Protein Phosphorylation and Dephosphorylation Are Central to Cellular Control
One common denominator in signal transductions—whether they
involve adenylate cyclase, a transmembrane receptor-tyrosine
kinase, phospholipase C, or an ion channel—is the eventual
regulation of the activity of a protein kinase. We have seen
examples of kinases activated by cAMP, insulin, Ca2+/calmodulin,
Ca2+/diacylglycerol, and by phosphorylation catalyzed by another
protein kinase. The number of known protein kinases has grown
remarkably since their discovery by Edwin G. Krebs and Edmond H.
Fischer in 1959. Hundreds of different protein kinases, each with
its own specific activator and its own specific protein
target(s), may be present in eukaryotic cells. Although many
other types of covalent modifications are known to occur on
proteins, it is clear that phosphorylations make up the vast
majority of known regulatory modifications of proteins.
The addition of a phosphate group to a Ser, Thr, or Tyr residue
introduces a bulky, highly charged group into a region that was
only moderately polar. When the modified side chain is located in
a region of the protein critical to its three-dimensional
structure, phosphorylation can be expected to have dramatic
effects on protein conformation and thus on the catalytic
activity of the protein. As a result of evolution, the
kinase-phosphorylated Ser, Thr, and/or Tyr residues of regulated
proteins occur within common structural motifs (consensus
sequences) that are recognized by their specific protein kinases
Lehninger-Nelson-Cox: Principles of Biochemistry, 777.o.
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