Phosphoinositide Phospholipase C - Reaction and Catalytic Mechanism

Reaction and Catalytic Mechanism

All family members are capable of catalyzing the hydrolysis of PIP₂, a phosphatidylinositol at the inner leaflet of the plasma membrane into the two second messengers, inositol triphosphate (IP₃) and diacylglycerol (DAG).

The chemical reaction may be expressed as:

1-phosphatidyl-1D-myo-inositol 4,5-bisphosphate + H₂O 1D-myo-inositol 1,4,5-trisphosphate + diacylglycerol

Thus, the two substrates of this enzyme are H₂O and 1-phosphatidyl-1D-myo-inositol 4,5-bisphosphate (PIP₂, phosphatidylinositol bisphosphate), whereas its two products are diacylglycerol and 1D-myo-inositol 1,4,5-trisphosphate (IP₃, inositol triphosphate).

PLCs catalyze the reaction in two sequential steps. The first reaction is a phosphotransferase step that involves an intramolecular attack between the hydroxyl group at the 2' position on the inositol ring and the adjacent phosphate group resulting in a cyclic IP₃ intermediate. At this point, DAG is generated. However, in the second phosphodiesterase step, the cyclic intermediate is held within the active site long enough to be attacked by a molecule of water, resulting in a final acyclic IP₃ product. It should be mentioned that bacterial forms of the enzyme, which contain only the catalytic lipase domain, produce cyclic intermediates exclusively, whereas the mammalian isoforms generate predominantly the acyclic product. However, it is possible to alter experimental conditions (e.g., temperature, pH) in vitro such that some mammalian isoforms will alter the degree to which they produce mixtures of cyclic/acyclic products along with DAG. This catalytic process is tightly regulated by reversible phosphorylation of different phosphoinositides and their affinity for different regulatory proteins.