phospholipases

Phospholipases are enzymes that hydrolyze phospholipids, releasing fatty acids and head group moieties. They act on glycerophospholipids and related lipids and participate in digestion, membrane remodeling, and cellular signaling. The major families are phospholipases A, B, C, and D, with multiple isoforms in animals, plants, and microbes. Distinct enzymes differ in their substrate preferences, cleavage sites, calcium dependence, and cellular localization.

Phospholipase A enzymes cleave the fatty acyl chain at one of the sn-1 or sn-2 positions of the glycerol backbone. Phospholipase A1 (PLA1) and phospholipase A2 (PLA2) release a free fatty acid and yield a lysophospholipid; PLA2 activity commonly releases arachidonic acid, a precursor of eicosanoids. Mammalian PLA2 enzymes include secreted (sPLA2) and cytosolic (cPLA2) forms with roles in inflammation and signaling. Phospholipase B can hydrolyze both acyl chains, generating glycerophosphodiesters, but it is less common in mammals.

Phospholipase C enzymes cleave the glycerophospholipid just before the phosphate, producing a diacylglycerol (DAG) and a phosphorylated head group (such as inositol trisphosphate, IP3). This activity is central to many signaling pathways that regulate calcium mobilization and protein kinase C activation.

Phospholipase D enzymes cleave after the phosphate, yielding a free head group and phosphatidic acid, a lipid second messenger involved in diverse signaling and membrane dynamics. PLD activity is important in plants and animals and can modulate vesicle trafficking and stress responses.

Together, phospholipases shape lipid composition, generate signaling molecules, and influence physiological and pathological processes.