Chapter 27: Biosynthesis of Membrane Lipids and Steroids

Biosynthesis of Membrane Lipids and Steroids

Phosphatidate: A Key Intermediate

  • Phosphatidate (diacylglycerol 3-phosphate) is a common intermediate in the synthesis of both phospholipids and triacylglycerols.
  • In mammals, phosphatidate synthesis occurs in the endoplasmic reticulum (ER) and the outer mitochondrial membrane.
  • Pathways involving phosphatidate:
    • Gluconeogenesis (Chapter 16)
    • Triacylglycerol breakdown (Chapter 22)
    • Triacylglycerol synthesis (Chapter 27)
    • Phospholipid synthesis (Chapter 27)
  • For phospholipid synthesis, either phosphatidate or the alcohol group must be activated by reaction with an NTP (nucleoside triphosphate).

Formation of Phosphatidate

  • Two fatty acids are added to glycerol-3-phosphate to yield phosphatidate.
  • This process requires two acyl coenzyme A molecules.
  • The reaction is catalyzed by glycerol phosphate acyltransferase.

Synthesis from Diacylglycerol

  • Phosphatidate can also be synthesized from diacylglycerol (DAG) in a salvage pathway.
  • This reaction is catalyzed by diacylglycerol kinase.
  • Reaction: Diacylglycerol + ATP → phosphatidate + ADP

Triacylglycerol Synthesis

  • Primarily occurs in the liver.
  • Phosphatidic acid phosphatase hydrolyzes phosphatidate, yielding diacylglycerol (DAG).
  • Diglyceride acyltransferase acetylates DAG to form triacylglycerol.
  • Both enzymes are associated in the triacylglycerol synthetase complex, bound to the ER membrane.

Phospholipid Synthesis: Activated Intermediates

  • Occurs in the ER and Golgi apparatus.
  • Requires phosphatidate to be combined with an alcohol; one component must be activated.

Formation of CDP-Diacylglycerol

  • In some phospholipid synthesis pathways, phosphatidate reacts with cytidine triphosphate (CTP) to form cytidine diphosphodiacylglycerol (CDP-diacylglycerol).

Formation of a Phosphodiester Linkage

  • CDP-diacylglycerol reacts with the hydroxyl group of an alcohol to form a phosphodiester linkage.
  • If the alcohol is inositol, the products are phosphatidylinositol and CMP.

Formation of Diphosphatidylglycerol (Cardiolipin)

  • Diphosphatidylglycerol (cardiolipin) plays a role in the organization of proteins participating in oxidative phosphorylation.
  • Abundant in the inner mitochondrial membrane.
  • If CDP-diacylglycerol reacts with phosphatidylglycerol, the products are cardiolipin and CMP.

Synthesis from Activated Alcohol

  • Phosphatidylethanolamine is the major phospholipid of the inner leaflet of cell membranes.
  • The alcohol ethanolamine is activated by phosphorylation and reacts with CTP to form CDP- ethanolamine.
  • The phosphorylethanolamine unit of CDP-ethanolamine is transferred to DAG to form phosphatidylethanolamine.

Phosphatidylcholine Synthesis

  • Phosphatidylcholine is the most common phospholipid in mammals, making up 50% of membrane mass.
  • CTP-phosphocholine cytidylyltransferase (CCT) catalyzes the formation of CDP-choline, the rate-limiting step in phosphatidylcholine synthesis, from dietary choline. Its regulator ligand is the membrane itself.

Phosphatidylcholine Synthesis When Dietary Choline Is Insufficient

  • Phosphatidylethanolamine methyltransferase is a liver enzyme that synthesizes phosphatidylcholine from phosphatidylethanolamine.
  • The amino group of phosphatidylethanolamine is methylated three times, with S-adenosylmethionine as the methyl donor.

Base-Exchange Reactions

  • Phosphatidylserine attracts phagocytes to consume cell remnants during apoptosis and constitutes ≈10% of phospholipids in mammals.
  • Normally located in the inner leaflet, it moves to the outer leaflet during apoptosis via an ATP-bind cassette translocase.
  • Synthesized by base-exchange reactions:
    • Phosphatidylcholine + serine → choline + phosphatidylserine
    • Phosphatidylethanolamine + serine → ethanolamine + phosphatidylserine

Sphingolipid Synthesis

  • Sphingolipids are membrane lipids with a sphingosine backbone found in all eukaryotic cells, highly concentrated in the central nervous system.
  • Ceramide is a lipid consisting of a fatty acid chain attached to the amino group of a sphingosine backbone, and it's the initial product of sphingolipid synthesis.

Sphingomyelin and Cerebroside

  • In sphingomyelin, the substituent is phosphorylcholine, derived from phosphatidylcholine.
  • In cerebroside, the substituent is glucose or galactose, with UDP-glucose or UDP-galactose as the sugar donor.

