29.1 Phosphatidate Is a Precursor of Storage Lipids and Many Membrane Lipids

✓ 5 Describe the relation between triacylglycerol synthesis and phospholipid synthesis.

Figure 29.1 provides a broad view of lipid synthesis. Both triacylglycerol synthesis and phospholipid synthesis begin with the precursor phosphatidate (diacylglycerol 3-phosphate). Phosphatidate is formed by the addition of two fatty acids to glycerol 3-phosphate. In most cases, glycerol 3-phosphate is first acylated by a saturated acyl CoA to form lysophosphatidate, which is, in turn, commonly acylated by unsaturated acyl CoA to yield phosphatidate:

Figure 29.1: PATHWAY INTEGRATION: Sources of intermediates in the synthesis of triacylglycerols and phospholipids. Phosphatidate, synthesized from dihydroxyacetone phosphate (DHAP) produced in glycolysis and fatty acids, can be further processed to produce triacylglycerol or phospholipids. Phospholipids and other membrane lipids are continually produced in all cells.

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Triacylglycerol Is Synthesized from Phosphatidate in Two Steps

The pathways diverge at phosphatidate. The synthesis of triacylglycerol is completed by a triacylglycerol synthetase complex that is bound to the endoplasmic reticulum membrane. Phosphatidate is hydrolyzed to give diacylglycerol (DAG), which is then acylated to a triacylglycerol:

The liver is the primary site of triacylglycerol synthesis. From the liver, triacylglycerols are transported to muscles for use as a fuel or to adipose tissue for storage. Approximately 85% of a nonobese person’s energy is stored as triacylglycerols, mainly in adipose tissue.

Phospholipid Synthesis Requires Activated Precursors

Phosphatidate is also a precursor for phospholipids. Phospholipid synthesis, which takes place in the endoplasmic reticulum, requires the combination of a diacylglycerol with an alcohol. As in most anabolic reactions, one of the components must be activated. In this case, either of the two components may be activated, depending on the source of the reactants.

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Synthesis from an activated diacylglycerolThis pathway starts with the reaction of phosphatidate with cytidine triphosphate (CTP) to form cytidine diphosphodiacylglycerol (CDP-diacylglycerol; Figure 29.2). This reaction, like those of many biosyntheses, is driven forward by the hydrolysis of pyrophosphate.

Figure 29.2: The structure of CDPdiacylglycerol. A key intermediate in the synthesis of phospholipids consists of phosphatidate (diacylglycerol 3-phosphate, or DAG-3P) and cytidine monophosphate (CMP).

The activated phosphatidyl unit then reacts with the hydroxyl group of an alcohol. If the alcohol is inositol, the products are phosphatidylinositol and cytidine monophosphate (CMP). Subsequent phosphorylations of phosphatidylinositol catalyzed by specific kinases lead to the synthesis of phosphatidylinositol 4,5-bisphosphate, a membrane lipid that is also an important molecule in signal transduction.

Synthesis from an activated alcoholPhosphatidylethanolamine in mammals can be synthesized from the alcohol ethanolamine through the formation of CDP-ethanolamine. In this case, ethanolamine is phosphorylated by ATP to form the precursor, phosphorylethanolamine. This precursor then reacts with CTP to form the activated alcohol, CDP-ethanolamine. The phosphorylethanolamine unit of CDP-ethanolamine is subsequently transferred to a diacylglycerol to form phosphatidylethanolamine.

