Phosphatidic acid (PA) serves as the foundational glycerophospholipid and a central metabolic intermediate. It consists of a diacylglycerol backbone esterified at the sn-3 position with a phosphate group (-PO4H2), without additional headgroup moieties that characterize phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS), phosphatidylinositol (PI), or phosphatidylglycerol (PG). Present at low steady-state levels (1–3% of membrane lipids), its conical geometry and rapid interconversion make PA a dynamic regulator of membrane curvature and intracellular signaling.

Molecular Structure

Phosphatidic acid comprises sn-glycerol-3-phosphate with acyl chains typically arranged as a saturated fatty acid at the sn-1 position (commonly 16:0 or 18:0) and an unsaturated chain at sn-2 (such as 18:1 or 20:4). This generates molecular species including 16:0/18:1-PA or 18:0/20:4-PA. The phosphomonoester group (pKa1 ~3, pKa2 ~8) confers one to two negative charges at physiological pH, enabling cation bridging interactions. Variants include lysophosphatidic acid (lyso-PA, containing a single acyl chain) and cyclic phosphatidic acid (cyclic-PA).

Biophysical Properties

Due to its small, highly charged headgroup, PA induces negative membrane curvature and favors the formation of non-bilayer hexagonal (HII) phases, particularly in the presence of Ca²⁺ or Mg²⁺ neutralization. This promotes membrane remodeling events such as vesicle budding, fission, fusion, and endoplasmic reticulum contact formation. PA is also pH-sensitive: protonation at acidic pH (<6) reduces headgroup size and enhances membrane fusion. Its thermal expansivity further contributes to lipid microdomain organization within biological membranes.