BB ch 17

Fatty Acids as High-Density Energy Stores

• Migratory birds showcase the power of fatty acid (FA) oxidation as a fuel source.
• Strategy: accumulate triacylglycerols (TAGs) until body mass ≈ $70\%$ fat.
• Two physicochemical reasons FAs are the premier storage form.
• Carbon is almost fully reduced (mostly -CH2)→ maximal energy release on oxidation.
• FAs are anhydrous (unlike glycogen) → pack tightly, yielding high "energy per gram".

Quantitative Perspective on Stored Fuels (70-kg Human)

• Adipose TAG: $37\ \text{kJ g}^{-1}\,(\text{dry})\times 15{,}000\ \text{g}=5.55\times10^{5}\ \text{kJ}$ .
• Protein (muscle): $1.02\times10^{5}\ \text{kJ}$ – generally not tapped unless fasting.
• Glycogen & blood glucose together < $0.35\times10^{4}\ \text{kJ}$ .
• Liver converts excess dietary carbohydrate → FAs for export.

Dietary Lipid Processing & Transport

• Digestion starts in the small intestine; bile salts (gallbladder) emulsify fats → mixed micelles.
• Pancreatic lipase: hydrolyzes C-1 & C-3 ester bonds → 2-monoacylglycerol + 2 FAs.
• Intestinal lipases remove the remaining acyl group.
• Enterocyte events:
    • FAs + monoacylglycerols re-esterified → TAGs.
    • TAG + cholesterol + apolipoproteins → chylomicrons (80% core TAG).
• Chylomicron travel:
    • Lymph → bloodstream.
    • ApoC-II activates capillary lipoprotein lipase → releases FAs & glycerol to tissues.
• Fate of liberated products:
    • Myocytes: immediate β-oxidation → ATP.
    • Adipocytes: re-esterification & storage.

Hormonal Mobilization of Stored TAG (Adipose)

• Key hormones: glucagon, epinephrine, ACTH.

Hormone → GPCR → $\text{adenylyl cyclase}$ → ↑ $[\text{cAMP}]$ .
$\text{PKA}$ phosphorylates:
• Hormone-sensitive lipase (HSL) → active.
• Perilipin → exposes TAG droplet surface.
TAG → DAG → MAG → 3 FAs + glycerol
• FAs leave adipocyte via FA transporter, bind serum albumin (up to 10 FAs per protein), travel to energy-needing tissues.
• Glycerol (5% energy) → liver; pathway
- $\text{glycerol kinase}$ → glycerol-3-P
  • $\text{glycerol-3-P dehydrogenase}$ → $\text{DHAP}$ (enters glycolysis/gluconeogenesis).

Historical Elucidation of FA Degradation (β-Oxidation)

• 1904 Knoop: stepwise 2-C removal (dog experiment with phenyl FA).
• 1940s Lehninger: occurs in mitochondria.
• Lynen & Reichart: product is acetyl-CoA.
• Key insight: first oxidation targets the β-carbon → "β-oxidation".

Overview of Fatty Acid Catabolism

• Three Stages

β-oxidation: long-chain acyl-CoA → multiple acetyl-CoA + FADH₂ + NADH.
Citric acid cycle: acetyl-CoA → 2\ \ce{CO2}+3\ \text{NADH}+\text{FADH2}+\text{GTP}
Oxidative phosphorylation: electron carriers → O₂; ATP synthesized.

Activation of FAs to Acyl-CoA (Cytosol or Outer Mito Membrane)

• Enzyme: acyl-CoA synthetase (thiokinase).

Fatty acid + ATP → acyl-adenylate + PPᵢ.
CoA-SH attacks acyl-AMP → acyl-CoA + AMP.
• Energetics: costs $2\ \text{high-energy}$ bonds (ATP → AMP + PPᵢ; PPᵢ → $2\,\text{P}_\text{i}$ ).
$\Delta G'^{\circ}\approx -19\ \text{kJ mol}^{-1}$ (after PPᵢ hydrolysis).

Carnitine Shuttle for Mitochondrial Entry (>C14)

• Steps:

Carnitine acyltransferase I (outer membrane) ⇒ acyl-carnitine.
Acyl-carnitine/carnitine antiporter traverses inner membrane.
Carnitine acyltransferase II (matrix) regenerates acyl-CoA + free carnitine.
• Regulation: malonyl-CoA (1st intermediate of FA synthesis) inhibits CAT I → prevents futile cycle.

β-Oxidation Cycle (Matrix)

• Repeats until FA fully converted; each turn shortens chain by 2 C and yields 1 acetyl-CoA, 1 FADH₂, 1 NADH.

