The major pathways of triacylglycerol and phosphoglycerol biosynthesis are outlined in Figure 24–1. Important substances such as triacylglycerols, phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol, and cardiolipin, a constituent of mitochondrial membranes, are formed from glycerol-3-phosphate. Significant branch points in the pathway occur at the phosphatidate and diacylglycerol steps. Phosphoglycerols containing an ether link (—C—O—C—), the best known of which are plasmalogens and platelet-activating factor (PAF), are derived from dihydroxyacetone phosphate. Glycerol 3-phosphate and dihydroxyacetone phosphate are intermediates in glycolysis, making a very important connection between carbohydrate and lipid metabolism (see Chapter 14).
Overview of acylglycerol biosynthesis. (PAF, platelet-activating factor.)
Phosphatidate Is the Common Precursor in the Biosynthesis of Triacylglycerols, Many Phosphoglycerols, & Cardiolipin
Both glycerol and fatty acids must be activated by ATP before they can be incorporated into acylglycerols. Glycerol kinase catalyzes the activation of glycerol to sn-glycerol 3-phosphate. If the activity of this enzyme is absent or low, as in muscle or adipose tissue, most of the glycerol-3-phosphate is formed from dihydroxyacetone phosphate by glycerol-3-phosphate dehydrogenase (Figure 24–2).
Biosynthesis of triacylglycerol and phospholipids. , Monoacylglycerol pathway; , glycerol phosphate pathway. Phosphatidylethanolamine may be formed from ethanolamine by a pathway similar to that shown for the formation of phosphatidylcholine from choline.
Biosynthesis of Triacylglycerols
Two molecules of acyl-CoA, formed by the activation of fatty acids by acyl-CoA synthetase (see Chapter 22), combine with glycerol-3-phosphate to form phosphatidate (1,2-diacylglycerol phosphate). This takes place in two stages, catalyzed by glycerol-3-phosphate acyltransferase and 1-acylglycerol-3-phosphate acyltransferase. Phosphatidate is converted by phosphatidate phosphohydrolase (also called phosphatidate phosphatase (PAP)) and diacylglycerol acyltransferase (DGAT) to 1,2-diacylglycerol and then triacylglycerol. Lipins, a family of three proteins, have PAP activity and they also act as transcription factors which regulate the expression of genes involved in lipid metabolism. DGAT catalyzes the only step specific for triacylglycerol synthesis and is thought to be rate limiting in most circumstances. In intestinal mucosa, monoacylglycerol acyltransferase converts monoacylglycerol to 1,2-diacylglycerol in the monoacylglycerol pathway. Most of the activity of these enzymes resides in the endoplasmic reticulum, but some is found in mitochondria. Although phosphatidate phosphohydrolase protein is found mainly in the cytosol, the active form of the enzyme is membrane bound.
Biosynthesis of Phospholipids
In the biosynthesis of phosphatidylcholine and phosphatidylethanolamine (Figure 24–2), choline or ethanolamine must first be activated by phosphorylation by ATP followed by linkage to CDP. The resulting CDP-choline or CDP-ethanolamine reacts with 1,2-diacylglycerol to form either phosphatidylcholine or phosphatidylethanolamine, respectively. Phosphatidylserine is formed from phosphatidylethanolamine directly by reaction with serine (Figure 24–2). Phosphatidylserine may re-form phosphatidylethanolamine by decarboxylation. An alternative pathway in liver enables phosphatidylethanolamine to give rise directly to phosphatidylcholine by progressive methylation of the ethanolamine residue. In spite of these sources of choline, it is considered to be an essential nutrient in many mammalian species, although this has not been established in humans.
The regulation of triacylglycerol, phosphatidylcholine, and phosphatidylethanolamine biosynthesis is driven by the availability of free fatty acids. Those that escape oxidation are preferentially converted to phospholipids, and when this requirement is satisfied, they are used for triacylglycerol synthesis.
