Protein Kinase and Phosphatase play a vital role in the transduction of stimuli received by the cell to generate specific response.

1. Signal Transduction:
Kinase & Phosphatase
Presented By:
Dr. Vibha Khanna
Associate Professor
(Botany)

2. Signal Transduction
• Signal transduction is based on two main principles.
• First is the posttranslational modification of pre-existing proteins, such as
phosphorylation, isoprenylation, lipidation, methylation, glycosylation, or
partial proteolysis.
• Second is production of new proteins or second messengers.
• Phosphorylation is a primary means of signal transduction. It can
function as an activating or inhibitory signal depending on the
substrate and site of phosphorylation.
• The second messenger stimulate protein kinases, which catalyze
phosphorylation.
• Protein kinases act as a means of signal amplification (e.g., the
activation of a single kinase molecule can result in the
phosphorylation of many proteins)

3. Protein Kinases (PTKs)
• Protein kinases (PTKs) are enzymes that regulate the biological
activity of proteins by phosphorylation of specific amino acids with
ATP as the source of phosphate, thereby inducing a conformational
change from an inactive to an active form of the protein.
• Eukaryotic organisms possess are three main types of PTKs, which are
classified according to the amino acid side chain that they
phosphorylate:
• Tyrosine kinases (TKs) that phosphorylate the Tyr phenolic hydroxyl
• Serine-threonine kinases that phosphorylate the hydroxy group of
these two amino acids
• Histidine kinases, recently discovered, that phosphorylate the nitrogen
of His residues.

4. Types of Protein Kinases
• PTKs are usually cytoplasmic enzymes, but others, known as receptor protein
kinases (RPTKs), transverse the cell membrane and have dual roles as enzymes and as
receptors.
• RPTKs proteins have an extracellular domain that recognizes an external messenger (growth
hormones, growth factors) and an intracellular kinase active site that becomes activated upon
binding of the messenger, triggering a signaling cascade that ultimately controls the transcription
of specific genes related to cellular proliferation and differentiation.
• Non-receptor PTKs have no extra-cellular domain, and are activated by upstream signalling
molecules such as G-protein–coupled receptors and immune system receptors, and also by
receptor TKs.
• The main protein kinases includes, PKA, PKG, and PKC as well as tyrosyl protein
kinases (part of tyrosine kinase receptors). They are distinguished from each other by the
different intracellular second messengers involved in their regulation and by the selective
substrates they use.
• They all have a binding site for Mg2+-ATP (phosphate donor) and for substrate protein as
well as various regulatory sites.
• Phosphorylation of these proteins is short lived because protein phosphatases rapidly
dephosphorylate proteins previously phosphorylated by protein kinases, thus
terminating the intracellular signal.

5. Substrate Identification by Kinases
• Kinases recognize their substrate
proteins through two different
mechanisms
• (1) proximal mode, where a kinase
recognize a peptide sequence as a target,
• (2) Distal mode, a domain on substrate
protein is recognized by a
recognition/cognate domain on kinase
molecules.
• Apart from these two modes of
substrate identification, the kinases
also find/move directly to their
substrate molecules with the help of
adaptor proteins such as Beta-
arrestins.

6. Regulation of Protein Kinase Activity
• Protein kinase activities are regulated by interaction with other
proteins.
• Some of these may be protein kinases themselves as occurs during
multilayer protein kinase cascades.
• Other protein–protein interactions contribute to controlling kinase activity in
a second messenger or receptor–ligand interaction dependent manner (e.g.
Ca2+/calmodulin kinases).
• A third category involves enzyme activities that directly control the level of
target protein phosphorylation.
• These enzymes are collectively termed protein phosphatases and are the functional
antagonists of protein kinases.
• Frequently, protein phosphatase activities are regulated by the same stimuli which
participate in controlling protein kinases activities.
• Thus protein phosphatases are sensitive to second messenger
systems and receptor–ligand interactions.

7. Kinase and Phosphatase
• Protein kinases and phosphatases are enzymes catalysing the
transfer of phosphate between their substrates.
• A protein kinase catalyses the transfer of -phosphate from ATP (or GTP) to its
protein substrates while
• a protein phosphatase catalyses the transfer of the phosphate from a
phosphoprotein to a water molecule.

8. Kinase Cascading
• Inside cells, kinases interact with
other kinases in a hierarchy
where kinases higher in
hierarchy bring about activation
of lower kinases and signal is
thereby amplified at each step
much like a cascade.
• Phosphatases usually don’t
work that way, and therefore to
maintain a balance, phosphatase
activity is present in several fold
excess of kinase activity,

9. Kinase and Phosphatase
• Protein kinases and phosphatases are both phosphotransferases,
however, their function is tightly regulated,
• phosphorylation is always catalysed by kinases whereas
• dephosphorylation is driven by phosphatases.
• Protein phosphorylation and dephosphorylation are critical for
signaling, cell division, protein translation, metabolism and survival.
• Cells maintain a basal kinase and phosphatase activity, however,
their activities are inducible in highly specific manner.