Content
⇒ Cell Communication
⇒ G Proteins
⇒ Kinases and Phosphatases
⇒ The Cell Cycle
Protein kinases phosphorylate other proteins. Some are stimulated at the
beginning of signal transduction pathways by specific growth factors,
whereas others are stimulated at later points during the signal pathway by
the binding of second messengers or by phosphorylation. Some kinases
are membrane-bound, but the majority are free in the cytoplasm. Most
protein kinases are multimeric, consisting of separate catalytic and regulatory
subunits. |
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Protein kinases are categorized on the basis of which amino acids
they phosphorylate (e.g., tyrosine and serine-threonine) as well as on the
basis of their activity (e.g., cAMP-dependent, cyclin-dependent, etc.).
Membrane tyrosine kinases and serine-threonine kinases are generally
stimulated directly by a chemical signal. Protein kinase α(PKA) and protein
kinase G (PKG) are soluble serine-threonine kinases activated by
cAMPand cGMP, respectively. PKC is used to label a large family of serine-
threonine protein kinases that are stimulated directly by the second
messengers DG and/or Ca2+. A protein kinase that requires the calcium
ion binding protein calmodulin and Ca2+ for its activity is known as
calmodulin-calcium-dependent protein kinase. Ca2+ pores in the plasma
membrane and in the endoplasmic reticulum are opened by the binding
of IP3 to the pores. A protein kinase that is required for progression
through the cell cycle is dependent upon a number of protein stimulators
called cyclins.
Because protein kinases are part of most signal pathways, they control
almost every aspect of cellular physiology. Thus, control of a cell’s
physiology is affected by the phosphorylated state of its proteins. If phosphorylation
affects a response, then there must be enzymes that reverse
that response. Protein phosphatases remove phosphate groups from proteins.
Some phosphatases are activated by phosphorylation, some by a
calmodulin-Ca2+ complex, and some by inhibitory proteins.
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