Cardiolipin (diphosphatidylglycerol; Figure 21–10) is a phospholipid present in mitochondria. It is formed from phosphatidylglycerol, which in turn is synthesized from CDP-diacylglycerol (Figure 24–2) and glycerol 3-phosphate according to the scheme shown in Figure 24–3. Cardiolipin, found in the inner membrane of mitochondria, has a key role in mitochondrial structure and function, and is also thought to be involved in programmed cell death (apoptosis).
Biosynthesis of cardiolipin.
Biosynthesis of Glycerol Ether Phospholipids
In glycerol ether phospholipids, one or more of the glycerol carbons is attached to a hydrocarbon chain by an ether linkage rather than an ester bond. Plasmalogens and platelet activating factor are important examples of this type of lipid. The biosynthetic pathway is located in peroxisomes. Dihydroxyacetone phosphate is the precursor of the glycerol moiety (Figure 24–4). It combines with acyl-CoA to give 1-acyldihydroxyacetone phosphate, and the ether link is formed in the next reaction, producing 1-alkyldihydroxyacetone phosphate, which is then converted to 1-alkylglycerol 3-phosphate. After further acylation in the 2 position, the resulting 1-alkyl-2-acylglycerol 3-phosphate (analogous to phosphatidate in Figure 24–2) is hydrolyzed to give the free glycerol derivative. Plasmalogens, which comprise much of the phospholipid in mitochondria, are formed by desaturation of the analogous 3-phosphoethanolamine derivative (Figure 24–4). Platelet-activating factor (PAF) (1-alkyl-2-acetyl-sn-glycerol-3-phosphocholine) is synthesized from the corresponding 3-phosphocholine derivative. It is formed by many blood cells and other tissues and aggregates platelets at concentrations as low as 10–11 mol/L. It also has hypotensive and ulcerogenic properties and is involved in a variety of biologic responses, including inflammation, chemotaxis, and protein phosphorylation.
Biosynthesis of ether lipids, including plasmalogens, and platelet-activating factor (PAF). In the de novo pathway for PAF synthesis, acetyl-CoA is incorporated at stage∗, avoiding the last two steps in the pathway shown here.
Phospholipases Allow Degradation & Remodeling of Phosphoglycerols
Although phospholipids are actively degraded, each portion of the molecule turns over at a different rate—eg, the turnover time of the phosphate group is different from that of the 1-acyl group. This is due to the presence of enzymes that allow partial degradation followed by resynthesis (Figure 24–5). Phospholipase A2 catalyzes the hydrolysis of glycerophospholipids to form a free fatty acid and lysophospholipid, which in turn may be reacylated by acyl-CoA in the presence of an acyltransferase. Alternatively, lysophospholipid (eg, lysolecithin) is attacked by lysophospholipase, forming the corresponding glyceryl phosphoryl base, which may then be split by a hydrolase liberating glycerol 3-phosphate plus base. Phospholipases A1, A2, B, C, and D attack the bonds indicated in Figure 24–6. Phospholipase A2 is found in pancreatic fluid and snake venom as well as in many types of cells; phospholipase C is one of the major toxins secreted by bacteria; and phospholipase D is known to be involved in mammalian signal transduction.
Metabolism of phosphatidylcholine (lecithin).
Sites of the hydrolytic activity of phospholipases on a phospholipid substrate.
Lysolecithin (lysophosphatidylcholine) may be formed by an alternative route that involves lecithin: cholesterol acyltransferase (LCAT). This enzyme, found in plasma, catalyzes the transfer of a fatty acid residue from the 2 position of lecithin to cholesterol to form cholesteryl ester and lysolecithin, and is considered to be responsible for much of the cholesteryl ester in plasma lipoproteins (see Chapter 25).
Long-chain saturated fatty acids are found predominantly in the 1 position of phospholipids, whereas the polyunsaturated fatty acids (eg, the precursors of prostaglandins) are incorporated more frequently into the 2 position. The incorporation of fatty acids into lecithin occurs in three ways; by complete synthesis of the phospholipid; by transacylation between cholesteryl ester and lysolecithin; and by direct acylation of lysolecithin by acyl-CoA. Thus, a continuous exchange of the fatty acids is possible, particularly with regard to introducing essential fatty acids into phospholipid molecules.