Fatty ACid Synthesis, Elongation, desaturation and regulation

Primary location and mechanism for FAS elongation

• Smooth ER membrane, cytosolic-facing leaflet, has the elongation complex

FAS Features

• Only produces palmitate C16:0, chain length diversity is from other parts of ER system

• Acyl chain is lodged into the ER membrane with embeedded membrane proteins

  • Only CoA headgroup is exposed to cytosol

• Produces long-chain FAS for membrane, sphingolipids and signaling

Er vs Mitochondrial contributions

• Er elongation

  • Located in smoot Er, cytoplasmic face

  • Substrates, Long-chain acyl-Coas + malonyl-CoA

  • Purpose, Membrane/signaling in lipids

  • Enzymes, 4 separation enzymes

• Mitochondrial elongation

  • Mitochondrial matrix

  • Acyl-ACPs, shorter chains

  • Minor role in mammals

  • Reversal of B-oxidation enzymes

Mitochondrial contributions

• FAS only runs when NADPH is plentiful,

•  most NADPH coms from PPP and from citrate export out of mitochondria

• Boots cytosolic NADPH production

Summary of steps

1. Condensation: Long-chain acyl-CoA + malonyl-CoA → β-ketoacyl-CoA (+ CO₂).

2. β-Keto reduction: β-ketoacyl-CoA + NADPH → β-hydroxyacyl-CoA.

3. Dehydration: β-hydroxyacyl-CoA → trans-Δ²-enoyl-CoA.

4. Enoyl reduction: trans-Δ²-enoyl-CoA + NADPH → elongated acyl-CoA (+2C)

FA desaturation in Er

• Uses mixed-function oxidases = acyl-CoA desaturases

  • Uses O2 in redox chemistry

  • O2 is an electron acceptor without incorporating directly into product

  • Catalyze 4 electron reaction

    • Remove 2 electrons as 2H from fatty acid to forma double bond

    • Receive 2 electron from NADH/NADPH

    • All 4 transfer to O2, fully reducing it to 2 molecules of H20 while creating a double bond

 

Substrates to reduce O2

• Fatty acyl-CoA being desaturated

• NADPH, provides electrons by cytochrome b5 reductase and cytochrome b5

  • Cytb5 is regenerated by NADPH through cytb5 reductase, uses FAD for single electron transfers

  • Each desat cycle couples substrate dehydrogenation with Cytb6 oxidation to deliver the 4 electrons needed to reduce O2 into H2O

Enzyme characteristics of Fatty acyl-CoA desaturases

• ER membrane, cytosolic facing active sites

• Introduce position specific cis double bonds, far from carboxyl and not conjugated with carbonyl

  • Only act at specific double bonds

3 component electron transport chain

Uses separates membrane proteins and are transient interactions

1. Cytb5 reductase oxidize: NADPH donates 2 electrons by FAD > creates FADH2

   1. Regenerates Cytochrome b5 reduced

2. Cytb5, 2 electrons transfer to Fe3+ creating Fe2+ in cytochrome b5 to desaturase

3. Desaturase, Di-iron centre removes 2 H from FA and receives 2 electrons from NADPH, transfer 4 electrons total to O2

Positions human can desaturate

• Palmitate (16:0) desaturated to palmitoleic acid (16:1 cis-9)

• Stearate (18:0) desaturated to oleic acid (18:1, cis-9)

  • Both are at the C9 position

• C6/C5, process essential Fas into longer PUFAs (eicosanoid precursor)

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  Key characteristics

  • Introduce cis double bonds at positions remove from carboxyl end (no conjugated with carbonyl)

  • Mixed function oxidases requires cytochrome b5 electron transfer chain

FA desaturations: Positions, species differences and essential fats

Positions

• Occurs are positions remote from carboxyl end like 5,6,7

• Double bonds aren't conjugated with acyl carbonyl

• Plants, algae, microbes and fish but not humans, can introduce double bonds farther from the carboxyl end of a FA like C12/C15 (omega 6/3)

In plants

• Oleate (18:1, cis-9) desaturation at C12 = linoleate 18:2 (cis-9,12)

