Garrett R.H., Grisham C.M. - Biochemistry (1999)(2nd ed.)(en)

25.2 ● Biosynthesis of Complex Lipids

821

Glycerolipid Biosynthesis

A common pathway operates in nearly all organisms for the synthesis of phosphatidic acid, the precursor to other glycerolipids. Glycerokinase catalyzes the phosphorylation of glycerol to form glycerol-3-phosphate, which is then acylated at both the 1- and 2-positions to yield phosphatidic acid (Figure 25.18). The first acylation, at position 1, is catalyzed by glycerol-3-phosphate acyltransferase, an enzyme that in most organisms is specific for saturated fatty acyl groups. Eukaryotic systems can also utilize dihydroxyacetone phosphate as a starting point for synthesis of phosphatidic acid (Figure 25.18). Again a specific acyltransferase adds the first acyl chain, followed by reduction of the backbone keto group by acyldihydroxyacetone phosphate reductase, using NADPH as the reductant. Alternatively, dihydroxyacetone phosphate can be reduced to glycerol-3-phosphate by glycerol-3-phosphate dehydrogenase.

Eukaryotes Synthesize Glycerolipids from

CDP-Diacylglycerol or Diacylglycerol

In eukaryotes, phosphatidic acid is converted directly either to diacylglycerol or to cytidine diphosphodiacylglycerol (or simply CDP-diacylglycerol; Figure 25.19). From these two precursors, all other glycerophospholipids in eukaryotes are derived. Diacylglycerol is a precursor for synthesis of triacylglycerol, phosphatidylethanolamine, and phosphatidylcholine. Triacylglycerol is synthesized mainly in adipose tissue, liver, and intestines and serves as the principal energy storage molecule in eukaryotes. Triacylglycerol biosynthesis in liver and adipose tissue occurs via diacylglycerol acyltransferase, an enzyme bound to the cytoplasmic face of the endoplasmic reticulum. A different route is used, however, in intestines. Recall (Figure 24.3) that triacylglycerols from the diet are broken down to 2-monoacylglycerols by specific lipases. Acyltransferases then acylate 2-monoacylglycerol to produce new triacylglycerols (Figure 25.20).

Phosphatidylethanolamine Is Synthesized from

Diacylglycerol and CDP-Ethanolamine

Phosphatidylethanolamine synthesis begins with phosphorylation of ethanolamine to form phosphoethanolamine (Figure 25.19). The next reaction involves transfer of a cytidylyl group from CTP to form CDP-ethanolamine and pyrophosphate. As always, PPi hydrolysis drives this reaction forward. A specific phosphoethanolamine transferase then links phosphoethanolamine to the diacylglycerol backbone. Biosynthesis of phosphatidylcholine is entirely analogous because animals synthesize it directly. All of the choline utilized in this pathway must be acquired from the diet. Yeast, certain bacteria, and animal livers, however, can convert phosphatidylethanolamine to phosphatidylcholine by methylation reactions involving S-adenosylmethionine (see Chapter 26).

Exchange of Ethanolamine for Serine Converts

Phosphatidylethanolamine to Phosphatidylserine

Mammals synthesize phosphatidylserine (PS) in a calcium ion–dependent reaction involving aminoalcohol exchange (Figure 25.21). The enzyme catalyzing this reaction is associated with the endoplasmic reticulum and will accept phosphatidylethanolamine (PE) and other phospholipid substrates. A mitochondrial PS decarboxylase can subsequently convert PS to PE. No other pathway converting serine to ethanolamine has been found.

