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:
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Conversion of TAG to Free Fatty Acids (FFAs)+Glycerol
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FATTY ACID OXIDATION
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 WHY THE COMPLEX SHUTTLE PROCESS? - 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 BYPRODUCT(S): NADH 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 |   |
 | <-OVERVIEW OF BETA-OXIDATION 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 |   |
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PRODUCTION OF KETONE BODIES
- 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 END: HMG-CoA STEP 3: Release of acetoacetate START: HMG-CoA ENZYME: HMG-CoA lyase END: acetoacetate STEP 4: (Spontaneous decarboxylation of acetoacetate to acetone *funny smell* and release of CO2) OR (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