• Desaturation at C15 creates alpha-linoleate 18:3(cis9, 12, 15), omega 3

• Desaturation at C6 creates gamma-linoleate 18:3 (cis 6,9, 12)

Essential fatty acids

• Required in diet due to missing desaturases

  • Linoleic acid omega 6 family (18:2 cis-9,12)

  • Alpha linolenic acid omega 3 family (18:3, cis-9,12,15)

• Deficiency consequences: impaired growth and physiological defects

 

Why dietary intake is essential

• Human desaturases act at fixed position, we cannot create double bonds at the omega end

• Eicosanoid Biosynthesis requires omega 3/6 precursors

  • No linoleic/alpha-linoleic= no eicosanoids = impaired inflammation/immunity

Eicosanoids: Balancing inflammation and vascular health from omega 6/omega3 fatty acids

• When omega 6/omega 3 fatty acids are elongated to 20 carbons they become precursors to eicosanoids

  • Eicosanoids is a major class of signaling molecules

Derivatives of Eicosanoids

• Thromboxane (TX)

• Prostaglandins (PG)

• Leukotriene (LT)

• Play a role in inflammation, vascular tone and immune response)

 

Omega 6/3 produce eicosanoids with opposite effects

• Omega 6 derive

  • Drive inflammation and clotting

  • Western diets are heavy with this, tips balance toward chronic inflammation, heart disease and arthritis

• Omega 3 derived

  • Calm inflammation and protect tissues

Synthesis of Eicosanoids, omega 6 pathways

1. Linoleate from diet

2. Desaturate at C6

3. Elongate to 20, double bonds are position 2 bonds later (6,9,12 > 8,11,14)

4. Desaturate at C5, Arachidonate

Synthesis of Eicosanoids, omega 3 pathways

1. Linoleate from diet

2. Desaturate at C15, alpha-Linolenate 18:3  cis 9, 12, 15

3. Elongate and desaturate C5/C8, produce Eicosapentaenoic acid, EPA, 20:5 (5,8,11,14,17)

4. Elongate and desaturate at C4 produces Docosahexaenoic acid DHA, 22:6 (4,7, 10, 13, 16, 19)

Eicosanoids: local signal for inflammation, immunity and homeostasis

• Arachidonic acid (20:4 cis 5,8, 11, 14; omega 6), produce when desaturases inserts a doble bond into linoleic acid, building its 4 characteristic double bonds

• Eicosanoids are signalling molecules device from 20C PUFM primarily from arachidonic acid (omega 6) and EPA/DHA (omega 3)

• They are released from membrane phospholipids by phospholipase A2 in response to stimuli like injury, infection or hormone

• Central roles in inflammation, immunity vascular functions and tissue repair

  • Critical local mediators that maintain homeostasis

  • Coordinate body's response to stress or injury  

Eicosanoid Classes

Prostaglandins

• Regulate inflammation, pain, fever and vascular tone

Thromboxane

• Control platelet aggregation and blood clotting

Leukotrienes

• Mediate allergic reaction, immune cell recruitment and inflammation

Lipoxins and resolvins

• Derived from omega-3 FAs, promote resolution of inflammation

Aspirin blocks COX-mediated Eicosanoid Production

Salicin

• Secondary metabolite produce in willow bark

• Used as pain reliver by indigenous communities

• Converts to active form salicylic acid in body

 

Aspirin

• Synthetic derivative of salicylic acid, aspirin, was used during the flu pandemic

• Clinical study for preventing heart disease

• Non-steroidal anti-inflammatory drug (NSAID)

• Irreversibly acetylates the active-site serine of cyclooxygenase (COX-1 and COX-2), blocks prostaglandin and thromxoane synthesis

• Reduce inflammation, pain and prevent thromboxane-dependent clot formation

Hormonal conditions

• Insulin

  • Increase synthesis of FAs, decrease b-oxidation

  • ATP surplus, energy is abundant, want to store energy

• Glucagon/epinephrine

  • Decrease synthesis, increase b-oxidation

  • ATP deficient, fasting or stress, need to break down storage

Main Regulatory: Acetyl-CoA Carboxylase

Rate limiting/committed step for synthesis: acetyl-CoA to malonyl-CoA

• Activated by citrate and insulin = synthesis

• Inhibited by palmitoyl-CoA and AMP activated kinase (AMPK) = b-oxidation

Malonyl-CoA: Dual role

FA synthesis substrate and beta-oxidation inhibitor

FA degradation Regulator: Carnitine palmitoyltransferase-1 (CPT-1)