Gangliosides

  • Gangliosides are the most complex sphingolipids, with an oligosaccharide chain linked to the terminal hydroxyl group of ceramide by a glucose residue.
  • The oligosaccharide chain contains at least one acidic sugar or sialic acid (N-acetylneuraminate or N-glycolylneuraminate).
  • The step-by-step addition of sugar residues to ceramide requires activated sugars and the CMP derivative of N-acetylneuraminate.

Tay-Sachs Disease

  • A genetic disease caused by an inability to degrade gangliosides.
  • Results in neurons swollen with lipid-filled lysosomes and elevated brain ganglioside content.
  • Can be identified by genetic testing.
  • Affected infants display weakness and limited psychomotor skills and usually die before age 3.

Phosphatidic Acid Phosphatase (PAP or Lipin 1)

  • Controls the extent to which triacylglycerols are synthesized relative to phospholipids and regulates the type of phospholipid synthesized.
  • High activity: phosphatidate is dephosphorylated, producing DAG and ultimately phospholipids or triacylglycerols.
  • Low activity: phosphatidate is used as a precursor for different phospholipids and acts as a signaling molecule and cofactor.

Cholesterol Synthesis

  • Cholesterol is a steroid that modulates the fluidity of animal cell membranes and is the precursor of steroid hormones.
  • The 27 carbon atoms of cholesterol are derived from acetyl CoA in a three-stage synthetic process.
    • Stage 1 occurs in the cytoplasm.
    • Stages 2 and 3 occur in the endoplasmic reticulum.

Regulation of Cholesterol Biosynthesis

  • Cholesterol can be obtained from the diet or synthesized de novo.
  • The liver and intestine are the primary sites of cholesterol biosynthesis.
  • The rate of cholesterol formation is mediated primarily by changes in the amount and activity of HMG CoA reductase, which catalyzes the synthesis of mevalonate, an intermediate in the cholesterol synthesis pathway.

SREBP

  • Sterol regulatory element binding protein (SREBP) is a transcription factor that regulates the proteins required for lipid synthesis.
  • Binds to the sterol regulatory element (SRE) on the reductase gene when cholesterol levels are low.
  • Resides in the ER membrane when inactive.
  • SREBP cleavage activating protein (SCAP) is an integral membrane protein that acts as a cholesterol sensor and associates with inactive SREBP in the ER membrane.

Activation of SREBP

  • When cholesterol levels fall, SCAP escorts SREBP to the Golgi complex, where it is cleaved to release SREBP from SCAP and the membrane.
  • SCAP migrates to the nucleus and binds the SRE of the HMG-CoA reductase gene and other genes in the cholesterol biosynthetic pathway, enhancing transcription.

Deactivation of SREBP

  • When cholesterol levels rise:
    • The proteolytic release of SREBP is blocked.
    • The SREBP in the nucleus is rapidly degraded by proteasomes located in the nucleus.
  • These events halt the transcription of genes of the cholesterol biosynthetic pathways.

Insig

  • When cholesterol is present, SCAP binds cholesterol, causing a structural change that allows it to bind to the ER protein Insig (insulin-induced gene).
  • Insig anchors SCAP and SREBP in the ER in the presence of cholesterol, preventing inappropriate movement of SCAP-SREBP to the Golgi complex.

Degradation of Reductase

  • Increasing concentrations of sterols induce structural changes in the membrane domain of the reductase.
  • Under these conditions, the reductase binds to a subset of Insigs associated with ubiquitinating enzymes.
  • Polyubiquitination occurs, and the enzyme is extracted from the membrane and degraded.

Additional Regulatory Strategies

  • Nonsterol metabolites derived from mevalonate inhibit the rate of translation of reductase mRNA, resulting in an 80% reduction in the rate of reductase protein production.
  • Phosphorylation by an AMP-activated protein kinase decreases the activity of the reductase, causing cholesterol synthesis to cease when ATP levels are low.

Lipoproteins

  • Lipoprotein particles are the means by which cholesterol and triacylglycerols are transported in body fluids to tissues for use as fuel or storage.
  • Fatty acid constituents of triacylglycerol components are incorporated into phospholipids for membrane synthesis.
  • Cholesterol is a vital component of membranes and is a precursor to steroid hormones.
  • Cells cannot degrade the steroid nucleus, so cholesterol must be used or excreted by the liver.

Lipoprotein Particle Composition

  • A core of hydrophobic lipids.
  • A shell of more-polar lipids and proteins.
  • Protein components (apoproteins) have two roles:
    • Solubilizing hydrophobic lipids.
    • Containing cell-targeting signals.

