Fatty acid Breakdown/Synthesis Flashcards
Where does Fatty acid breakdown/b-oxidation occur in the cell?
Where do they come from?
In the mitochondrion
Fatty Acids → Acetyl-CoA
Origins:
1. Uptaken into the cells from circulation in form of lipoproteins
2. Lipolysis (stored in lipid droplets inside the cell
3. Phospholipase A1 & A2 (minor)
What is Lang’s cycle?
Phospholipids → {Phospholipase A2} → Lysophospholipd (lose hydrocarbure chain)
*In most cases, FA are reused to go back to phospholipid, so doesn’t really release a FA for breakdown (b-oxidation)
How do Fatty acids get into the mitochondria?
Problem: Long chain of FA (>16C) can’t cross the inne mitochondrial membrane
- Activation by CoA via Acyl-CoA Synthetase
- high-energy
- Uses 1 ATP → reaction driven PPi hydrolysis
- Keeps FA inside the cell + graident to allow more FA in
- Acyl-CoA is bound to the outer mitochondrial membrane - Coupling to Carnitine vis Carnitine Palmitoyl Transferase I (CPT I) → RATE-LIMITING STEP
- CPT I on outer mito membrane facing cytosol
- Transfers Acyl from CoA → Acylcarnitine (high-energy) → makes Acylcarnitine - Transit towards mitochondrial matrix via Carnitine:acylcarnitine translocase
- Carnitine out
- Transported freely across the outer membrane, translocase found in the inner membrane - FA release into matrix via CPT II → regenerates Acyl-CoA inside the matrix
- CPT II located on matrix side of inner membrane
- Carnitine is recycled to the cytosol
- Acyl-CoA → b-oxidation → Acetyl-CoA
- CoA prevents going back in the cytosol
What is the rate limiting step of Fatty Acid breakdown?
How is it regulated?
Fatty acid breakdown is limited by substrate availability → entry of Fatty acids in the mitochondrion
Acyl-CoA → {CPT I} → Acylcarnitine → transport into mitochondrion
Inhibited by high [malonyl-CoA] in the cytosol:
- High energy state of the cell
- Stimulates biosynthesis of FA, must stop breaking down
- Binds in the same site of CPT I as Acyl-CoA
Steps:
1. CPT1 → Aceylcarnitine
2. Diffuses through outer membrane
3. IMM transporter Carnitine:acylcarnitine translocase
4. CPT2 inside the matrix converts back to acyl-CoA
What are the steps for b-oxidation of saturated FA?
*Start with Acyl-CoA
1. Dehydrogenation → makes FADH2 → ETFreduced → QH2 (Ox. Phos.)
- Oxidizes the C at the b-position (makes double bond between a and b Carbons)
2. Hydration
3. Dehydrogenation → generates NADH
4. Thiolic Cleavage → releases 1x Acetyl-CoA (-2C) and add a new CoA to the end of the new chain (2C shorter)
5. Repeat until no Carbons left
What is the balance sheet of b-oxidation for a 16:0 fatty acids like?
16:0 = saturated 16C fatty acid
*Palmotoyl-CoA = Acyl-CoA
1. Activation of palmitate to palmotoyl-CoA → -2 ATPs
2. Oxidation of 8 acetyl-CoA (CAC) → 8x10 = 80 ATP
3. Oxidation of 7 FADH2 → 7x1.5 = 10.5 ATP
4. Oxidation of 7 NADH → 7x2.5 = 17.5 ATP
Net: Palmitate → CO2 + H2O = 106 ATP
What are the steps for b-oxidation of unsaturated FA (odd) ?
*Dehydrogenation replaced by isomerisation → no FADH2 produced
1. Isomerization cis → trans double bond
2. Hydration (double bond becomes C-OH)
3. Dehydrogenation
4. Thiolic cleavage
5. Repeat
*For each odd unsaturation → no FADH2 (-1.5 ATP)
What is the cost of even vs odd unsaturations?
ODD unsaturations → no FADH2 produced (isomerization instead)
EVEN unsaturations → costs 1 NADPH/even unsaturations
- produce FADH2 (2 double bonds) → reductase (costs NADPH) → ismoerase → good to go
What are the general steps of b-oxidation of Odd-chain fatty acids?
- Normal b-oxidation until last 3 carbons → last 3 carbons released as propinoyl-CoA
- Last 5 Carbons → Propionyl-CoA + Acetyl-CoA - Propionyl-CoA (3C) converted to succinyl-CoA (4C)
- Succinyl-CoA → CAC
Which reactions allow conversion of Propionyl-CoA → Succinyl-CoA
Propionyl-CoA → S-Methylmalonyl-CoA (high-energy intermediate)
- Uses ATP + HCO3- (which is transported on malonyl-CoA)
- Irreversible
- Catalysed by carboxylase
…
→ Succinyl-CoA
What are the steps of b-oxidation of branched-chain FA?
Broken down into Propionyl-CoA for branched sections (3C) and Acetyl-CoA for unbranched sections (2C)
Which 2 branched chain fatty acids do we generally find in humans?
*Mostly in milk
1. Phytanic acid (20 C)
2. Pristanic acid (19 C)
What are the 3 ketone bodies found in humans?
- Acetoacetate
- Acetone
- b-Hydroxybutarate
*Acetyl-CoA can be used to regenerate Ketone bodies
*Wat to deliver water-soluble “fat derivatives” to another tissue
When/Where does ketogenesis occur?
*Primarily in the liver
During fasting/starvation, when [glucose] is low → Oxaloacetate is depleted → CAC stops
Problem: Acetyl-CoA doesn’t transfer between tissues
Solution: Ketone bodies serve as “shuttles” for Acetyl-CoA between tissues → bring Acetyl-CoA to the heart and brain so they can run their CAC by regenerating Acetyl-CoA by Thiolase reaction
*Oxaloacetate stores are used for gluconeogenesis as body tries to keep ~5.5mM [glucose]
What reactions are involved in ketogenesis?
How do we go from 1 ketone body to another?
Acetyl-CoA → Acetate
Acetyl-CoA + Acetate → Acetoacetyl-CoA → HMG-CoA → {HMG-CoA lyase} → Acetoacetate
Ketone bodies:
Acetoacetate → {dehydrogenase} → b-Hydroxybutarate
Acetoacetate → {release CO2} → Acetone
What reactions in the heart and brain allows for conversion of ketone bodies back to Acetyl-CoA?
b-Hydroxybutarate → Acetoacetate → {Succinyl-CoA → Succinate) → Acetoacetyl-CoA → {add CoA-SH} → 2x Acetyl-CoA
*Each ketone body contains 2 Acetyl groups → 2x Acetyl-CoA is have the CoA available
*No acetone can be consumed because goes to the lungs → wasted breath
What are outputs of b-oxidation?
- Acetyl-CoA
- FADH2
- NADH
Which metabolic circumstances require the synthesis of fatty acids?
- To initiate cell division
- After a glucose-rich meal
- After a fructose-containing meal
In which cell compartment does Fatty Acid synthesis occur?
In the cytosol → need to shuttle the Acetyl-CoA from the mitochondria to the cytosol, because it can’t cross with its CoA group
How is Acetyl-CoA transported from the mitochondria to the cytosol for Fatty Acid synthesis?
Through the Citrate Shuttle:
1. Acetyl-CoA + Oxaloacetate → {Citrate Synthase} → Citrate Releases the CoA group
2. Citrate/Pi Antiport across the mitochondrial membrane to the cytosol
3. Citrate → {Citrate Lyase, ATP + CoA-SH} → Acetyl-CoA + Oxaloacetate
4. Acetyl-CoA used for FA synthesis + Oxaloacetate → Malate → Pyruvate → transported back to mitochondria
What happens to Oxaloacetate when Acetyl-CoA is released in the cytosol after being shuttled as citrate?
*No oxaloacetate transporter
Oxaloacetate (CYTOSOL) → {Malate dehydrogenase, NADH} → Malate → {Malic enzyme, NADP+} → Pyruvate → shuttled back to the mitochondria → {Pyruvate carboxylase, ATP + CO2} → Oxaloacetate (MITO)
NADPH produced goes to Fatty Acid synthesis
What is the role of Acetyl-CoA Carboxylase (ACC)?
What is its cofactor?
Converts Acetyl-CoA (2C) → Malonyl-CoA (3C)
- Irreversible, Rate-limiting step of FA synthesis
- Driven by ATP breakdown which makes it exergonic
- 1 enzyme/2 reactions
- Biotin = cofactor which provides Carbon
*Reminder, PEP and PEPCK uses biotin as carbon provider
Which reaction is an activation step of fatty acid synthesis?
Adding ACP to Acetyl-CoA (by MAT enzyme)
Malonyl/Acetyl-CoA Transacylase?
What is ACP?
Couples acetyl-CoA/Malonyl-CoA with ACP → keeps acetyl and malonyl “activated” → prevents leakage to other compartments
*Switches the CoA for ACP group → primed molecule is now ready for elongation
ACP = Acyl Carrier Protein (similar to CoA)
What molecule is made by Fatty Acyl Synthase ?
What are the steps?
Palmitate (C16:0) → used as a basic building block for other fatty acids
- Acetyl-ACP + Malonyl-ACP →… → Butyryl-ACP
- Butyryl-KS (4C) + Malonyl-ACP (3C) - CO2 → Acetoacetyl-ACP → … → Butyryl-ACP (goes in cycles 7x)
→ Palmitoyl-ACP → Palmitate
*2 NADPH invested/2C added (/Malonyl-ACP)
What is the structure of Fatty Acyl Synthase?
- 1 protein with many domain → all catalytic activities on 1 monomer
- 2 domains make thioester bonds (KS and ACP (main one))
- 2 homodimers organized in upside-down orientation → domains interact with each other
VERY FAST
What is the general stoichiometric equation of the synthesis of Palmitate?
- 7 malonyl-CoA are derived from Acetyl-CoA:
7 Acetyl-CoA + 7 CO2 + 7 ATP → 7 malonyl-CoA + 7 ADP + 7 Pi + 7 H+
*CO2 are released in 1st step - Overall Palmitate biosynthesis (by Fatty Acyl Synthase):
8 Acetyl-CoA + 14 NADPH + 7 ATP → palmitate + 14 NADP+ + 8 COA + 6 H2O + 7 ADP + 7 Pi
Which reactions allow elongation of Palmitate to form longer Fatty Acids? (Where does it occur?)
*In the mitochondria → Addition of Acetyl-CoA to palmitate (reverse of FA oxidation)
- Consumes 1x NADH + 1x NADPH
- Adds 2C/ cycle
- Many different enzymes (not like Fatty Acyl Synthase)
How is ACC regulated?
*By polymerisation and phosphorylation
Allosteric:
- Activated by Citrate → promotes polymerization
- Inhibited by fatty acids
Hormonal:
Activated by Insulin → stimulates glucose uptake + pyruvate dehydrogenase (more Acetyl-CoA → more Citrate → allosteric polymerization)
→ Dephosphorylation of ACC by Insulin pathway
Glucagon/Epinephrine → inhibits (by PKA-mediated phosphorylation)
Which reactions allow desaturation of palmitate or longer fatty acids?
Only 4 different terminal desaturase in mammals → ∆9, ∆6, ∆5 and ∆4 Fatty Acyl-CoA Desaturases
*No desaturase beyond ∆9 in mammals
What is the consequence of mammals having no desaturase beyond ∆9?
Some essential fatty acids can’t be produced so need to be acquired from diet (plants)
- Linoleic acid
- Linolenic acid
What is the rate limiting step of fatty acid synthesis?
Acetyl-CoA Carboxylase (ACC) → produces Malonyl-CoA
What type of reaction in Citrate Lyase from the point of view of the CAC?
Cataplerotic reaction → depletes the CAC
Citrate → Acetyl-CoA
*Produces Acetyl-CoA which is not an intermediate of the CAC
*Cytosolic enzyme
What is the importance of fatty acids in immune response?
Some FA are precursor molecules for inflammatory mediators
Pharmacological NSAID (non-steroidal anti-inflammatory drugs) inhibit production these mediators from fatty acids
What are some of the general COX inhibitors?
COX = cyclooxygenase: Arachidonic acid (FA) → Prostaglandin F2 (involved in inflammatory response)
COX inhibitors prevent inflammatory responses → Aspirin, Ibuprofen, Naproxen
Imbalanced lipid metabolism can cause health issues. Which are true?
1)
