LEHNINGER PRINCIPLES OF BIOCHEMISTRY - VOL. 2. BIOENERGETICS AND METABOLISM - 2014

CHAPTER II. BIOENERGETICS AND METABOLISM

21. BIOSYNTHESIS OF LIPIDS

21.3. Biosynthesis of Membrane Phospholipids

In Chapter 10, we introduced the two Major Classes of membrane Phospholipids: Glycerophospholipids and Sphingolipids. A vast array of phospholipid molecules can be constructed by combining different Fatty acids and polar HEAD groups with a glycerol or sphingosine backbone (see Figs. 10-9, 10-13). All biosynthetic pathways follow a few basic patterns. In general, the assembly of phospholipids from primary precursors requires (1) Synthesis of the backbone molecule (glycerol or sphingosine); (2) attachment of fatty acid(s) to the backbone via an ester or amide linkage; (3) attachment of a hydrophilic head group to the backbone through a phosphodiester bond; and, finally, in some cases, (4) modification or replacement of the polar head group to yield the final phospholipid product.

In Eukaryotic Cells, phospholipid synthesis takes place primarily on the cytosolic surface of the smooth Endoplasmic reticulum and on The inner mitochondrial membrane. Some newly synthesized phospholipids remain at the site of synthesis, but the majority are destined for other cellular compartments. The mechanism by which Water-insoluble phospholipids move from their site of synthesis to their subsequent functional destinations is not yet fully understood, but we will conclude this section by discussing several mechanisms elucidated in recent years.

Cells Use Two Strategies for the Attachment of Polar Head Groups to Phospholipids

The initial steps in glycerophospholipid synthesis are identical to those in triacylglycerol synthesis (Fig. 21-17): two fatty acyl residues form ester linkages at the C-1 and C-2 positions of L-glycerol 3-phosphate, yielding phosphatidic acid. Typically, a saturated fatty acid is attached at C-1 and an unsaturated fatty acid at C-2. A secondary pathway leading to phosphatidic acid involves the phosphorylation of diacylglycerols by a specific kinase.

The polar head group of glycerophospholipids is attached via a phosphodiester bond, formed when each of two alcohol hydroxyls (one on the polar head group and one at C-3 of glycerol) forms an ester with phosphoric acid (Fig. 21-23). First, one of the hydroxyl groups is activated by Condensation with cytidine diphosphate (CDP). Subsequent nucleophilic attack by the other hydroxyl group displaces cytidine monophosphate (CMP) (Fig. 21-24). CDP is attached either to the diacylglycerol, yielding activated phosphatidic acid and CDP-diacylglycerol (Strategy 1), or to the hydroxyl group of the head group (Strategy 2). Eukaryotic cells employ both strategies, whereas prokaryotes use only Strategy 1. The pivotal role of cytidine NUCLEOTIDES in Lipid Biosynthesis was discovered by Eugene Kennedy in the early 1960s.

Class="center">Eugene P. Kennedy

FIGURE 21-23 Attachment of the polar head group. The polar head group is attached to diacylglycerol in a phospholipid via a phosphodiester bond formed by the condensation of phosphoric acid with two alcohol groups (two molecules of H₂O are released).

FIGURE 21-24 Two major strategies for forming the phosphodiester bond in phospholipids. In both strategies, the phosphate group for the phosphodiester linkage is provided by CDP.

CDP-Diacylglycerols Serve as Precursors for Phospholipid Synthesis in E. coli

Strategy 1 for polar head group attachment is illustrated by the synthesis of phosphatidylserine, phosphatidylethanolamine, and phosphatidylglycerol in E. coli. Diacylglycerol is activated by the condensation of phosphatidic acid with cytidine triphosphate (CTP) to form CDP-diacylglycerol, with the release of pyrophosphate (Fig. 21-25). Displacement of CMP via nucleophilic attack by the hydroxyl group of Serine or the hydroxyl group at C-1 of glycerol 3-phosphate yields phosphatidylserine or phosphatidylglycerol 3-phosphate, respectively. The latter product undergoes further conversion by the hydrolytic Cleavage of a phosphate monoester (releasing P_i) to form phosphatidylglycerol.

