Biosynthesis of Triacylglycerol
Introduction
Triacylglycerols are formed by linking fatty acids with an ester linkage to three alcohol groups in glycerol. Triacylglycerols are the form in which fat energy is stored in adipose tissue. Triacylglycerols are sometimes referred to as triglycerides.
Triacylglycerol is the major form of dietary lipid in fats and oils, whether derived from plants or animals. The physical properties of the triacylglycerol are determined by the specific fatty acids esterified to the glycerol moiety and the actual position the fatty acids occupy.
Each of the three carbons comprising the glycerol molecule allows for a stereochemically distinct fatty acid bond position: sn-1, sn-2, and sn-3.
Simple Triacylglycerol: A triacylglycerol with three identical fatty acids is termed a simple triacylglycerol. These are exceedingly rare in nature.
Mixed Triacylglycerol: A triacylglycerol with two or three different fatty acids is termed a mixed triacylglycerol and makes up the bulk of the fat.
Melting Point:
The melting point of a triacylglycerol is determined by:
Physical characteristics
Position of the fatty acids esterified to glycerol
Their chain length; number, position, and conformation of the double bonds.
Stereochemical position.
Biosynthesis of Triacylglycerols
Animals can synthesize and store large quantities of triacylglycerols, to be used later as fuel. In humans only a few hundred grams of glycogen can be stored in the liver and muscles, barely enough to supply the body's energy needs for 12 hours. In contrast, the total amount of stored triacylglycerol in a 70 kg man of average build is about 15 kg, enough to supply his basal energy needs for as long as 12 weeks.
Whenever carbohydrate is ingested in excess of the capacity to store glycogen, it is converted into triacylglycerols and stored in adipose tissue. Plants also manufacture triacylglycerols as an energy-rich fuel, stored especially in fruits, nuts, and seeds.
Biosynthesis Process
Glycerol-3-phosphate can be formed in two ways. It can arise from dihydroxyacetone phosphate generated during glycolysis by the action of the cytosolic NAD-linked glycerol-3phosphate dehydrogenase, and in liver and kidney it is also formed from glycerol by the action of glycerol kinase.
The other precursors of triacylglycerols are fatty acyl-CoAs, formed from fatty acids by acylCoA synthetases, the same enzymes responsible for the activation of fatty acids for β oxidation.
The first stage in the biosynthesis of triacylglycerols is the acylation of the two free hydroxyl groups of glycerol-3-phosphate by two molecules of fatty acyl-CoA to yield diacylglycerol-3-phosphate, more commonly called phosphatidate.
Phosphatidate occurs in only trace amounts in cells, but is a central intermediate in lipid biosynthesis; it can be converted either to a triacylglycerol or to a glycerophospholipid.
In the pathway to triacylglycerols, phosphatidate is hydrolyzed by phosphatidate phosphatase to form a 1,2-diacylglycerol.
Diacylglycerols are then converted into triacylglycerols by transesterification with a third fatty acyl-CoA.
Fig: Biosynthesis of triacylglycerol
Mobilization and utilization of triacylglycerol
During fat mobilization, triglycerides are broken down into free fatty acids and glycerols in a process called lipolysis. Lipolysis is stimulated by the hormones epinephrin, glycagon, or adrenalcorticaltropic hormone (ACTH). These hormones bind to receptors on the plasma membrane of the cell and initiate a signal cascade.
The first step of the signal cascade is the activation of adenylyl cyclase, which is the enzyme required to synthesize cyclic AMP from ATP. High levels of cyclic AMP activate protein kinase A. The protein kinase then uses ATP to activate the triaclyglycerol lipase. The phosphorylated lipase can then catalyze the hydrolysis of triglycerides to free fatty acids. Multiple lipases work to hydrolyze the fatty acids off of the glycerol.
Fig: Mobilization of Triacylglycerols
Dietary fats are mobilized in the duodenum, by lipases secreted by the pancreas.
The Fate of Glycerol and Fatty Acids
Once a triglyceride is completely broken down, the free fatty acids bind to serum albumin in the blood stream and carries the free fatty acids to the tissues that need energy. Glycerol is absorbed by the liver.
The glycerol represents 5% of the energy available from triglycerides. In order to use the glycerol we need to activate it by phosphorylating it. Glycerol 3-phosphate is oxidized into Dihydroxyacetone phosphate (DHAP), which is then isomerized into Glyceraldehyde 3- phosphate (G3P). You should recognize G3P as an intermediate in the glycolytic and gluconeogenic pathways and can be converted into pyruvate or glucose.
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