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!clinic! CLINICAL INSIGHT: Phosphatidylcholine Is an Abundant Phospholipid

The most common phospholipid in mammals is phosphatidylcholine, comprising approximately 50% of the membrane mass. Dietary choline is activated in a series of reactions analogous to those in the activation of ethanolamine. CTP-phosphocholine cytidylyltransferase (CCT) catalyzes the formation of CDP-choline, the rate-limiting step in phosphatidylcholine synthesis. CCT is an amphitropic enzyme, a class of enzymes whose regulator ligand is the membrane itself. A portion of the enzyme, normally associated with the membrane, detects a fall in phosphatidylcholine as an alteration in the physical properties of the membrane. When this occurs, another portion of the enzyme is inserted into the membrane leading to enzyme activation. Indeed, the kcat/KM value increases three orders of magnitude upon activation, resulting in the restoration of phosphatidylcholine levels. Earlier we examined how cancer cells increase fatty acid synthesis to meet the fatty acid needs for membrane synthesis. Evidence is accumulating that CCT is specifically activated in some cancers to generate the required phosphocholine.

The importance of phosphatidylcholine is attested to by the fact that the liver possesses an enzyme, phosphatidylethanolamine methyltransferase, that synthesizes phosphatidylcholine from phosphatidylethanolamine when dietary choline is insufficient. The amino group of this phosphatidylethanolamine is methylated three times to form phosphatidylcholine. S-Adenosylmethionine is the methyl donor (Chapter 31):

Thus, phosphatidylcholine can be produced by two distinct pathways in mammals, ensuring that this phospholipid can be synthesized even if the components for one pathway are in limited supply.

Sphingolipids Are Synthesized from Ceramide

Phospholipids, with their glycerol backbones, are not the only type of membrane lipid. Sphingolipids, with a backbone of sphingosine rather than glycerol, are found in the plasma membranes of all eukaryotic cells, although the concentration is highest in the cells of the central nervous system. To synthesize a sphingolipid, palmitoyl CoA and serine condense to form 3-ketosphinganine, which is then reduced to dihydrosphingosine (Figure 29.3). Dihydrosphingosine is converted into ceramide with the addition of a long-chain acyl CoA to the amino group of dihydrosphingosine followed by an oxidation reaction to form the trans double bond.

Figure 29.3: The synthesis of ceramide from palmitoyl CoA and serine. Subsequent to the condensation of palmitoyl CoA with serine, a sequence of three reactions—a reduction, an acylation, and an oxidation—yields ceramide.

The terminal hydroxyl group of ceramide is also substituted to form a variety of sphingolipids (Figure 29.4):

Figure 29.4: Sphingolipid synthesis. Ceramide is the starting point for the formation of sphingomyelin and gangliosides.

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!quickquiz! QUICK QUIZ 1

Describe the roles of glycerol 3-phosphate, phosphatidate, and diacylglycerol in triacylglycerol synthesis and phospholipid synthesis.

!clinic! CLINICAL INSIGHT: Gangliosides Serve as Binding Sites for Pathogens

Ganglioside-binding by cholera toxin is the first step in the development of cholera, a pathological condition characterized by severe diarrhea. Enterotoxigenic E. coli, the most common cause of diarrhea, including traveler’s diarrhea, produces a toxin that also gains access to the cell by first binding to gangliosides. Gangliosides are also crucial for binding immune-system cells to sites of injury in the inflammatory response.

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!clinic! CLINICAL INSIGHT: Disrupted Lipid Metabolism Results in Respiratory Distress Syndrome and Tay–Sachs Disease

Disruptions in lipid metabolism are responsible for a host of diseases. We will briefly examine two such conditions. Respiratory distress syndrome is a pathological condition resulting from a failure in the biosynthesis of dipalmitoylphosphatidylcholine. This phospholipid, in conjunction with specific proteins and other phospholipids, is found in the extracellular fluid that surrounds the alveoli of the lung, where it decreases the surface tension of the fluid to prevent lung collapse at the end of the expiration phase of breathing. Premature infants may suffer from respiratory distress syndrome because their immature lungs do not synthesize enough dipalmitoylphosphatidylcholine.