Phosphatidylserine and phosphatidylglycerol can serve as precursors for other bacterial Membrane Lipids (Fig. 21-25). Decarboxylation of the serine residue of phosphatidylserine, catalyzed by phosphatidylserine decarboxylase, yields phosphatidylethanolamine. In E. coli, the condensation of two molecules of phosphatidylglycerol, accompanied by the release of glycerol, produces cardiolipin, in which two diacylglycerol moieties are linked through a shared polar head group.

Eukaryotes Synthesize Anionic Phospholipids from CDP-Diacylglycerols

In eukaryotes, phosphatidylglycerol, cardiolipin, and phosphatidylinositol (all anionic phospholipids; see Fig. 10-9, Vol. 1) are synthesized via the same strategy used by Bacteria. Phosphatidylglycerol is formed in exactly the same manner as in bacteria. The synthesis of cardiolipin in eukaryotes differs slightly: phosphatidylglycerol condenses with CDP-diacylglycerol (Fig. 21-26) rather than with another molecule of phosphatidylglycerol, as occurs in E. coli (Fig. 21-25).

Phosphatidylinositol is synthesized by the condensation of CDP-diacylglycerol with Inositol (Fig. 21-26). Specific phosphatidylinositol Kinases subsequently convert phosphatidylinositol into its phosphorylated derivatives (see Fig. 10-17, Vol. 1). Phosphatidylinositol and its phosphorylated derivatives in the eukaryotic Cell/33.html">Plasma Membrane play a central role in signal Transduction (see Figs. 12-10, 12-16, Vol. 1).

FIGURE 21-25 Formation of polar head groups in phospholipids of E. coli. Attachment of the polar head group (serine or glycerol 3-phosphate) initially forms a CDP-diacylglycerol intermediate (Strategy 1 in Fig. 21-24). For all phospholipids except phosphatidylserine, the polar head group undergoes further modification as shown here. PG, phosphatidylglycerol; PS, phosphatidylserine.

FIGURE 21-26 Synthesis of cardiolipin and phosphatidylinositol in eukaryotes. These glycerophospholipids are synthesized via Strategy 1 (Fig. 21-24). Phosphatidylglycerol is synthesized just as in bacteria (see Fig. 21-25). PI, phosphatidylinositol.

In eukaryotes, the metabolic pathways for the synthesis of phosphatidylserine, phosphatidylethanolamine, and phosphatidylcholine are interconnected

Like bacteria, Yeast can produce phosphatidylserine via the condensation of CDP-diacylglycerol and serine, and can also synthesize phosphatidylethanolamine from phosphatidylserine in a reaction catalyzed by phosphatidylserine decarboxylase (Fig. 21-27). In mammalian cells, an alternative pathway for phosphatidylserine formation involves polar head group exchange reactions, in which free serine replaces ethanolamine. Phosphatidylethanolamine can also be converted into phosphatidylcholine (lecithin) by The addition of three methyl groups to its amino group; S-adenosylmethionine serves as the methyl group donor for all three methylation reactions (see Fig. 18-18). These represent the primary pathways for the synthesis of phosphatidylethanolamine and phosphatidylcholine in eukaryotic cells.

Fig. 21-27. Main metabolic pathway from phosphatidylserine to phosphatidylethanolamine and phosphatidylcholine in all eukaryotes. AdoMet, S-adenosylmethionine; adoHcy, S-adenosylhomocysteine.

