Lecture 9: Fatty Acid Catabolism

Importance of Lipids
Lipids or "fats" have a bad reputation, but they're important in...
* Energy Metabolism
* Components of...
  * Membranes
  * Hormones
  * Fat-Soluble vitamins
  * Thermal insulators
  * Signalling Molecules

Total weight of Triacylglycerides (TAGs) in human of 70kg ~= 15 kg which holds 555 000 kJ of energy stores
* Recall glucose ~= 20g, glycogen ~= 400g)

Lipid = amphipathic molecule and can be saturated or unsaturated:

Name them based on trans/cis bonds (e.g. all-cis-9,12,15-Octadecanoic acid)

Triacylglycerol: Glycerol molecules plus 3 FAs via ester bonds

Sources of TAGs:
  1. The diet
  2. de novo synthesis (from the liver)
  3. Storage deposits in adipocytes

- Unlimited storage capacity; unregulated storage in adipocytes leads to obesity; release of stored fats is hormonally-regulated.

Conversion of TAG to Free Fatty Acids (FFAs)+Glycerol
  1. Adipose Triglyceride Lipase (ATGL) from tri-to di-
  2. Hormone-Sensitive Lipase (HSL) from di- to mono-
  3. Monoacylglycerol Lipase (MGL) from mono- to glycerol

Part 1: Activation of FA
2-step mech, requires ATP and CoA, produces Fatty acyl-CoAEnzyme = acyl-CoA synthetase
- Multiple genes code for it; gene generates multiple protein isoforms
Depends on chain length:
- Short-medium FA chains (0-5,6-10-C) activated in cytoplasm
- long-chain (10-21-C) activated by acyl-CoA synthetase which is bound to the outer membrane of the mitochondria
Formation @ end = on the cytosolic face of the outer mitochondrial membrane

Part 2: FA Transport via the Carnitine Shuttle

Step 1: Bind Fatty acyl-CoA to carnitine using carnitine palmitoyltransferase I (CPT I)
- Result = Fatty-acyl carnitine + CoA-SH
Step 2: Fatty-acyl carnitine shuttle goes through cytoplasm into intermembrane space via porins pores, into mito matrix via translocase protein
Step 3: Deacylate FA-Carnitine via carnitine palmitoyltransferase II (CPT II)
- Result = Fatty acyl-CoA + Carnitine

- CPT I strongly inhib'd by malonyl-CoA (first committed intermediate of FA synthesis)
- Conditions favouring FA synthesis prevent FAs from entering the mitocondria where beta-oxidation occurs.

Part 3: Beta-Oxidation
- WHERE: Mitochondrial matrix; @ the beta-carbon of fatty acyl-CoA
- WHAT: Chain is shortened by 2 carbons at a time
- HOW LONG: cyclic pathway of 4 rxns, "chopping" from the CoA-side

STEP 1 (left): Dehydrogenation of acyl-CoA into trans-2-Enoyl-CoA
START product = acyl-CoA
ENZYME: Acyl-CoA Dehydrogenases (ACAD) which are bound to FAD
END product: trans-2-Enoyl-CoA
BYPRODUCT(S): Generate FADH2; Electrons are transferred to electron-transferring flavoprotein (ETF) also bound to FAD and will be passed to CoQ via ETF-Q oxidoreductase
MECHANISM: Dehydrogenation

STEP 2 (left): Hydration of trans-2-Enoyl
START product = trans-2-Enoyl-CoA
ENZYME: Enoyl-CoA hydratase (EH)
END product = L-3-hydroxyacyl-CoA
MECHANISM: Hydration
STEP 3 (left): Dehydrogenation of L-3-hydroxyacyl-CoA
START product = L-3-hydroxyacyl-CoA
ENZYME: 3-L-hydroxyacyl-CoA dehydrogenase
END product: 3-ketoacyl-CoA
MECHANISM: Dehydrogenation

STEP 4 (right): Thiolytic cleavage of 3-ketoacyl-CoA
START product: 3-ketoacyl-CoA
ENZYME: thiolase
END product: C(n-2) acyl-CoA and acetyl-CoA
MECHANISM: thiolytic cleavage


Total yield of a C16 Acyl-CoA?
- 1 cycle of beta-oxidation produces: 1 FADH2, 1NADH + (H+), 1 acetyl-CoA
- 1 acetyl-CoA yields: 3 NADH, 1 FADH2, 1 GTP
- In oxphos: 1 FADH2 = 1.5 ATP, 1 NADH+ (H+) = 2.5 ATP, 1 GTP = 1 ATP
After 1 round of Beta-oxidation, remaining is C14...
7 total rounds of Beta-oxidation result in a C2 Acyl-CoA = all beta-oxidation*7, all acetyl-CoA*7
-> 7th round creates 2 acetyl-CoAs, so 7 total all beta-oxidation but 8 acetyl-CoAs
- Subtract 1 ATP for activation results in 107 ATP

Problem for Unsaturated FAs...?
Problem 1: Enoyl-CoA Hydratase (from Reaction 2 in beta-oxidation) CANNOT ACT ON CIS DOUBLE-BONDS
- Solution: enoyl-CoA isomerase flips the bond from cis to trans before interaction w/ EH

Problem 2: In two double-bonds one after the other, enoyl-CoA isomerase is not enough
- Solution: use 2,4-Dienoyl-CoA reductase in conjunction with the isomerase (NOTE: USES 1 MOLECULE OF NADPH)

Take-home message: Unsaturated FAs are also catabolized, but require other enzymes:
1. Enoyl-CoA Isomerase
2. 2,4-Dienoyl-CoA reductase

- Acetyl-CoA is a key intermediate between fat and carbohydrate metabolism
- Ketone bodies are produced via Ketogenesis by the liver cells in the mitochondrial matrix when blood glucose levels are low and other carb stores are exhausted (i.e.: glycogen).
- Decrease of oxaloacetate b/c it's part of the TCA cycle which is slowed down when fasting/starving; acetyl-CoA is supposed to react with oxaloacetate, but instead it accumulates
- Solution? Convert acetyl-CoA from FA catabolism into Ketone bodies: Acetoacetate and beta-hydroxybutyrate

STEP 1: first condensation (i.e. reversal of step 4 in beta-oxidation)
START: Acetyl-CoA
ENZYME: beta-ketothiolase
END: Acetoacetyl-CoA

STEP 2: second condensation
START: Acetoacetyl-CoA
ENZYME: HMG-CoA synthase

STEP 3: Release of acetoacetate
END: acetoacetate

STEP 4: 
(Spontaneous decarboxylation of acetoacetate to acetone *funny smell* and release of CO2)
(Reduction of acetoacetate to beta-hydroxybutyrate via dehydrogenase)
1)2)  3) 
4) 5)

Role of ketone bodies in fasting and starving?
- Acetoacetate and beta-hydroxybutyrate are generated in the liver in this time; these molecules can be used by peripheral tissues as alternative fuel under ketogenic conditions
- SUMMARY: liver will produce ketone bodies for the peripheral tissues (mainly the brain) to consume as alternative fuels
- Diabetic patients will use intermediates from TCA to fuel gluconeogenesis, so the acetyl-CoA being produced will be diverted to acetoacetyl-CoA to ketone bodies into bloodstream to be used as alternative fuel

Case Study
A mutation in a gene that produces the translocase of the carnitine shuttle system causes long FAs to no longer be transported into the mitochondrial matrix.
- Results? Arrythmia and poor muscle contractions to name two; diet consists of high-carbs and low-fats