Protein kinases play a predominant regulatory role in nearly every aspect of cell biology and they can modify the function of a
protein in almost every conceivable way.
Protein phosphorylation can increase or decrease
enzyme activity and it can alter other
biological activities such as transcription and translation. Moreover, some phosphorylation sites on a given
protein are stimulatory while others are inhibitory. The human
protein kinase gene family consists of 518 members along with 106 pseudogenes. Furthermore, about 50 of the 518 gene products lack important catalytic residues and are called
protein pseudokinases. The non-catalytic allosteric interaction of
protein kinases and pseudokinases with other
proteins has added an important regulatory feature to the biochemistry and cell biology of the
protein kinase superfamily. With rare exceptions, a divalent
cation such as Mg2+ is required for the reaction. All
protein kinases exist in a basal state and are activated only as necessary by divergent regulatory stimuli. The mechanisms for switching between dormant and active
protein kinases can be intricate.
Phosphorylase kinase was the first
protein kinase to be characterized biochemically and the mechanism of its regulation led to the discovery of
cAMP-dependent protein kinase (
protein kinase A, or PKA), which catalyzes the phosphorylation and activation of
phosphorylase kinase. This was the first
protein kinase cascade or signaling module to be elucidated. The
epidermal growth factor receptor-Ras-Raf-
MEK-ERK signaling module contains
protein-
tyrosine,
protein-
serine/
threonine, and dual specificity
protein kinases. PKA has served as a prototype of this
enzyme family and more is known about this
enzyme than any other
protein kinase. The inactive PKA
holoenzyme consists of two regulatory and two catalytic subunits. After binding four molecules of cAMP, the
holoenzyme dissociates into a regulatory subunit dimer (each monomer binds two cAMP) and two free and active catalytic subunits. PKA and all other
protein kinase domains have a small amino-terminal lobe and large carboxyterminal lobe as determined by X-ray crystallography. The N-lobe and C-lobe form a cleft that serves as a docking site for
MgATP. Nearly all active
protein kinases contain a K/E/
D/D signature sequence that plays important structural and catalytic roles.
Protein kinases contain hydrophobic catalytic and regulatory spines and collateral shell residues that are required to assemble the active
enzyme. There are two general kinds of conformational changes associated with most
protein kinases. The first conformational change involves the formation of an intact regulatory spine to form an active
enzyme. The second conformational change occurs in active
kinases as they toggle between open and closed conformations during their catalytic cycles. 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.
Imatinib was approved by the United States FDA for the treatment of
chronic myelogenous leukemia in 2001; this small molecule inhibits the BCR-Abl
protein kinase oncoprotein that results from the formation of the
Philadelphia chromosome. More than two dozen other orally effective mechanism-based small molecule
protein kinase inhibitors have been subsequently approved by the FDA. These drugs bind to the
ATP-binding site of their target
enzymes and extend into nearby hydrophobic pockets. Most of these
protein kinase inhibitors prolong survival in
cancer patients only weeks or months longer than standard cytotoxic
therapies. In contrast, the clinical effectiveness of
imatinib against
chronic myelogenous leukemia is vastly superior to that of any other targeted
protein kinase inhibitor with overall survival lasting a decade or more. However, the near universal and expected development of drug resistance in the treatment of neoplastic disorders requires new approaches to solve this therapeutic challenge.
Cancer is the predominant indication for these drugs, but disease targets are increasing. For example, we can expect the approval of new drugs inhibiting other
protein kinases in the treatment of illnesses such as
hypertension,
Parkinson's disease, and
autoimmune diseases.