Chylomicrons

  • Very low-density particles with ≈90% triacylglycerols.
  • Apolipoprotein B-48 (apo B-48) is a large protein that forms an amphipathic spherical shell around the fat globule.
  • Lipoprotein lipases hydrolyze triacylglycerols in chylomicrons for release; they are located on the lining of blood vessels in muscle and other tissues.

Very Low-Density Lipoprotein (VLDL)

  • Particles that export triacylglycerols and cholesterol in excess of the liver’s own needs into the blood.
  • Stabilized by two apolipoproteins: apo B-100 and apo E.

Intermediate-Density Lipoprotein (IDL)

  • Particles formed after lipases hydrolyze triacylglycerols from VLDL and chylomicrons.
  • Rich in cholesteryl esters.
  • Fates:
    • Taken up by the liver for processing.
    • Converted into low-density lipoprotein by the removal of more triacylglycerol.

Low-Density Lipoprotein (LDL)

  • Major carrier of cholesterol in blood, regulates de novo cholesterol synthesis at peripheral tissues.
  • Contains a core of ≈1500 cholesterol molecules esterified to fatty acids (most commonly linoleate).
  • Contains a shell of phospholipids, unesterified cholesterol, and a single copy of apo B-100, which is recognized by target cells.

High-Density Lipoprotein (HDL)

  • Particles that pick up cholesterol released into the plasma and deliver it to the liver for excretion.
  • An acyltransferase in HDL esterifies the cholesterols.

LDL and Cholesterol Metabolism

  • Cells outside the liver and intestine obtain cholesterol from LDL in the plasma rather than synthesizing it de novo.
  • The process of LDL uptake is called receptor-mediated endocytosis.

Familial Hypercholesterolemia

  • A genetic disease resulting from the absence or deficiency of functional LDL receptors, leading to elevated cholesterol and LDL in blood plasma.
  • Excess cholesterol collects in various tissues of the body.
  • Excess LDL becomes oxidized and taken up by macrophages, which become engorged and form foam cells.
  • Foam cells become trapped in blood vessels and contribute to the formation of atherosclerotic plaques.

LDL Receptor Mutations

  • The human LDL receptor is a multidomain membrane-spanning glycoprotein.
  • Receptor exists in two states:
    • An open (extended) state that binds LDL.
    • A closed state that releases LDL.
  • The conversion from open to closed state takes place on exposure to the acidic environment of the endosome.
  • Mutations that disrupt the interconversion of the closed and open states are responsible for many cases of familial hypercholesterolemia.

Regulation of LDL Receptor Cycling

  • PCSK9 is a protease that plays a crucial role in regulating LDL receptor cycling and is secreted by the liver.
  • PCSK9 binds to the LDL receptor, locking it in the open conformation.
  • In the open conformation, the LDL receptor is degraded in the lysosome along with its LDL cargo.

Treatment of High LDL Levels

  • Reduced levels of PCSK9 allow more receptor cycling and more efficient removal of LDL from the blood.
  • Several monoclonal antibodies to PCSK9 have been approved for treatment for high LDL levels, though their use has been limited by their high cost.

HDL and Atherosclerosis

  • In reverse cholesterol transport, HDL removes cholesterol from cells, especially macrophages, and returns it to the liver for excretion.
  • When transport fails, macrophages become foam cells and facilitate the formation of plaques.
  • The more HDL, the more readily this transport takes place and the less likely that the macrophages will develop into foam cells.

Clinical Management of Cholesterol Levels

  • The goal is to increase the number of LDL receptors to reduce blood cholesterol levels, using a two-pronged approach:
    • Reabsorption of bile salts from the intestine is inhibited by polymers that bind bile salts.
    • De novo synthesis of cholesterol is blocked by statins, competitive inhibitors of HMG-CoA reductase.

Cholesterol and Isoprene

  • Bile salts are the major breakdown products of cholesterol.
  • Bile salts are synthesized in the liver, stored and concentrated in the gallbladder, and released into the small intestine.
  • Bile salts effectively solubilize dietary lipids because they contain both polar and nonpolar regions.
  • Solubilization increases the effective surface area of lipids, maximizing exposure to lipases and leading to more ready absorption by the intestine.

Steroid Hormones

  • Different classes of steroid hormones are derived from cholesterol.

Vitamin D

  • Vitamin D3 is formed from cholesterol in a series of steps, one of which requires ring-splitting by ultraviolet light.
  • Vitamin D3 is converted to the hormone calcitriol, the active form of vitamin D, by hydroxylation reactions in the liver and kidneys.
  • Vitamin D3 binds to receptors to form complexes that function as transcription factors, regulating gene expression.

Vitamin D Deficiency

  • Rickets is a disease characterized by inadequate calcification of cartilage and bone, caused by inadequate intake of vitamin D or insufficient photolysis of 7-dehydrocholesterol to previtamin D3.
  • Today, the most reliable dietary sources of vitamin D are fortified foods or supplements.