Plant Triacylglycerol Synthesis
- Precursors for Triacylglycerol Biosynthesis. ...
- Triacylglycerol Can Be Synthesized through Sequential Acylation of the Glycerol Backbone. ...
- Acyl Editing in the Endoplasmic Reticulum Can Affect the Fatty Acid Composition of Triacylglycerol. ...
- Metabolic Engineering of Triacylglycerol Biosynthesis. ...
- Triacylglycerol Biosynthesis in Vegetative and Root Tissue. ...
What is the biosynthesis of triacylglycerol?
The biosynthesis of triacylglycerol occurs in the endoplasmic reticulum (ER) and involves acyl-editing of fatty acyl chains within the nitrogenous phospholipids of the ER. Depending on the plant species, particular reactions of triacylglycerol assembly and acyl-editing may be catalyzed by one or more forms of an enzyme.
How is glycerol synthesized from acyl CoA?
Glycerol accepts fatty acids from acyl‐CoAs to synthesize glycerol lipids. Glycerol phosphate comes from glycolysis—specifically from the reduction of dihydroxyacetone phosphate using NADH as a cofactor.
What is triacylglycerol (TAG)?
Triacylglycerols or triglycerides are formed by ester linkage of fatty acids to three alcohol groups in glycerol. Animals can synthesize and store large quantities of Triacylglycerols in the form of fats in adipose tissue, to be used later as fuel. TAG is also the main constituent of vegetable oils.
What is the function of triacylglycerols in animal cells?
Animals can synthesize and store large quantities of Triacylglycerols in the form of fats in adipose tissue, to be used later as fuel. TAG is also the main constituent of vegetable oils. TAGs are hydrophobic and hence are not soluble in the aqueous environment of the bloodstream.
Where does triacylglycerol biosynthesis occur?
A generalized scheme for triacylglycerol biosynthesis in developing seeds of oleaginous plants is depicted in Figure 1. The de novo biosynthesis of fatty acids occurs in the plastid through the catalytic action of acetyl-CoA carboxylase (EC 6.4.1.2) and the fatty acid synthase complex. Monounsaturated fatty acids from C 4 to C 18 are formed in the plastid while the fatty acyl chain is attached to the acyl carrier protein of the fatty acid synthase complex. Thioesterase (acyl-acyl carrier protein hydrolase) (EC 3.1.2.14) catalyzes the release of saturated or monounsaturated fatty acyl chain from acyl-acyl carrier protein. In Arabidopsis and many other oilseed species, oleic acid (18:1 Δ9- cis) is a major product of plastidial fatty acid synthesis. Following export from the plastid, fatty acids are re-esterified to CoA to form acyl-CoA through the catalytic action of acyl-CoA synthetase (EC 6.2.1.3) on the outer membrane of the plastid. Further elongation of acyl-CoAs can occur on the ER. For example, in high-erucic-acid rapeseed ( Brassica napus ), oleoyl-CoA is sequentially elongated to form eicosenoyl (20:1 Δ11- cis )-CoA and erucoyl (22:1 Δ13- cis )-CoA. Both plastidially-exported and -elongated acyl-CoAs constitute the acyl-CoA pool, which can be further modified through acyl-exchange reactions in the ER. Acyl-exchange reactions will be described in more detail later in this contribution.
How is triacylglycerol synthesized?
Triacylglycerol Can Be Synthesized through Sequential Acylation of the Glycerol Backbone. Triacylglycerol can be formed through three sequential acyl-CoA-dependent acylations of the glycerol backbone beginning with sn -glycerol-3-phosphate. Removal of the phosphate group occurs prior to the final acylation.
What is the acyl-coa pool?
The acyl-CoA pool provides acyl-donor substrate for the acyltransferase reactions of triacylglycerol assembly in the ER. The glycerol backbone for triacylglycerol assembly is in the form of sn -glycerol-3-phosphate, which is generated through the catalytic action of sn -glycerol-3-phosphate dehydrogenase (EC 1.1.1.8). The dihydroxyacetone phosphate substrate for this reaction is derived from glycolysis.
How are fatty acids modified?