Whereas respiratory distress syndrome results from failure in biosynthesis, Tay–Sachs disease, a congenital disease that afflicts infants soon after birth, is caused by a failure of lipid degradation: specifically, an inability to degrade gangliosides. Gangliosides are found in highest concentration in the nervous system, particularly in gray matter, where they constitute 6% of the lipids. Gangliosides are normally degraded inside lysosomes by the sequential removal of their terminal sugars, but, in Tay–Sachs disease, this degradation does not take place. The terminal residue of the ganglioside is removed very slowly or not at all. The missing or deficient enzyme is a specific β-N-acetylhexosaminidase (Figure 29.5).

Figure 29.5: Tay–Sachs disease results from the inability to degrade a ganglioside. A particular ganglioside, GM2, accumulates in Tay–Sachs patients because a key step in its degradation, conversion into ganglioside GM3, cannot take place, because of insufficient β-N-acetylhexosaminidase. Abbreviations: GalNAc, N-acetylgalactosamine; NAN, N-acetylneuraminate; Gal, galactose; Glc, glucose.

As a consequence, neurons become significantly swollen with lipid-filled lysosomes (Figure 29.6). An affected infant displays weakness and retarded psychomotor skills before 1 year of age. The child is demented and blind by age 2 and usually dies before age 3.

Figure 29.6: Lysosome with lipids. An electron micrograph of a lysosome engorged with lipids. Such lysosomes are sometimes described as being “onion-skin”-like because the layers of undigested lipids resemble a sliced onion.

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Tay–Sachs disease was especially prominent among Ashkenazi Jews (descendants of Jews from central and eastern Europe). A genetic testing program, initiated in the early 1970s upon the development of a simple blood test to identify carriers, has virtually eliminated the disease in the population. Tay–Sachs disease can also be diagnosed in the course of fetal development.

Phosphatidic Acid Phosphatase Is a Key Regulatory Enzyme in Lipid Metabolism

Although the details of the regulation of lipid synthesis remain to be elucidated, evidence suggests that phosphatidic acid phosphatase (PAP), working in concert with diacylglycerol kinase, plays a key role in lipid synthesis regulation. Phosphatidic acid phosphatase, also called lipin 1 in mammals, controls the extent to which triacylglycerols are synthesized relative to phospholipids and regulates the type of phospholipid synthesized (Figure 29.7). For instance, when PAP activity is high, phosphatidate is dephosphorylated and diacylglycerol is produced, which can react with the appropriate activated alcohols to yield phosphatidylethanolamine, phosphatidylserine, or phosphatidylcholine. Diacylglycerols can also be converted into triacylglycerols, and evidence suggests that the formation of triacylglycerols may act as a fatty acid buffer, which helps to regulate the levels of diacylglycerol and sphingolipids, both of which serve signaling functions.

Figure 29.7: Regulation of lipid synthesis. Phosphatidic acid phosphatase (PAP) is the key regulatory enzyme in lipid synthesis. When active, PAP generates diacylglycerol, which can react with activated alcohols to form phospholipids or with fatty acyl CoA to form triacylglycerols. When PAP is inactive, phosphatidate is converted into CMP-DAG for the synthesis of different phospholipids. PAP also controls the amount of DAG and phosphatidate, both of which function as second messengers.

When PAP activity is lower, phosphatidate is used as a precursor for different phospholipids, such as phosphatidylinositol and cardiolipin. Moreover, phosphatidate is a signal molecule itself. Phosphatidate regulates the growth of endoplasmic reticulum and nuclear membranes and acts as a cofactor that stimulates the expression of genes in phospholipid synthesis.

What are the signal molecules that regulate the activity of PAP? CDP-diacylglycerol, phosphatidylinositol, and cardiolipin enhance PAP activity, and sphingosine and dihydrosphingosine inhibit it.

Studies in mice clearly show the importance of PAP for the regulation of fatty acid synthesis. The loss of PAP function prevents normal adipose-tissue development, leading to lipodystrophy (severe loss of body fat) and insulin resistance. Excess PAP activity results in obesity. Understanding the regulation of phospholipid synthesis is an exciting area of research that will be active for some time to come.