A diglyceride, or diacylglycerol ( DAG), is a glyceride consisting of two fatty acid chains covalent bond to a glycerol molecule through ester linkages.
Production
Diglycerides are a minor component of many
and are normally present at ~1–6%; or in the case of
cottonseed oil as much as 10%.
Industrial production is primarily achieved by a
glycerolysis reaction between
triglycerides and glycerol. The raw materials for this may be either
or
.
Food additive
Diglycerides, generally in a mix with
(E471), are common food additives largely used as
Emulsion. The values given in the nutritional labels for total fat, saturated fat, and
trans fat do not include those present in mono- and diglycerides. They often are included in bakery products, beverages,
ice cream,
peanut butter,
chewing gum,
shortening, whipped toppings,
margarine, confections, and some snack products, such as
Pringles.
Biological functions
Protein kinase C activation
In biochemical signaling, diacylglycerol functions as a
second messenger lipid signaling, and is a product of the
hydrolysis of the phospholipid phosphatidylinositol 4,5-bisphosphate (PIP
2) by the enzyme
phospholipase C (PLC) (a
cell membrane-bound enzyme) that, through the same reaction, produces inositol trisphosphate (IP
3). Although inositol trisphosphate diffuses into the
cytosol, diacylglycerol remains within the
plasma membrane, due to its
hydrophobe properties. IP
3 stimulates the release of calcium ions from the smooth endoplasmic reticulum, whereas DAG is a physiological activator of protein kinase C (PKC). The production of DAG in the membrane facilitates translocation of PKC from the cytosol to the
plasma membrane.
Munc13 activation
Diacylglycerol has been shown to exert some of its excitatory actions on vesicle release through interactions with the presynaptic priming protein family Munc13. Binding of DAG to the C1 domain of Munc13 increases the fusion competence of synaptic vesicles resulting in potentiated release.
Diacylglycerol can be mimicked by the tumor-promoting compounds phorbol esters.
Other
In addition to activating PKC, diacylglycerol has a number of other functions in the cell:
-
a source for
-
a precursor of the endocannabinoid 2-arachidonoylglycerol
-
an activator of a subfamily of TRPC (TRPC) cation channels, TRPC3/6/7.
Metabolism
Synthesis of diacylglycerol begins with glycerol-3-phosphate, which is derived primarily from dihydroxyacetone phosphate, a product of
glycolysis (usually in the cytoplasm of liver or adipose tissue cells). Glycerol-3-phosphate is first
acylation with
acyl-
coenzyme A (
acyl-CoA) to form lysophosphatidic acid, which is then acylated with another molecule of acyl-CoA to yield phosphatidic acid. Phosphatidic acid is then de-phosphorylated to form diacylglycerol.
Dietary fat is mainly composed of . Because triglycerides cannot be absorbed by the digestive system, triglycerides must first be enzymatically digested into Monoglyceride, diacylglycerol, or free fatty acids. Diacylglycerol is a precursor to triacylglycerol (triglyceride), which is formed in the addition of a third fatty acid to the diacylglycerol under the catalysis of diglyceride acyltransferase.
Since diacylglycerol is synthesized via phosphatidic acid, it will usually contain a saturated fatty acid at the C-1 position on the glycerol moiety and an unsaturated fatty acid at the C-2 position.
Diacylglycerol can be phosphorylated to phosphatidic acid by diacylglycerol kinase.
Insulin resistance
Activation of
PRKCQ by diacylglycerol may cause insulin resistance in muscle by decreasing IRS1-associated PI3K activity.
Similarly, activation of
PRKCE by diacyglycerol may cause insulin resistance in the liver.
See also