FIGURE 25.19

822 Chapter 25 ● Lipid Biosynthesis

O

R1

C

O

CH2

R2

C

O

CH

O

O

O

CH2

O

P

O

P

O

Cytidine

CDP diacylglycerol

O–

O–

+

HOCH2CH2NH3

P P

Ethanolamine

Phosphatidate

+

cytidylyltransferase

HOCH2CH2N(CH3)3

ATP

CTP

Choline

Ethanolamine kinase

ADP

ATP

O

Choline kinase

O

O

ADP

–O

+

R1

C

CH2

P

OCH2CH2NH3

R2

C

O

O

O

CH

+

–O

O–

–O

P

OCH2CH2N(CH3)3

O

CH2

O

P

Phosphoethanolamine

–O

CTP: Phospho-

CTP

O–

Phosphocholine

ethanolamine

Phosphatidic acid

CTP

cytidylyltransferase

P P

CTP: Phosphocholine

Cytidine

ADP

H2O

P

P

cytidylyltransferase

Diacylglycerol

Phosphatidic acid

Cytidine

O

O

+

kinase

phosphatase

O

O

O

P

O

P

OCH2CH2NH3

ATP

P

+

–

–

O

P

O

P

OCH2CH2N(CH3)3

O

O

CDPcholine:

1,2-diacylgly-

CDPethanolamine

O–

O–

cerol phospho-

O

choline

CDPcholine

CDPethanolamine: 1,2-

transferase

R1

C

O

CH2

diacylglycerol phospho-

ethanolamine transferase

CMP

CMP

R2

C

O

CH

O

O

CH2OH

Diacylglycerol

O

R1

C

O

CH2

O

R2

C

O

CH

O

R1

C

O

CH2

+

R3

C

CoA

O

CH2

O

P

OCH2CH2N(CH3)3

R2

C

O

CH

O

Diacylglycerol

+

O–

acyltransferase

O

CH2

O

P

OCH2CH2NH3

CoA

Phosphatidylcholine

O–

O

Phosphatidylethanolamine

R1

C

O

CH2

O

O

R2

C

C H

O CH2 O C R3

Triacylglycerol

● Diacylglycerol and CDP-diacylglycerol are the principal precursors of glycerolipids in eukaryotes. Phosphatidylethanolamine and phosphatidylcholine are formed by reaction of diacylglycerol with CDP-ethanolamine or CDP-choline, respectively.

FIGURE 25.21

FIGURE 25.20

● Triacylglycerols are formed primarily by the action of acyltransferases on monoand diacylglycerol. Acyltransferase in E. coli is an integral membrane protein (83 kD) and can utilize either fatty acyl-CoAs or acylated acyl carrier proteins as substrates. It shows a particular preference for palmitoyl groups. Eukaryotic acyltransferases use only fatty acyl-CoA molecules as substrates.

O

R1

C

O

CH2

O

R2

C

CH

O

+

O

CH2

O

P

O

CH2

CH2

NH3

CO2

–

O

Phosphatidylethanolamine

Serine

Serine

Phosphatidylserine

decarboxylase

Base exchange enzyme

(mitochondria)

(endoplasmic reticulum)

Ethanolamine

Ethanolamine

O

R1

C

O

CH2

Serine

O

R2

C

CH

O

H

+

O

CH2

O

P

O

CH2

C

NH3

O–

COO–

Phosphatidylserine

25.2 ● Biosynthesis of Complex Lipids

823

▲

Dietary triacylglycerols

Lipases

O CH2OH

R2 C O C H

CH2 OH

2-Monoacylglycerol

O

R1 C S CoA

Monoacylglycerol acyltransferase

CoA

O

R1 C O CH2

O

R2 C O CH

CH2 OH

Diacylglycerol

O

R3 C SCoA

Diacylglycerol acyltransferase

CoA

● The interconversion of phosphatidylethanolamine and phosphatidylserine in mammals.

O

O

R1

C

CH2

O

R2

C

O

CH

O

Eukaryotes Synthesize Other Phospholipids via CDP-Diacylglycerol

Eukaryotes also use CDP-diacylglycerol, derived from phosphatidic acid, as a precursor for several other important phospholipids, including phosphatidylinositol (PI), phosphatidylglycerol (PG), and cardiolipin (Figure 25.22). PI accounts for only about 2 to 8% of the lipids in most animal membranes, but breakdown products of PI, including inositol-1,4,5-trisphosphate and diacylglycerol, are second messengers in a vast array of cellular signaling processes.