• Control mitochondrial entry of FA

• Inhibited by malonyl-CoA

• Links synthesis to suppression of beta oxidation

Regulation of ACC

• ACC in eukaryotes exists as dimer and polymerize into filamentous structures = active form

• Equilibrium between inactive monomers and active filaments is regulated by metabolites and hormones

• Structural Regulation

  • Palmitoyl-CoA: shifts ACC toward inactive monomeric state

    • Feedback inhibition, product inhibit upstream enzyme

  • Citrate: allosteric activator

    • Promote filament formation, stimulate enzyme activity when energy and substrates are abundant

• Hormonal

  • Glucagon/epinephrine cause AMP-activated protein kinase (AMPK), phosphorylates ACC ensuring fatty acid synthesis is suppressed during fasting or stress when energy is mobilised

  • Phosphorylated = inactive, Dephosphorylated = active

• Cross-Pathway coupling with Malonyl-CoA

  • High citrate = energy abundance = ACC active = increase malonyl-CoA

  • ACC active inhibits CPT-1 = blocks FA entry into mitochondria = lower b-oxidation

• Acetyl-CoA has many fates, when it goes to ACC it is committed to FA synthesis

• Once FA synthase release Palmitate it get CoA-tagger for packaging and also inhibits ACC

Integrated Regulatory Network Fed States

Fed state (synthesis ON, Oxidation OFF)

• Insulin increase, increase glucose uptake, increase glycolysis = pyruvate > acetyl-CoA > increase in citrate

  • Citrate actives ACC > malonyl-CoA increase

    • Malonyl-CoA increase FA synthesis and inhibit CPT-1, decrease beta-oxidation

      • Inhibition of CPT-1 will prevent acyl-carnitine and therefore less acetyl-CoA in matrix for beta-oxidation

• Carnitine acyl-transferase 1 is a step unique to FA catabolism

  • Malonyl-CoA inhibits this

  • Favour anabolism over catabolism

Integrated Relay network Fasted State

• Glucagon/epinephrine > AMPK active > phosphorylate and inactive ACC

• Malonyl-CoA decrease, CPT-1 uninhibited, beta-oxidation increase

When fat stores are full

• Decrease FA synthesis

• Increase b-oxidation

• Palmitoyl-CoA inhibits ACC

Upstream control pathways

Glucogenic pathway:

Glucose ──► PFK-1 ──► Pyruvate ──► PDH ──► Acetyl-CoA ──► ACC ──► Malonyl-CoA

     ↑ Insulin stimulates all steps

Hormonal integration:

• Insulin: ↑ Glucose uptake, ↑ PDH, ↑ ACC (dephosphorylation) • Glucagon: ↑ AMPK → ↓ ACC

Metabolic wisdom

• Malonyl-CoA solve the futile cycle problem

  • When building fat = don't burn fat (CPT-1 blocked)

  • When burning fat = don't build fat (ACC inactive)

• High insulin + citrate = synthesis dominant

• High glucagon + low malonyl-CoA = oxidation dominant

 

Summary of How cells switch on and off fatty acid synthesis

On

• Insulin drives fatty acid synthesis by boosting glucose uptake = push glycolysis forward and feed PDH to generate the acetyl-CoA for lipogenesis

• Fatty acyl-CoA accumulates, it feeds back to block ACC polymerization

• This stops fatty acid synthesis when fat stores are sufficient

 

Off

• ACC commits acetyl-CoA to fatty acid synthesis by converting it to malonyl-CoA

• Malonyl-COA shuts down beta-oxidation by inhibiting CPT-1, ensures newly made fats are immediately burned

• Citrates actives ACC, signals high energy and abundant carbon to ramp up lipogenesis

 

FA synthesis

• Runs only when NADPH is plentiful

• Most NADPH comes from PPP and from citrate export out of mitochondria and boosts cytosolic NADPH production