26 Kinases and phosphatases in cell signalling
Regulation of enzyme and protein activity is often achieved by phosphorylation or dephosphorylation directed. The presence of a negatively charged phosphate group at specific residues in the target protein affects the conformation of the polypeptide, which can cause either activation or inhibition depending on the protein.
Phosphorylation
Protein kinases
Protein kinases transfer the terminal phosphate from ATP onto residues containing a hydroxyl group within a specific consensus amino acid sequence. Most protein phosphorylation is catalysed by serine and theonine kinases with only a small proportion (0.1%) catalysed by tyrosine kinases.
All protein kinases have regulatory and catalytic elements. These may be contained on separate polypeptide subunits (e.g. cyclic AMP-dependent protein kinase (cA-PK)) (Fig. 3.26.1) or combined in a single polypeptide (e.g. protein kinase C). Protein kinase C is maintained in an inactive form by binding of a regulatory domain with a pseudosubstrate motif to block the active site of the catalytic domain. Binding of diacylglycerol (DAG) and Ca2+
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