In many instances, reactions in both fatty acid and triacylglycerol biosynthesis were modified via the introduction of cDNAs encoding enzymes with desired substrate selectivities/specificities from other sources. A major obstacle to producing industrially useful fatty acids in Brassica oilseed species remains the inability of the resident LPAAT to incorporate these fatty acyl groups at the sn -2 position of the glycerol backbone. Triacylglycerols enriched in lauric acid (12:0) can serve as feedstock for the detergent industry. Researchers at Calgene introduced a lauroyl-ACP thioesterase from the California bay laurel ( Umbellularia californica) into B. napus to facilitate production of lauric acid and its subsequent incorporation into seed triacylglycerol. Incorporation of lauroyl moieties, however, was limited to the sn -1 and sn -3 positions of the triacylglycerol of the transgenics because the endogenous B. napus LPAAT discriminated against saturated acyl-CoA substrates. This obstacle was overcome by co-expressing, in B. napus, the cDNA encoding the California bay lauroyl-ACP thioesterase in combination with a cDNA encoding an LPAAT from coconut ( Cocus nucifera ). The endogenous LPAAT of high-erucic-acid B. napus also discriminates against erucoyl moieties. In order to overcome this problem, biotechnologists have transformed high-erucic-acid B. napus with cDNAs encoding LPAATs from other species which are capable of incorporating erucoyl moieties at the sn -2 position.
What is triacylglycerol?
Triacylglycerol is composed of three fatty acyl groups esterified to a glycerol backbone at the sn -1, sn -2 and sn -3 positions. In higher plants, triacylglycerol is the predominant component of the oil of the seeds or fruits of oleaginous plants and primarily serves as an energy store to support the growth of the young seedling during ...
How are polyunsaturated fatty acids formed?
Polyunsaturated fatty acids are mainly formed using phosphatidylcholine as a substrate for the catalytic action of membrane-bound fatty acid desaturase 2 and 3 (FAD2 and FAD3) , which leads to the conversion of oleoyl moieties to linoleoyl and linolenoyl, respectively (see the web page on fatty acid biosynthesis). The formation of unusual fatty acids, such as ricinoleic and epoxy fatty acids, also occurs on phosphatidylcholine and involves the action of enzymes which are evolutionarily related to desaturases. In some plant species, however, polyunsaturated fatty acid formation can also occur through the action of desaturases that use acyl-CoA as substrates. Polyunsaturated fatty acids formed on phosphatidylcholine are eventually incorporated into triacylglycerol via a number of possible routes.
What is the name of the enzyme that catalyzes the dephosphorylation of phosphatidic acid?
Phosphatidic acid phosphatase (PAP) catalyzes the dephosphorylation of phosphatidic acid (PA) to produce sn -1,2-diacylglycerol (DAG) prior to the final acylation catalyzed by acyl-CoA:diacylglycerol acyltransferase (DGAT).
Where are triacylglycerols synthesized?
The major sites for the synthesis of TAG are liver, adipose tissues, intestinal mucosa, mammary glands and muscles. In animal tissues, TAGs and glycerophopholipids such as phosphatidylethanolamine share two precursors; fatty acyl-CoA ...
How are diacylglycerols converted to triacylglycerols?
Conversion of diacylglycerols into triacylglycerols: Diacylglycerols obtained from step 2 are then converted to Triacylglycerols with the incorporation of a third fatty acyl-CoA by an enzyme acyl transferase, the process called transesterification. Phosphatidic acid in lipid biosynthesis.
How are triglycerides formed?
Triacylglycerols or triglycerides are formed by ester linkage of fatty acids to three alcohol groups in glycerol. Animals can synthesize and store large quantities of Triacylglycerols in the form of fats in adipose tissue, to be used later as fuel. TAG is also the main constituent of vegetable oils.
What are the precursors of glycerophopholipids?
In animal tissues, TAGs and glycerophopholipids such as phosphatidylethanolamine share two precursors; fatty acyl-CoA and L-glycerol 3-phosphate and several biosynthetic steps.
How is the synthesis of tag biosynthesis regulated?
Regulation of TAG biosynthesis in animals: The synthesis and breakdown of TAG are regulated by several hormones. Insulin stimulates the conversion of dietary carbohydrates and proteins to fat.
Where does glycerol 3-phosphate come from?
The vast majority of the glycerol 3-phosphate is derived from the glycolytic intermediate dihydroxyacetone phosphate (DHAP) by the action of cytosolic NAD-linked glycerol 3-phosphate dehydrogenase. In liver and kidney, a small amount of glycerol 3-phosphate is also formed from glycerol by the action of glycerol kinase.