In mammals, phosphatidylserine is not synthesized from CDP-diacylglycerol; instead, it is derived from phosphatidylethanolamine through an exchange reaction (Fig. 21-28, a). The synthesis of phosphatidylethanolamine and phosphatidylcholine in mammals follows strategy 2 (Fig. 21-24): phosphorylation and activation of the head group, followed by condensation with diacylglycerol. For example, Choline is salvaged (reutilized) via phosphorylation and subsequent conversion to CDP-choline through condensation with CTP. Diacylglycerol displaces CMP from CDP-choline to yield phosphatidylcholine (Fig. 21-28, b). A similar salvage pathway converts dietary ethanolamine into phosphatidylethanolamine. Phosphatidylcholine is also formed in the Liver by the methylation of phosphatidylethanolamine (using S-adenosylmethionine, as described above), whereas in all other Tissues it is produced solely via the condensation of diacylglycerol and CDP-choline. The metabolic pathways for The formation of phosphatidylcholine and phosphatidylethanolamine across various organisms are summarized in Fig. 21-29.

Fig. 21-28. Pathways for the synthesis of phosphatidylserine and phosphatidylcholine in mammals. (a) Phosphatidylserine is synthesized via a Ca²⁺-dependent exchange reaction catalyzed by phosphatidylserine synthase 1 (PSS1) or phosphatidylserine synthase 2 (PSS2). The type 1 enzyme utilizes either phosphatidylethanolamine or phosphatidylcholine as a substrate. The Metabolic Pathways in bacteria and yeast correspond to those in Fig. 21-27. (b) The same strategy demonstrated for phosphatidylcholine synthesis (strategy 2 in Fig. 21-24) is also employed for ethanolamine utilization in the synthesis of phosphatidylethanolamine.

While the exact influence of Lipid Composition on membrane function remains incompletely understood, it has been established that this factor can exert profound effects. Researchers have isolated fruit flies with Mutations in the Gene encoding ethanolamine kinase (analogous to choline kinase; Fig. 21-28, b). A deficiency in this enzyme blocks one of the pathways for phosphatidylethanolamine synthesis, thereby reducing the levels of this lipid in cellular membranes. Mutant flies carrying this genetic defect (representing an easily stressed genotype) exhibit transient paralysis following electrical stimulation or mechanical Shock, whereas wild-type flies are unaffected by these treatments.

Fig. 21-29. General Overview of the biosynthetic pathways for major phospholipids. The metabolic pathways for phospholipid synthesis vary among different classes of organisms. Mammalian pathways are highlighted in yellow, those in bacteria and yeast in pink, and overlapping pathways in orange. In mammals, phosphatidylethanolamine and phosphatidylcholine are synthesized from diacylglycerols and the appropriate CDP-derivatized head group. The conversion of phosphatidylethanolamine to phosphatidylcholine in mammals occurs exclusively in the liver. The pathways for phosphatidylserine synthesis in various organisms are illustrated in detail in Figs. 21-27 and 21-28.

The synthesis of plasmalogens requires the formation of an ether-linked fatty alcohol

The metabolic pathway for The biosynthesis of ether lipids, including plasmalogens and platelet-activating factor (see Fig. 10-10, Vol. 1), involves the replacement of an esterified fatty acyl group with a long-chain alcohol to form an ether bond (Fig. 21-30). This is followed by the attachment of a head group through a mechanism very similar to that for conventional ester-linked phospholipids. Finally, the characteristic double bond in plasmalogens (highlighted in cyan in Fig. 21-30) is introduced by the action of a mixed-function oxidase analogous to the fatty acid desaturase oxidase (Fig. 21-13). Peroxisomes serve as the primary site for plasmalogen synthesis.

Fig. 21-30. Synthesis of ether lipids and plasmalogens. The newly formed ether bond is highlighted in pink. The intermediate 1-alkyl-2-acylglyceryl-3-phosphate is an ether analog of phosphatidic acid. The mechanisms for attaching polar head groups to ether lipids are generally the same as those for their ester-linked counterparts. The characteristic double bond in plasmalogens (highlighted in cyan) is introduced at a late stage by a mixed-function oxidase system similar to that shown in Fig. 21-13.

