DFG, the residues that constitute the spines were not identified by sequence analyses per se. Rather, the
PKA signaling
As noted previously, the PKA holoenzyme consists of two general types (I and II) originally based upon their order of elution from DEAE-cellulose anion exchange resin; the type I enzyme elutes first at a lower salt concentration than the type II enzyme. As noted previously, PKA holoenzymes exist as an inactive tetrameric complex composed of two catalytic and two regulatory subunits (R2C2) [60]. Mammals possess four non-redundant R-subunits (RIα, RIβ, RIIα, and RIIβ), which differ in their
Secondary and tertiary structures of Abl
The small lobe of all protein kinases including Abl is dominated by a five-stranded antiparallel β-sheet (β1–β5) and an important regulatory αC-helix [74], [98]. The first X-ray structure of a protein kinase (PKA) [72], [73] contained an αA and an αB-helix proximal to αC (PDB ID:2CPK), but these first two helices are not conserved in the protein kinase family. The large lobe of the Abl protein kinase domain is mainly α-helical with six conserved segments (αD–αI) that occur in all protein
Pseudokinase properties
In their comprehensive description of the protein kinase complement of the human genome (kinome), Manning et al. [1] reported that 50 of the 518 protein kinases lack amino acid residues important for catalysis. Thus, the absence of the αC-Glu or β3-Lys or alterations in the DFG or HRD signatures was ascribed to catalytically inactive kinases, or pseudokinases. These pseudokinases are scattered throughout the various protein kinase subfamilies, suggesting that they have evolved from diverse
Overview of inhibitors
Because mutations and dysregulation of protein kinases play causal roles in human disease, this family of enzymes has become one of the most important drug targets over the past two decades [108]. Trastuzumab was the first FDA-approved protein kinase inhibitor (1999); this biologic is a monoclonal antibody that inhibits ErbB2 and is used for the treatment of ErbB2-positive breast, gastric, and gastroesophageal cancers [45], [77]. Several other large molecule protein kinase inhibitors have been
Ph+ chronic myelogenous leukemia
Rowley discovered that the Philadelphia chromosome is formed from a reciprocal translocation t(9;22)(q34;q11.2) that results in a lengthened chromosome 9 and a shortened chromosome 22 (the Philadelphia chromosome) [132]. The Philadelphia chromosome occurs in about 95% of chronic myelogenous leukemia patients. The BCR-Abl oncogene results from this translocation where Abl is the human ortholog of the murine Abelson leukemia virus, which was originally on chromosome 9, and BCR refers to the
The role of serendipity in drug development
Imatinib was developed as a PDGFR inhibitor, but its initially approved therapeutic targets included BCR-Abl and Kit [109], [164]. Imatinib was approved in 2001 for the treatment of BCR-Abl-positive CML and in 2002 for the treatment of GIST with KIT mutations. The drug has subsequently been FDA-approved for the treatment of a variety of other diseases including adults with (i) relapsed or refractory Ph+ ALL, (ii) myelodysplastic/myeloproliferative diseases associated with PDGFR gene
Epilog
Human immunodeficiency virus infection and acquired immune deficiency syndrome (HIV/AIDS) result in a disease with variable signs and symptoms [180]. HIV is a retrovirus that primarily infects components of the human immune system such as CD4+ T cells, macrophages, and dendritic cells. It directly and indirectly destroys CD4+ T cells. HIV1 and HIV2 contain genes that encode the proteins required for virus replication: gag encodes the proteins that form the core of the virion, pol encodes
Conflict of interest
The author is unaware of any affiliations, memberships, or financial holdings that might be perceived as affecting the objectivity of this review.
Acknowledgement
The author thanks Laura M. Roskoski for providing editorial and bibliographic assistance.
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