Fluorescent Sensors of Protein Kinases: From Basics to Biomedical Applications - Progress in Molecular Biology and Translational Science - page 220

signal transduction pathways in healthy cells, and their activity is normally
tightly controlled and finely regulated. Phosphorylation affects substrate
proteins in a variety of ways, by affecting their activity, subcellular
localization, or their stability through routing to the ubiquitin-proteasome
pathway for degradation, by promoting a conformational change in the
structure of the substrate, its association with cofactors, partner proteins,
inhibitors, or gene promoters 12 ( Fig. 6.1B ).
Protein kinases present a very similar structural fold, yet a high degree of
structural plasticity. 13-16 Indeed, the structural conformation of active
kinases differs from that of inactive kinases, yet the catalytic domains of
active kinases adopt similar structures ( Fig. 6.1C ). Despite this structural
homology, the primary sequences and modes of regulation vary widely
from one kinase to another. Their specificity toward substrate recognition
is equally very fine-tuned, and many kinases use binding sites that are
remote from the catalytic site to recruit substrates with higher affinity,
thereby leading to greater catalytic efficiency and specificity. 17,18
Protein kinase activity is finely regulated through a variety of mecha-
nisms, including activating phosphorylation (cyclin-dependent kinases,
CDKs) or inhibitory phosphorylation (Src); through dimerization or
multimerization (insulin receptor, EGF receptor); through allosteric regula-
tion, upon binding of activator or inhibitor proteins, cofactors, or small
molecules; through control of accessibility to their substrates; or through
autoinhibition (PKA/PKI). 16 Protein kinases are “molecular switches”
which may adopt at least two conformations corresponding to an “ON”
state and an “OFF” state. Activation is associated with a net conformational
change of a dynamic segment, termed “the activation loop”, thereby
providing an accessible binding site for the substrate. 16,19
Protein kinases have been the focus of many studies not only because of
their central role in major signaling pathways and biological processes but
also because of their implication in disease. Indeed, a large number of
genetic alterations result in deregulated protein kinase activity, thereby
prompting or contributing to the development of pathological conditions,
including cardiovascular diseases, neurodegenerative and endocrinological
disorders, immune deficiency and viral infection, psoriasis, cancer, and
diabetes. 20-23 As such, protein kinases constitute disease biomarkers and
pharmacological targets in a variety of human diseases, and several
small-molecule inhibitors have been developed to interfere with their
function, some of which have become therapeutics administered in
the clinic. 21-28