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Phosphatidylinositol 3-kinase (PI3K) adds a phosphate group to the 3C position in a inosital ring in Phosphatidylinositol.

For example:

PI-4,5-phosphate (PIP2) + ATP ----> PI-3,4, 5-bisphosphate (PIP3) + ADP

This in the context of the Insulin pathway allows the resulting PIP3 to remain bound to the plasma membrane. This initiates a signal cascade which can cause GLUT4 translocation, decreased synthesis of glycerol and fatty acids, increased glycogen synthesis and increased protein synthesis in the cell.

The enzyme has 2 subunits:

  1. Regulatory subunit, p85 (85 kDa), contains 2 SH2 domains and 1 SH3 domain.
  2. Catalytic subunit, p110 (110 kDa).

The SH2 domain in the context of the Insulin pathway binds p85 to phosphorylated tyrosines on the Insulin receptor substrate (IRS). The SH3 domain targets the proline rich section of p85 binding both regulatory and catalytic subunits together.

There are three classes of P13K enzymes. Class I can phosphorylate 3 different substrates: non-phosphorylated phosphatidylinositol, inositol monophosphate and bisphosphate inositol. The catalytic subunit is p100 and the regulatory subunit is p85. This class of P13K are associated with being of oncogenic nature, as the catalytic subunit can become mutated, unlike class II and III[1].

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