BCH 333 Chapter 26: Biosynthesis of Membrane Lipids and Steroids

Lipid

• largely nonpolar biomolecule that is non-water soluble, but soluble in organic solvent

• Defined by solubility, not a common structural feature → lipids are much more varied than protein, carbohydrate, and nucleic acid structures

Triacylglycerols (TAGs)

• fat storage

• Glycerol esterified to three fatty acids

• Synthesized in the ER

• Need to know general structure (be able to draw)

Phospholipids

• share a common synthetic intermediate with TAGs

• Cell membrane component

• 2 nonpolar/hydrophobic tails and a polar/hydrophilic head (amphipathic)

Sphingolipids

• cell membrane component

• Similar structure to phospholipids, but based on a sphingosine backbone instead of glycerol

• Found in membranes and lipid rafts

Cholesterol

• cell membrane component

• Moderates cell fluidity

• Steroid—4 fused ring system base is cholesterol

Bile salts

• lipid solubilizing agents

• Steroid 4 fused ring system base

Steroid hormones

• signaling molecules

• 4 fused ring system base

Terpenes

• built from isopentenyl or isoprene units linked together

• Monoterpene = 10C

• Sesquiterpene = 15C

  • Sesqui = 1.5; originally thought it was fundamental units of 10, not 5 carbons when the terminology was established

• Diterpene = 20 C

• Triterpene = 30C

Phosphatidate

• common intermediate for TAG and phospholipid synthesis

• G3P + RCO-CoA → CoA + acylated intermediate

  • G3P from reduction of DHAP or phosphorylation of glycerol

• Acylated intermediate + RCO—CoA → CoA + phsophatidate (Need to know name and recognize structure)

• R’s can vary

• Mechanism is nucleophilic acyl substitution; OH from G3P attacks carbonyl carbon of RCO-CoA, tetrahedral intermediate forms and collapses, SCoA leaves → ester linkages to glycerol

TAG synthesis

• phosphatidate + H2O → diacylglycerol (DAG) + Pi

  • Phosphatidic acid phosphatase: key regulatory enzyme in lipid metabolism; “commitment step” to TAG synthesis

• DAG + RCO-CoA → CoA + TAG (third round of acylation)

Phospholipid synthesis

• not covering in depth this year

• Either the alcohol of the phosphatidate or the DAG phosphate must be activated in order to react → alcohol is activated to be a better LG

• Phosphatidate → → phosphatidylinositol (polycalcohol; no O in the ring = not a carb/cyclized sugar)

• Phosphatidate → → phosphatidylethanolamine → -(3 SAM) → → phosphatidylcholine

  • Note: the use of methylating agent SAM to convert ethanolamine to choline; this is one of two different pathways to make phosphatidylcholine, the most common phospholipid in mammals

Sphingolipid synthesis

• sphingolipids, membrane lipids found in all eukaryotic cells, have a sphingosine backbone instead of a glycerol backbone

• Ceramide is the initial product of sphingolipid synthesis

• In all sphingolipids, the amino group of the ceramide is acylated; the terminal hydroxyl group of ceramide is substituted to form other sphingolipids

• sphingomyelin and cerebroside (activated sugars can be added to synthesize gangliosides)

Ceramide synthesis

• palmitoyl CoA + serine → CoA + CO2 + 3-ketosphinganine

  • CO2 from serine carboxylate group → C-C bond breaks, electrons attack palmitoyl CoA carbon C, displacing SCoA

  • Nucleophilic acyl substitution

• 3-ketosphinganine + NADPH → NADP+ + dihydrosphingosine

  • Reduction of the carbonyl group

• Dihydrosphingosine + RCO-CoA → CoA + dihydroceramide

  • Acylation of the amine (forms an amide group)

• Dihydroceramide + FAD → FADH2 + ceramide

  • Oxidation

  • Ceramide has one long hydrocarbon tail from the original palmitic acid (fatty acid); the other is from the acylation step that adds a new FA by an amide bond (rather than the ester bond seen in the phospholipids based on glycerol)

Sphingomyelin

• component of the myelin sheath surrounding nerve cells

• Derivative of ceramide in which phosphorylcholine is bound to the terminal hydroxyl group

• Ceramide + phosphatidylcholine → DAG + sphingomyelin

• Add stuff onto the OH of the ceramide

• Should know what choline looks like

Cerebroside

• also a component of myelin

• Is derived from ceramide by the attachment of glucose or galactose to the terminal hydroxyl group

• Ceramide + UDP-glucose → UDP + cerebroside

Cholesterol structure

• Not completely flat/planar; some puckering

• Note A, B, C, D ring system of steroids (derived from cholesterol)