Dihydroxyacetone Phosphate Is a Precursor to the Plasmalogens

Certain glycerophospholipids possess alkyl or alkenyl ether groups at the 1-position in place of an acyl ester group. These glyceroether phospholipids are synthesized from dihydroxyacetone phosphate (Figure 25.23). Acylation of dihydroxyacetone phosphate (DHAP) is followed by an exchange reaction, in which the acyl group is removed as a carboxylic acid and a long-chain alcohol

R3 C O CH2

Triacylglycerol

824 Chapter 25 ● Lipid Biosynthesis

O

NH2

R1

C

O

CH2

O

N

O

O

R2

C

O

C

H

O N

CH2

O

P

O

P

O

CH2

O

O–

O–

Inositol

Phosphatidylinositol synthase

CMP

		O
R1		C			O		CH2
		O
											O
R2		C			O		C			H

					O		P				O–
		CH2
			OH OH
	H						O
			H		H
					H
	HO						H
			H		OH

Phosphatidylinositol

OH OH

CDP-diacylglycerol

Glycerol-3-P

Glycerophosphate phosphatidyltransferase

CMP

O

R1

C

O

CH2

O

Phosphoglycerol

O

H

R2

C

O

H

O

C

O

O–

O

P

O

CH2

C

CH2

P

CH2

O–

OH

O–

Phosphatidylglycerol-P

Phosphatidylglycerol-P

phosphatase

P

O

R1

C

O

CH2

O

O

Glycerol

R2

O

H

H

C

C

CH2

O

P

O

CH2

C

CH2OH

O–

OH

Phosphatidylglycerol

CDP-diacylglycerol

Cardiolipin synthase

CMP

O

O

R1

C

O

CH2

CH2

O

C

R4

O

O

O

Glycerol

O H

R2

C

O

C

H

H

C

O

C

R3

CH2

O

P

O

CH2

C

CH2

O

P

O

CH2

O–

OH

O–

Cardiolipin

FIGURE 25.22 ● CDP-diacylglycerol is a precursor of phosphatidylinositol, phosphatidylglycerol, and cardiolipin in eukaryotes.