Sphingolipid and glycerophospholipid biosyntheses share several precursors and mechanisms

Sphingolipid biosynthesis proceeds in four stages: (1) synthesis of the 18-carbon amine sphinganine from palmitoyl-CoA and serine; (2) attachment of a fatty acid via an amide bond to yield an N-acylsphinganine; (3) desaturation of the sphinganine moiety to form N-acylsphingosine (ceramide); and (4) attachment of a head group to yield a sphingolipid, such as a cerebroside or sphingomyelin (Fig. 21-31). The initial steps take place in The endoplasmic reticulum, whereas head group attachment (step 4) occurs in the Golgi apparatus.

Fig. 21-31. Biosynthesis of sphingolipids. The condensation of palmitoyl-CoA and serine (yielding β-ketosphinganine), followed by NADPH-dependent reduction, produces sphinganine, which is subsequently acylated to form N-acylsphinganine (ceramide). In animals, a double bond (highlighted in pink) is introduced by a mixed-function oxidase, after which a polar head group is attached: phosphatidylcholine in the formation of sphingomyelin, or glucose in the formation of cerebroside.

This metabolic pathway shares several features with the pathways leading to glycerophospholipid synthesis: reducing equivalents are provided by NADPH, and Fatty acids are incorporated as activated CoA derivatives. In cerebroside formation, sugars are introduced as activated nucleotide derivatives. Several novel aspects of polar head group attachment during sphingolipid synthesis have also been uncovered. Most notably, phosphatidylcholine, rather than CDP-choline, serves as the phosphocholine donor in sphingomyelin synthesis.

In Glycolipids, cerebrosides, and gangliosides (see Fig. 10-12, Vol. 1), the head group sugar is linked directly to the C-1 hydroxyl group of sphingosine, forming a glycosidic bond rather than a phosphodiester bond. The sugar donor in this reaction is a UDP-sugar (UDP-glucose or UDP-galactose).

Polar lipids are localized in specific cellular membranes

Following their synthesis in the smooth endoplasmic reticulum (ER), polar lipids—including glycerophospholipids, sphingolipids, and glycolipids—are incorporated into specific cellular membranes in defined proportions through mechanisms that remain poorly understood. Membrane lipids are water-insoluble and therefore cannot simply diffuse from their site of synthesis (the ER) to their destination membrane. Instead, they are packaged into transport vesicles that bud off from the Golgi complex, move through the Cytosol, and fuse with the target membrane (see Fig. 11-22, Vol. 1). Although many details have been elucidated, the full picture of these metabolic pathways is not yet complete. A 68 kDa protein known as CERT (ceramide transfer protein) mediates The transport of ceramide from the endoplasmic reticulum to the Golgi apparatus.

Cytosolic Proteins also bind phospholipids and sterols and transfer them between cellular membranes; collectively, these mechanisms ensure that the membranes of different Organelles maintain their characteristic lipid compositions (see Fig. 11-2, Vol. 1).

Summary of Section 21.3 Biosynthesis of Membrane Phospholipids

■ Diacylglycerols serve as the primary precursors for glycerophospholipids.

■ In bacteria, phosphatidylserine is synthesized by the condensation of serine with CDP-diacylglycerol, and its subsequent decarboxylation yields phosphatidylethanolamine. Phosphatidylglycerol is formed through the condensation of CDP-diacylglycerol with glycerol-3-phosphate, followed by the cleavage of phosphate from the ester bond.

■ In yeast, the biosynthetic pathways for phosphatidylserine, phosphatidylethanolamine, and phosphatidylglycerol are similar to those in bacteria, whereas phosphatidylcholine is produced by the methylation of phosphatidylethanolamine.

■ Mammalian cells share certain pathways with bacteria, though the synthesis routes for phosphatidylcholine and phosphatidylethanolamine differ to some extent. The head-group alcohol (choline or ethanolamine) is activated as a CDP derivative and subsequently condensed with diacylglycerol. Phosphatidylserine is generated exclusively from phosphatidylethanolamine.

■ Plasmalogen synthesis involves the formation of their characteristic double bond via a mixed-function oxidase. The head groups of sphingolipids are attached through specific mechanisms.

■ Phospholipids are transported to their intracellular destinations either via transport vesicles or with the assistance of specific proteins.