What are the two fates of fatty acids?
Biosynthesis of TAG: Most of the fatty acids either synthesized in body or ingested through diet by an organism have one of two fates; incorporation into triacylglycerols for the storage of metabolic energy. incorporation into the phospholipid components of membranes. The organism’s current needs determine the alternative fates of fatty acids.
What are the three enzymes involved in lipolysis?
The current model of lipolysis identifies three major enzymes involved: ATGL, HSL, and MGL. Catecholamines , particularly norepinephrine, are the primary activators of fasting-induced lipolysis, while other hormones also have an effect. These include cortisol, glucagon, growth hormone (GH), and adrenocorticotropic hormone (ACTH).
What are the proteins that stimulate lipolysis?
A number of lipid droplet–associated proteins are known to modulate rates of basal (non-stimulated) and stimulated lipolysis. These proteins include CGI-58 and perilipin. Perilipin is the major protein found in association with lipid droplets in adipocytes.[4] In WAT, there are two important mechanisms regulating lipolysis: the activation of ATGL by CGI-58 and the protein kinase A (PKA)-mediated phosphorylation of HSL and perilipin.
What is lipolysis in the body?
Lipolysis is the metabolic process through which triacylglycerols (TAGs) break down via hydrolysis into their constituent molecules: glycerol and free fatty acids (FFAs). Fat storage in the body is through adipose TAGs and is utilized for heat, energy, and insulation. The body uses fat stores as its main source of energy during starvation, conserving protein. Overall, fats are quantitatively the most important fuel in the body, and the length of time that a person can survive without food depends mainly on the amount of fat stored in the adipose tissue. Thus, lipolysis is especially important in the fasting state of metabolism when blood glucose levels have decreased. However, it also occurs under non-stimulated (basal conditions). [1]
What are the two methods of estimating lipolysis?
There are currently several strategies in place to estimate lipolysis and these generally fall into two categories: non-activity-based methods and activity-based methods . The non-activity-based methods involve determining the quantity of the associated enzymes and regulatory proteins. The activity-based methods involve measuring the activity of the associated enzymes directly. [2]
Which fatty acids have double bonds?
All naturally occurring fatty acids possess double bonds in the cis configuration. Polyunsaturated fatty acid classification is often according to the position of the first double bond from the omega-end (the carbon farthest from the carboxyl group). Common examples of these are omega-3 and omega-6 fatty acids. Monoacylglycerols (monoglycerides), diacylglycerols (diglycerides), and triacylglycerols (triglycerides) contain one, two, and three fatty acids esterified to glycerol, respectively.
What is the effect of lipolysis on non-adipose tissue?
Defective lipolysis in non-adipose tissues impairs their normal function, leading to excessive TAG accumulation and lipid storage disease.[2] Conversely, an overabundance of FFAs due to unregulated lipolysis results in lipotoxicity in non-adipose tissues. Failure to package FFAs into lipid droplets causes chronic elevation of circulating FFAs, which can lead to chronic inflammation, mitochondrial dysfunction, and cell death. [3]
Precursors For Triacylglycerol Biosynthesis
- A generalized scheme for triacylglycerol biosynthesis in developing seeds of oleaginous plants is depicted in Figure 1. The de novo biosynthesis of fatty acids occurs in the plastid through the catalytic action of acetyl-CoA carboxylase (EC 6.4.1.2) and the fatty acid synthase complex. Monounsaturated fatty acids from C4 to C18 are formed in the pl...
Metabolic Engineering of Triacylglycerol Biosynthesis
- Modification of the fatty acid composition of major oilseed crops has been achieved through manipulation of enzyme-catalyzed reactions in lipid biosynthesis (see also our web page on production of unusual fatty acids). In many instances, reactions in both fatty acid and triacylglycerol biosynthesis were modified via the introduction of cDNAs encoding enzymes with …
Triacylglycerol Biosynthesis in Vegetative and Root Tissue
- Numerous plant species have been shown to contain relatively low levels of triacylglycerol in the cytosol of leaves. Labelling experiments with 14C-acetate have suggested that leaf triacylglycerol may serve as a diurnal photosynthetic store, similar to starch. Seven- to 20-fold increases in the triacylglycerol content of leaves (Nicotiana tabacum and Arabidopsis) have been achieved throu…