25.2 ● Biosynthesis of Complex Lipids

825

CH2 OH

C O O

CH2 O P O–

O–

Dihydroxyacetone phosphate

O

Dihydroxyacetone

CoA

S

R

C

phosphate

acyltransferase

CoASH

O

R

C

O

CH2

O

C

O

O–

CH2

O

P

O–

1-Acyldihydroxyacetone phosphate

1-Acyldihydroxyacetone

HO

CH2CH2R1

R

C

O–

phosphate synthase

O

R1CH2CH2

O

CH2

C

O

O

O–

CH2

O

P

O–

R1

H

H

O

CH2

C

C

O

R2

C

O

CH

O

+

CH2

O

P

O

CH2CH2NH3

O–

Plasmalogen

1-Alkyl-2-acylglycero-

NAD+

+ 2

H2O

phosphoethanolamine

desaturase

NADH +

O2 + H+

R1CH2CH2

O

CH2

O

R2

O

H

C

C

O

+

CH2

O

P

O

CH2CH2NH3

O–

1-Alkyl-2-acylglycero-3-phosphoethanolamine

1-Alkyldihydroxyacetone phosphate

CDP-ethanolamine

CDP

NADPH

+ H+

1-Alkyldihydroxyacetone

transferase

CDP-

ethanolamine

phosphate

NADP+

oxidoreductase

O

R1CH2CH2

O

CH2

R1CH2CH2 O

CH2

S

CoA

O

R2

CoASH

O

C

HOCH

O–

R2

C

O

C

H

O

CH2

O

P

–

CH2

O

P

O

1-Alkylglycerophosphate

–

O

acyltransferase

O–

1-Alkylglycero-3-phosphate

1-Alkyl-2-acylglycero-3-phosphate

adds to the 1-position. This long-chain alcohol is derived from the corresponding acyl-CoA by means of an acyl-CoA reductase reaction involving oxidation of two molecules of NADH. The 2-keto group of the DHAP backbone is then reduced to an alcohol, followed by acylation. The resulting 1-alkyl-2- acylglycero-3-phosphate can react in a manner similar to phosphatidic acid to produce ether analogs of phosphatidylcholine, phosphatidylethanolamine, and so forth (Figure 25.23). In addition, specific desaturase enzymes associated with the endoplasmic reticulum can desaturate the alkyl ether chains of these lipids

FIGURE 25.23 ● Biosynthesis of plasmalogens in animals. Acylation at C-1 is followed by exchange of the acyl group for a long-chain alcohol. Reduction of the keto group at C-2 is followed by transferase reactions, which add an acyl group at C-2 and a polar head-group moiety, and a desaturase reaction that forms a double bond in the alkyl chain. The first two enzymes are of cytoplasmic origin, and the last transferase is located at the endoplasmic reticulum.

826 Chapter 25 ● Lipid Biosynthesis

FIGURE 25.24 ● Platelet activating factor, formed from 1-alkyl-2-lysophosphatidylcholine by acetylation at C-2, is degraded by the action of acetylhydrolase.

as shown. The products, which contain , -unsaturated ether-linked chains at the C-1 position, are plasmalogens; they are abundant in cardiac tissue and in the central nervous system. The desaturases catalyzing these reactions are distinct from but similar to those that introduce unsaturations in fatty acyl-CoAs. These enzymes use cytochrome b5 as a cofactor, NADH as a reductant, and O2 as a terminal electron acceptor.

Platelet Activating Factor

A particularly interesting ether phospholipid with unusual physiological properties has recently been characterized. As shown in Figure 25.24, 1-alkyl-2-acetyl- glycerophosphocholine, also known as platelet activating factor, possesses an alkyl ether at C-1 and an acetyl group at C-2. The very short chain at C-2 makes this molecule much more water-soluble than typical glycerolipids. Platelet activating factor displays a dramatic ability to dilate blood vessels (and thus reduce blood pressure in hypertensive animals) and to aggregate platelets.

Sphingolipid Biosynthesis

Sphingolipids, ubiquitous components of eukaryotic cell membranes, are present at high levels in neural tissues. The myelin sheath that insulates nerve axons is particularly rich in sphingomyelin and other related lipids. Prokaryotic organisms normally do not contain sphingolipids. Sphingolipids are built upon sphingosine backbones rather than glycerol. The initial reaction, which involves condensation of serine and palmitoyl-CoA with release of bicarbonate, is catalyzed by 3-ketosphinganine synthase, a PLP-dependent enzyme (Figure 25.25). Reduction of the ketone product to form sphinganine is catalyzed by 3-keto- sphinganine reductase, with NADPH as a reactant. In the next step, sphinganine is acylated to form N-acyl sphinganine, which is then desaturated to form ceramide. Sphingosine itself does not appear to be an intermediate in this pathway in mammals.

RCH2CH2

O

CH2

HO

C

H

O

+

CH2

O

P

O

CH2CH2N(CH3)3

O–

1-Alkyl-2-lysophosphatidylcholine

O

O–

O

CH3C

CH3C

CoA

Acetylhydrolase

Acetyl-CoA: 1-alkyl-2-lysoglycero-

phosphocholine transferase

CoA

H2O

RCH2CH2

O

CH2

O

CH3C

C

H

O

+

O

CH2

O

P

O

CH2CH2N(CH3)3

O–

1-Alkyl-2-acetylglycerophosphocholine (platelet activating factor, PAF)

