Phosphatidate phosphatase is a key regulatory enzyme in lipid metabolism, catalyzing the conversion of phosphatidate to diacylglycerol. The two substrates of PAP are phosphatidate and H2O, and its two products are diacylglycerol and phosphate, as shown here. The reverse reaction is catalyzed by the enzyme diacylglycerol kinase, which replaces the hydroxyl group on diacylgylcerol with a phosphate from ATP, generating ADP in the process. While ATP is used by DGK in mammalian cells, yeast cells tend to use CTP as the high-energy phosphate donor instead. Mechanistically speaking, this has no effect on the overall reaction. In yeast, the forward direction is Mg^2+-dependent, while the reverse direction is Ca^2+-dependent. PAP1, a cytosolic phosphatidate phosphatase found in the lung, is also Mg^2+-dependent, but PAP2, a six-transmembrane-domain integral protein found in the plasma membrane, is not. and products of the reaction catalyzed by the enzyme phosphatidate phosphatase, and thus also those of the reverse reaction, which is catalyzed by the enzyme diacylglycerol kinase.
Role in the regulation of lipid flux
PAP regulates lipid metabolism in several ways. In short, PAP is a key player in controlling the overall flux of triacylglycerols to phospholipids and vice versa, also exerting control through the generation and degradation of lipid-signaling molecules related to phosphatidate. When PAP is active, diacylglycerols formed by PAP can go on to form any of several products, including phosphatidylethanolamine, phosphatidylcholine, phosphatidylserine, and triacylglycerol. Phospholipids can be formed from diacylglycerol through reaction with activated alcohols, and triacylglycerols can be formed from DAG through reaction with fatty acyl CoA molecules. When PAP is inactive, DGK drives the reaction in reverse, allowing phosphatidate to accumulate as it brings down DAG levels. Phosphatidate can then be converted into an activated form, CDP-DAG, through the liberation of a pyrophosphate from a CTP molecule, or into cardiolipin. CDP-DAG is a principal precursor used by the body in phospholipid synthesis. Furthermore, because both phosphatidate and DAG function as secondary messengers, PAP is able to exert extensive and intricate control of lipid metabolism far beyond its local effect on phopshatidate and DAG concentrations and the resulting effect on the direction of lipid flux as outlined above.
Enzyme regulation
PAP is upregulated by CDP-diacylglycerol, phosphatidylinositol, and cardiolipin. PAP is downregulated by sphingosine and dihydrosphingosine. This makes sense in the context of the discussion above. Namely, a build up of products that are formed from phosphatidate serves to upregulate PAP, the enzyme that consumes phosphatidate, thereby acting as a signal that phosphatidate is in abundance and causing its consumption. At the same time, a build up of products that are formed from DAG serves to downregulate PAP, the enzyme that forms DAG, thereby acting as a signal that DAG is in abundance and its production should be slowed.
There are several different genes that code for phosphatidate phosphatases. They fall into one of two types, depending on their cellular localization and substrate specificity.
Type I
Type I phosphatidate phosphatases are soluble enzymes that can associate to membranes. They are found mainly in the cytosol and the nucleus. Encoded for by a group of genes named Lipin, they are substrate specific only to phosphatidate. There are speculated to be involved in the de novo synthesis of glycerolipids. Each of the 3 Lipin proteins found in mammals—Lipin1, Lipin2, and Lipin3—has unique tissue expression motifs and distinct physiological functions.
Regulation
Regulation of mammalian Lipin PAP enzymes occurs at the transcriptional level. For example, Lpin1 is induced by glucocorticoids during adipocyte differentiation as well as in cells that are experiencing ER proliferation. Lipin2, on the other hand, is repressed during adipocyte differentiation.
Type II
Type II phosphatidate phosphatases are transmembrane enzymes found mainly in the plasma membrane. They can dephosphorylate other substrates besides phosphatidate, and therefore are also known as lipid phosphate phosphatases. Their main role is in lipid signaling and in phospholipid head-group remodeling. One example of a type II phosphatidate phosphatase is PgpB. PgpB is one of three integral membrane phosphatases in Escherichia coli that catalyzes the dephosphorylation of phosphatidylglycerol phosphate to PG. The other two are PgpA and PgpC. While all three catalyze the reaction from PGP to PG, their amino acid sequences are dissimilar and it is predicted that their active sites open to different sides of the cytoplasmic membrane. PG accounts for approximately 20% of the total membrane lipid composition in the inner membrane of bacteria. PgpB is competitively inhibited by phosphatidylethanolamine, a phospholipid formed from DAG. This is therefore an example of negative feedback regulation. The enzyme active site contains a catalytic triad Asp-211, His-207, and His-163 that establishes a charge relay system. However, this catalytic triad is essential for the dephosphorylation of lysophosphatidic acid, phosphatidic acid, and sphingosine-1-phosphate, but is not essential in its entirety for the enzyme's native substrate, phosphatidylglycerol phosphate; His-207 alone is sufficient to hydrolyze PGP. In the cartoon depiction of PgpB below, one can see its six transmembrane alpha helices, which are here shown horizontally. Of the three PGP phosphatases discussed above, PgpB is the only to have multiple transmembrane alpha helices.
Genes
Human genes that encode phosphatidate phosphatases include:
