Lipid synthesis requires the coordinated action of gluconeogenesis and fatty acid metabolism, as illustrated in Figure 26.1. The common step in the synthesis of both phospholipids for membranes and triacylglycerols for energy storage is the formation of phosphatidate (diacylglycerol 3-phosphate). In mammalian cells, phosphatidate is synthesized in the endoplasmic reticulum and the outer mitochondrial membrane. The pathway begins with glycerol 3-phosphate, which is formed primarily by the reduction of dihydroxyacetone phosphate (DHAP) synthesized by the gluconeogenic pathway, and to a lesser extent by the phosphorylation of glycerol. The addition of two fatty acids to glycerol-3-phosphate yields phosphatidate. First, acyl coenzyme A contributes a fatty acid chain to form lysophosphatidate and, then, a second acyl CoA contributes a fatty acid chain to yield phosphatidate.

These acylations are catalyzed by glycerol phosphate acyltransferase. In most phosphatidates, the fatty acid chain attached to the C-1 atom is saturated, whereas the one attached to the C-2 atom is unsaturated. Phosphatidate can also be synthesized from diacylglycerol (DAG), in what is essentially a salvage pathway, by the action of diacylglycerol kinase:

Diacylglycerol + ATP → phosphatidate + ADP

The phospholipid and triacylglycerol pathways diverge at phosphatidate. In the synthesis of triacylglycerols, a key enzyme in the regulation of lipid synthesis, phosphatidic acid phosphatase, hydrolyzes phosphatidate to give a diacylglycerol. This intermediate is acylated to a triacylglycerol through the addition of a third fatty acid chain in a reaction that is catalyzed by diglyceride acyltransferase. Both enzymes are associated in a triacylglycerol synthetase complex that is bound to the endoplasmic reticulum membrane.

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The liver is the primary site of triacylglycerol synthesis. From the liver, the triacylglycerols are transported to the muscles for energy conversion or to the adipose cells for storage.

Membrane-lipid synthesis continues in the endoplasmic reticulum and in the Golgi apparatus. Phospholipid synthesis 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 the diacylglycerol or the alcohol may be activated, depending on the source of the reactants.

The synthesis of some phospholipids begins with the reaction of phosphatidate with cytidine triphosphate (CTP) to form the activated diacylglycerol, cytidine diphosphodiacylglycerol (CDP-diacylglycerol). This reaction, like those of many biosyntheses, is driven forward by the hydrolysis of pyrophosphate.

The activated phosphatidyl unit then reacts with the hydroxyl group of an alcohol to form a phosphodiester linkage. If the alcohol is inositol, the products are phosphatidylinositol and cytidine monophosphate (CMP).

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Subsequent phosphorylations catalyzed by specific kinases lead to the synthesis of phosphatidylinositol 4,5-bisphosphate, the precursor of two intracellular messengers—diacylglycerol and inositol 1,4,5-trisphosphate (Section 14.2). If the alcohol is phosphatidylglycerol, the products are diphosphatidylglycerol (cardiolipin) and CMP. In eukaryotes, cardiolipin is synthesized in the mitochondria and is located exclusively in inner mitochondrial membranes. Cardiolipin plays an important role in the organization of the protein components of oxidative phosphorylation. For example, it is required for the full activity of cytochrome c oxidase (Section 18.3).

The fatty acid components of phospholipids may vary, and thus cardiolipin, as well as most other phospholipids, represents a class of molecules rather than a single species. As a result, a single mammalian cell may contain thousands of distinct phospholipids. Phosphatidylinositol is unusual in that it has a nearly fixed fatty acid composition. Stearic acid usually occupies the C-1 position and arachidonic acid the C-2 position.

Phosphatidylethanolamine, the major phospholipid of the inner leaflet of cell membranes, is synthesized from the alcohol ethanolamine. To activate the alcohol, 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 transferred to a diacylglycerol to form phosphatidylethanolamine.

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 k_cat/K_M value (Section 8.4) increases three orders of magnitude upon activation, resulting in the restoration of phosphatidylcholine levels.

Earlier (Section 22.4) we examined how cancer cells increase lipogenesis 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.

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The importance of phosphatidylcholine is attested to by the fact that the liver possesses an enzyme, phosphatidylethanolamine methyltransferase, which 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 (Section 24.2).

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.

Choline is a popular dietary supplement that is believed by some to enhance liver and neuronal function. Although the effectiveness of choline supplements is not established, the dangers of excess choline consumption are becoming clear. Gut bacteria convert excess choline into trimethylamine (TMA), a gas that smells like rotten fish, and the liver converts the absorbed TMA into trimethylamine-N-oxide (TMAO). TMAO stimulates cholesterol uptake by macrophages, a process that can result in atherosclerosis. Foods rich in phosphatidylcholine, such as red meats and dairy products, may also result in TMAO production.

Phosphatidylserine makes up 10% of the phospholipids in mammals. This phospholipid is synthesized in a base-exchange reaction of serine with phosphatidylcholine or phosphatidylethanolamine. In the reaction, serine replaces choline or ethanolamine.

Phosphatidylserine is normally located in the inner leaflet of the plasma membrane bilayer but is moved to the outer leaflet in apoptosis (Section 18.6). There, it serves to attract phagocytes to consume the cell remnants after apoptosis is complete. Phosphatidylserine is translocated from one side of the membrane to the other by an ATP-binding cassette translocase (Section 13.2).

Note that a cytidine nucleotide plays the same role in the synthesis of these phosphoglycerides as a uridine nucleotide does in the formation of glycogen (Section 21.4). In all of these biosyntheses, an activated intermediate (UDP-glucose, CDP-diacylglycerol, or CDP-alcohol) is formed from a phosphorylated substrate (glucose 1-phosphate, phosphatidate, or a phosphorylalcohol) and a nucleoside triphosphate (UTP or CTP). The activated intermediate then reacts with a hydroxyl group (the terminus of glycogen, an alcohol, or a diacylglycerol).

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We now turn from glycerol-based phospholipids to another class of membrane lipid—the sphingolipids. These lipids are found in the plasma membranes of all eukaryotic cells, although the concentration is highest in the cells of the central nervous system. The backbone of a sphingolipid is sphingosine, rather than glycerol. Palmitoyl CoA and serine condense to form 3-ketosphinganine. The serine–palmitoyl transferase catalyzing this reaction is the rate-limiting step in the pathway and requires pyridoxal phosphate, revealing again the dominant role of this cofactor in transformations that include amino acids. Ketosphinganine is then reduced to dihydrosphingosine before conversion into ceramide, a lipid consisting of a fatty acid chain attached to the amino group of a sphingosine backbone (Figure 26.2).

In all sphingolipids, the amino group of ceramide is acylated. The terminal hydroxyl group also is substituted (Figure 26.3). In sphingomyelin, a component of the myelin sheath covering many nerve fibers, the substituent is phosphorylcholine, which comes from phosphatidylcholine. In a cerebroside, the substituent is glucose or galactose. UDP-glucose or UDP-galactose is the sugar donor.

Gangliosides are the most complex sphingolipids. In a ganglioside, an oligosaccharide chain is linked to the terminal hydroxyl group of ceramide by a glucose residue (Figure 26.4). This oligosaccharide chain contains at least one acidic sugar, either N-acetylneuraminate or N-glycolylneuraminate. These acidic sugars are called sialic acids. Their nine-carbon backbones are synthesized from phosphoenolpyruvate (a three-carbon unit) and N-acetylmannosamine 6-phosphate (a six-carbon unit).

Gangliosides are synthesized by the ordered, step-by-step addition of sugar residues to ceramide. The synthesis of these complex lipids requires the activated sugars UDP-glucose, UDP-galactose, and UDP-N-acetylgalactosamine, as well as the CMP derivative of N-acetylneuraminate. CMP-N-acetylneuraminate is synthesized from CTP and N-acetylneuraminate. The sugar composition of the resulting ganglioside is determined by the specificity of the glycosyltransferases in the cell. Almost 200 different gangliosides have been characterized (see Figure 26.4 for the composition of ganglioside G_M1).

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Ganglioside-binding by the cholera toxin is the first step in the development of cholera, a pathological condition characterized by severe diarrhea (Section 14.5). 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.

The structures of sphingolipids and the more abundant glycerophospholipids are very similar (Figure 12.8). Given the structural similarity of these two types of lipids, why are sphingolipids required at all? Indeed, the prefix “sphingo” was applied to capture the “sphinxlike” properties of this enigmatic class of lipids. Although the precise role of sphingolipids is not firmly established, progress toward solving the riddle of their function is being made. As discussed in Chapter 12, sphingolipids are important components of lipid rafts, highly organized regions of the plasma membrane that are important in signal transduction. Sphingosine, sphingosine 1-phosphate, and ceramide serve as second messengers in the regulation of cell growth, differentiation, and death. For instance, ceramide derived from a sphingolipid initiates programmed cell death in some cell types and may contribute to the development of type 2 diabetes (Chapter 27).

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Respiratory distress syndrome is a pathological condition resulting from a failure in the biosynthetic pathway for dipalmitoylphosphatidylcholine. This phospholipid, in conjunction with specific proteins and other phospholipids, is found in the extracellular fluid that surrounds the alveoli of the lung. Its function is to decrease 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.

Tay–Sachs disease is caused by a failure of lipid degradation: 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. As a consequence, neurons become significantly swollen with lipid-filled lysosomes (Figure 26.5). 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.

The ganglioside content of the brain of an infant with Tay–Sachs disease is greatly elevated. The concentration of ganglioside G_M2 is many times higher than normal because its terminal N-acetylgalactosamine residue is removed very slowly or not at all. The missing or deficient enzyme is a specific β-N-acetylhexosaminidase.

Tay–Sachs disease can be diagnosed in the course of fetal development. Cells obtained from either the villi of the placenta (chorionic villus sampling) or from the amniotic fluid (amniocentesis) are tested for the presence of the defective gene. 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.

Ceramide is a precursor for sphingomyelin, cerebroside, and gangliosides. Ceramide itself, however, induces programmed cell death or apoptosis (Section 18.6). Recall that cancer cells require all types of lipids for membrane formation (Section 22.4). How do cancer cells prevent ceramide-induced cell death? It appears that these cells destroy the apoptotic signal by converting it into a pro-mitotic signal. Ceramidase removes the fatty acid from the amino group of ceramide, generating sphingosine. Sphingosine is then converted into sphingosine 1-phosphate by sphingosine kinase, which stimulates cell division.

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Thus, cancer cells convert a potentially lethal signal molecule into one that promotes tumor growth. Efforts are underway to develop inhibitors of ceramidase for use as chemotherapeutic agents.

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 the regulation of lipid synthesis. PAP, 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 26.6). 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. Diacylglycerol can also be converted into triacylglycerols, and evidence suggests that the formation of triacylglycerols may act as a fatty acid buffer. This buffering helps to regulate the levels of diacylglycerol and sphingolipids, which serve signaling functions.

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.

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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. The enzyme also undergoes extensive phosphorylation and dephosphorylation. When phosphorylated, the enzyme resides in the cytoplasm. Upon dephosphorylation, the enzyme associates with the endoplasmic reticulum, the location of its substrate phosphatidate. Details of the covalent modification are under investigation.

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. The regulation of phospholipid synthesis is an exciting area of research that will be active for some time to come.