FIGURE 25.25

m e

25.2 ● Biosynthesis of Complex Lipids

827

H

O

–OOC

CH2OH

C

CH3(CH2)14

C

S

CoA

+NH3

Palmitoyl-CoA

Serine

3-Ketosphinganine

H2O

synthase

CoASH

HCO3–

O

H

CH2OH

CH3(CH2)14

C

C

+NH3

2S-3-Ketosphinganine

NADPH + H+

3-Ketosphinganine

reductase

NADP+

OH

H

CH2OH

CH3(CH2)14

C

C

H

+NH3

2S,3R-Sphinganine

Acyl-CoA

CoA

N -acyl-sphinganine

X

XH2

H

OH H

CH2OH

CH3(CH2)12

C

C

C

C

H

H

NH

C

O

(CH2)n

CH3

Ceramide

● Biosynthesis of sphingolipids in animals begins with the 3-ketosphinga- nine synthase reaction, a PLP-dependent condensation of palmitoyl-CoA and serine. Subsequent reduction of the keto group, acylation, and desaturation (via reduction of an electron acceptor, X) form ceramide, the precursor of other sphingolipids.

828

Chapter 25 ●

Lipid Biosynthesis

H

CH3

(CH2)12

C

C

CHOH

H

H

R1

C

N

CH

CH3

H

Phosphatidyl-

CH3

H

O

CH2

UDP-

Gal

(CH2)12

C

C

CHOH

(CH2)12

C

C

CHOH

choline

CH2OH

H

H

H

H

R1

C

N

CH

R1

C

N

C

HO

O

O

OH

UDP

O

CH2OH

1,2-Diacylglycerol

O

CH2

Ceramide

O

OH

UDPgalacto-

UDP-

Glu

Sphingomyelin

syltransferase

–O

O

β -D-Galactosylceramide

UDP

UDP-

Gal

P

UDP

O

CH2OH

CH2

CH2OH

O

H

CH2

O

H

O

CH2

O

HO

OH

H

O

HC

N

C

R1

H3C N CH3

H

H

OH

H

OH

H

H

OH

CH3

H

OH

H

HC

H

R2

β -D-Galactosyl-(1

4)-β -D-glucosylceramide

UDP-

N-Acetylgalactosamine

UDP

N-Acetylgalactosaminyltransferase

CH2OH

CH2OH

HO

H

O

O

CH2OH

H

O

O

CH2

O

H

OH

H

O

O

OH

H

H

H

H

H

HC

N

C

R1

H

NH

OH

H

H

OH

H

OH

H

H

HC

CH3

C

O

H

OH

R2

β-D-N-Acetylgalactosamine-(1 4)-

β-D-galactosyl-(1 4)-β -D-glucosylceramide

CH3

CH2OH

CMP-

sialic acid

O

C H HCOH

COO–

Sialytransferase

O

HN

COH

CMP

H H

H

H

OH

H

CH2OH

CH2OH

CH2OH

H

O

O

O

H

O

CH2

O

HO

OH

H

O

O

HC

N

C

R1

H

H

H

OH

H

O

H

H

OH

H

H

H H

H

OH

HC

H

NH

H

OH

R2

CH3

C

O

Ganglioside GM2

FIGURE 25.26 ● Glycosylceramides (such as galactosylceramide), gangliosides, and sphingomyelins are synthesized from ceramide in animals.

25.3 ● Eicosanoid Biosynthesis and Function

829

Ceramide Is the Precursor for Other

Sphingolipids and Cerebrosides

Ceramide is the building block for all other sphingolipids. Sphingomyelin, for example, is produced by transfer of phosphocholine from phosphatidylcholine (Figure 25.26). Glycosylation of ceramide by sugar nucleotides yields cerebrosides, such as galactosylceramide, which makes up about 15% of the lipids of myelin sheath structures. Cerebrosides that contain one or more sialic acid (N-acetylneuraminic acid) moieties are called gangliosides. Several dozen gangliosides have been characterized, and the general form of the biosynthetic pathway is illustrated for the case of ganglioside GM2 (Figure 25.26). Sugar units are added to the developing ganglioside from nucleotide derivatives, including UDP-N-acetylglucosamine, UDP-galactose, and UDP-glucose.