• All 27 carbons are ultimately derived from acetyl CoA—based on the fact that acetyl CoA’s alpha hydrogens are a little more acidic than the typical hydrogen on C due to resonance stabilization from the O (enolate), meaning that the carbon has some nucleophilic character

Stages of cholesterol synthesis

• isopentyl pyrophosphate synthesis

• Condensation of six five-carbon units to generate squalene

• Cyclization and subsequent modification

Isopentyl pyrophosphate synthesis

• should know these steps, names, structures

• Enol/enolate of acetyl CoA (the carbon with some nucleophilic character due to resonance) attacks the acetoacetyl CoA ketone carbonyl to give the addition product; water hydrolyzes the thioester to release CoA

• Acetyl CoA + acetoacetyl CoA → CoA + 3-hydroxy-3-methylglutaryl CoA (HMG-CoA—need to know structure)

• HMG-CoA can follow one of two pathways:

  • Mitochondria: HMG-CoA → acetyl CoA + acetoacetate

  • Cytosol: HMG-CoA → mevalonate

• Formation of mevalonate (reduction) is the first committed step in cholesterol biosynthesis (→ highly regulated)

  • 1st NADH converts thioester to an aldehdye via nucleophilic acyl substitution/kicks out SCoA, 2nd NADH reduces carbonyl to primary alcohol via nucleophilic addition (acts as a hydride donor)

  • HMG-CoA + 2 H+ + 2 NADPH → CoA + 2 NADP+ + mevalonate

  • Catalyzed by HMG-CoA reductase

• Mevalonate + 3 ATP → 3 ADP + intermediate

• Intermediate → CO2 + Pi + 3-isopentyl pyrophosphate

  • Is a decarboxylation, which converts the 6C structure into a 5C structure

Condensation of six five-carbon units to generate squalene

• 3-isopentyl pyrophosphate will be isomerized by an isomerase enzyme to a new structure that can easily ionize to a resonance-stabilized cation (dimethylallyl pyrophosphate)

• Ionization

  • Allylic substrate → allylic carbocation + PPi

  • Loss of the phosphates; carbocation is resonance stabilized due to the allyl group/double bond

• Condensation

  • Allylic carbocation + 3-isopentyl pyrophosphate → intermediate

  • Double bond of 3-isopentyl pyrophosphate attacks carbocation carbon

• Elimination

  • Intermediate → H+ + geranyl pyrophosphate (10C)

  • Formation of double bond where cation is

• Process repeats with another isopentyl pyrophosphate adding onto the geranyl pyrophosphate in the same way → farnesyl pyrophosphate (15C)

• 2 farnesyl phosphate + NADPH → squalene + 2 PPi + NADP+ + H+

  • Combination of 2 15C units → 30C molecule

  • Need to be able to recognize squalene; don’t need to know intermediates, but should understand sequence of events

Cyclization and subsequent modification

• don’t need to draw

• Squalene is epoxidized into squalene epoxide

  • Requires aerobic conditions because the epoxide oxygen comes from molecular oxygen

• Once the epoxide is protonated, it is “opened” in a mechanism that forms the four rings, leaving a cation off of the D ring

• Concerted elimination and four methyl/hydride shifts result in lanosterol

• Lanosterol is converted into cholesterol in 19 additional steps

Regulation of cholesterol biosynthesis

• occurs at several levels

• Feedback inhibition is mediated primarily by changes in the amount and activity of HMG-CoA reductase (by controlling transcription of its gene, translation of the mRNA, degradation of the enzyme, and regulation by phosphorylation)

Lovastatin (mevacor)

• cholesterol drug that acts as a competitive inhibitor of HMG-CoA reductase

• Part of the structure looks super similar to HMG-CoA, so it competes

• Isolated from a mushroom

Important biochemicals synthesized from cholesterol and isoprene

• cholesterol is precursor to steroid hormones, vitamin D, and bile salts

• Bile slats are detergents that render dietary lipids more accessible for digestion by lipases and thus facilitate formation of the fatty acid products

• Bile salts are synthesized in the liver and stored in the gallbladder until secreted into the small intestine

• Synthesis of bile salts

  • Cholesterol → glycocholate or taurocholate

• 5-carbon isopentyl pyrophosphate is used to form a wide variety of compounds (isoprenoids), including C10 and C15 plant fragrances, natural rubber, the side chains of vitamin K, coenzyme Q10, and chlorophyll, as well as the pigments that give color to foods such as carrots and tomatoes (are formed in a pathway similar to squalene, excpet that C20 instead of C15 units are formed and condensed)

  • Some isoprenoids include myrcene, limonene, zingiberene, natural rubber, and menthol