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Receptor Tyrosine Kinases
Another important type of second-messenger system involves a receptor type called a receptor tyrosine kinase. These receptors span the membrane of the cell and have a hormone receptor on the outside and a tyrosine kinase portion on the inside. There are several subclasses of these receptor kinases, as shown in Figure 24.13. The best known of these is class II, which includes the insulin receptor.
These kinases are allosteric enzymes. When the hormone binds to the bind-ing region on the outside of the cell, it induces a conformational change in the tyrosine kinase domain that activates the kinase activity.
The activated tyrosine kinases phosphorylate tyrosines on a variety of target proteins, causing altera-tions in membrane transport of ions and amino acids and the transcription of certain genes. Phospholipase C (seen in Figure 24.12) is one of the targets of tyrosine kinases. Another is an insulin-sensitive protein kinase, which phos-phorylates and activates protein phosphatase 1.
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