25.3 ● Eicosanoid Biosynthesis and Function

Eicosanoids, so named because they are all derived from 20-carbon fatty acids, are ubiquitous breakdown products of phospholipids. In response to appropriate stimuli, cells activate the breakdown of selected phospholipids (Figure 25.27). Phospholipase A2 (Chapter 8) selectively cleaves fatty acids from the C-2 position of phospholipids. Often these are unsaturated fatty acids, among which is arachidonic acid. Arachidonic acid may also be released from phospholipids by the combined actions of phospholipase C (which yields diacylglycerols) and diacylglycerol lipase (which releases fatty acids).

Eicosanoids Are Local Hormones

Animal cells can modify arachidonic acid and other polyunsaturated fatty acids, in processes often involving cyclization and oxygenation, to produce so-called local hormones that (1) exert their effects at very low concentrations and

(2) usually act near their sites of synthesis. These substances include the prostaglandins (PG) (Figure 25.27) as well as thromboxanes (Tx), leukotrienes, and other hydroxyeicosanoic acids. Thromboxanes, discovered in blood platelets (thrombocytes), are cyclic ethers (TxB2 is actually a hemiacetal; see Figure 25.27) with a hydroxyl group at C-15.

Prostaglandins Are Formed from Arachidonate by

Oxidation and Cyclization

All prostaglandins are cyclopentanoic acids derived from arachidonic acid. The biosynthesis of prostaglandins is initiated by an enzyme associated with the endoplasmic reticulum, called prostaglandin endoperoxide synthase, also known as cyclooxygenase. The enzyme catalyzes simultaneous oxidation and cyclization of arachidonic acid. The enzyme is viewed as having two distinct activities, cyclooxygenase and peroxidase, as shown in Figure 25.28.

A Variety of Stimuli Trigger Arachidonate Release

and Eicosanoid Synthesis

The release of arachidonate and the synthesis or interconversion of eicosanoids can be initiated by a variety of stimuli, including histamine, hormones such as epinephrine and bradykinin, proteases such as thrombin, and even serum albumin. An important mechanism of arachidonate release and eicosanoid syn-

FIGURE 25.27

830 Chapter 25 ● Lipid Biosynthesis

Hormone

(or other

Receptor

stimulus)

	Activation of	Activation of
	PLC and	PLA2
	diacylglycerol
	lipase				Phospholipids
	Arachidonate
	2 O2
	O
		COO–	H
		HO
	O				COO–
	OH
		H	S CH2
	PGH2				N CH2COO–
			CH	C
HO		Leukotriene C		O	H
	COO–		NH	C	CH2CH2	CH
				O	H+N	COO–
					3
O	OH	H
	PGD2					COO–
	O
	COO–	HO	O
					OH
	HO			TXB2
		–OOC
	OH
	PGE2	O
	OH
		COO–
	HO	HO	OH
	OH		PGI2

● Arachidonic acid, derived from breakdown of phospholipids (PL), is the precursor of prostaglandins, thromboxanes, and leukotrienes. The letters used to name the prostaglandins are assigned on the basis of similarities in structure and physical properties. The class denoted PGE, for example, consists of -hydroxyketones that are soluble in ether, whereas PGF denotes 1,3-diols that are soluble in phosphate buffer. PGA denotes prostaglandins possessing , -unsaturated ketones. The number following the letters refers to the number of carbon–carbon double bonds. Thus, PGE2 contains two double bonds.

PGF2a

thesis involves tissue injury and inflammation. When tissue damage or injury occurs, special inflammatory cells, monocytes and neutrophils, invade the injured tissue and interact with the resident cells (e.g., smooth muscle cells and fibroblasts). This interaction typically leads to arachidonate release and eicosanoid synthesis. Examples of tissue injury in which eicosanoid synthesis has been characterized include heart attack (myocardial infarction), rheumatoid arthritis, and ulcerative colitis.