Acyl-CoA dehydrogenase: alkane → trans-Δ²-enoyl-CoA + $\text{FADH}_2$ .
• Mechanism: α-C proton abstraction, β-C hydride to FAD.
Enoyl-CoA hydratase (crotonase): trans-Δ²-enoyl-CoA + \ce{H2O} → L-β-hydroxyacyl-CoA (stereospecific).
β-Hydroxyacyl-CoA dehydrogenase: hydroxyl → keto; produces $\text{NADH}$ (→ $2.5\ \text{ATP}$ each).
Thiolase (β-ketothiolase): enzyme Cys-S⁻ attacks β-carbonyl; CoA-SH cleaves → acetyl-CoA + acyl-CoA (n-2).

Energetics Example: Palmitate (C16)

• 7 β-oxidation cycles.
• Products: 8 acetyl-CoA + 7 FADH₂ + 7 NADH.
• Complete oxidation (β-Ox + TCA + OxPhos): $108\ \text{ATP}$ (net $106$ if subtracting 2-ATP equivalent for activation).
• Assumes $1.5\ \text{ATP}/\text{FADH}_2$ , $2.5\ \text{ATP}/\text{NADH}$ .
• High yield & metabolic water: \ce{C16H32O2} + 23\ \ce{O2} → 16\ \ce{CO2} + 16\ \ce{H2O} .
• Essential for desert animals, marine mammals, camels.

Oxidation of Unsaturated FAs

• Problem: cis double bonds interrupt normal pathway; auxiliary enzymes fix geometry.
• Monounsaturated FA (e.g., oleate, Δ9):
• After 3 cycles, cis-Δ³-enoyl-CoA appears.
• Enoyl-CoA isomerase → trans-Δ²-enoyl-CoA → resumes standard β-oxidation.
• Polyunsaturated FA (e.g., linoleate, Δ9,12):
• Sequence: β-oxidation ×3 → cis-Δ³; isomerase → trans-Δ².
• One more cycle → trans-Δ², cis-Δ⁴-dienoyl-CoA (dead end).
• 2,4-Dienoyl-CoA reductase (NADPH) converts to trans-Δ³.
• Isomerase converts trans-Δ³ → trans-Δ²; β-oxidation continues.
• Net cost: 1 NADPH (≈ -2.5 ATP penalty) for every double bond beyond the first.

Oxidation of Odd-Chain FAs (Plants, Marine Organisms)

• Normal β-oxidation proceeds until 5-carbon acyl-CoA remains.
• Final thiolase cleavage: acetyl-CoA + propionyl-CoA (3 C).
• Propionyl-CoA → succinyl-CoA via 3-enzyme route:

Propionyl-CoA carboxylase (biotin; ATP) → D-methylmalonyl-CoA.
Racemase → L-methylmalonyl-CoA.
Methylmalonyl-CoA mutase (vitamin B₁₂) → succinyl-CoA (enters TCA or gluconeogenesis).

Integrated Regulation of FA Metabolism

• When dietary carbohydrate abundance ↑, need for FA oxidation ↓.
• Key regulatory nodes:
• Acetyl-CoA carboxylase (ACC) – makes malonyl-CoA.
• Activated by insulin/citrate; inhibited by phosphorylation (AMPK, glucagon, epinephrine).
• Carnitine acyltransferase I – inhibited by malonyl-CoA; rate-limiting for β-oxidation entry.

TAGs → Glucose in Germinating Seeds (Glyoxylate Cycle)

• Stored oil bodies hydrolyzed → FAs.
• β-Oxidation (peroxisome) → acetyl-CoA.
• Glyoxylate cycle bypasses decarboxylations of TCA, keeping carbon skeleton for gluconeogenesis → sucrose for developing plant.

Ketone Bodies: Formation, Utilization & Pathology

• Ketone bodies (KB): acetoacetate, D-β-hydroxybutyrate, acetone.
• Produced in liver mitochondria when acetyl-CoA > OAA (starvation, uncontrolled type I diabetes).

Thiolase: $2\ \text{acetyl-CoA} \rightarrow \text{acetoacetyl-CoA}.$
HMG-CoA synthase & lyase: acetoacetate ± reduction → β-OH-butyrate; spontaneous decarboxylation → acetone.
• Extrahepatic tissues (brain, muscle) convert KB → acetyl-CoA.
• β-hydroxybutyrate dehydrogenase (NAD+).
• 3-ketoacyl-CoA transferase (absent in liver!) swaps CoA from succinyl-CoA.
• Thiolase splits acetoacetyl-CoA → 2 acetyl-CoA.
• Clinical notes:
• Diabetes: "cells starve in the midst of plenty"; excessive gluconeogenesis drains OAA → ↑KB (ketoacidosis, fruity breath from acetone).
• Starvation: KB spare glucose for brain, reduce protein catabolism.

Ethical & Physiological Implications

• Modern high-fat diets vs. evolutionary energy demands.
• Role of gut microbiome metabolite TMAO (from carnitine) in atherosclerosis – highlights diet–microbe–host interactions.
• Understanding FA oxidation disorders guides newborn screening (e.g., medium-chain acyl-CoA dehydrogenase deficiency).

Numerical & Stoichiometric Highlights

• Activation cost: $\text{ATP} \rightarrow \text{AMP} + \text{PP}<em>\text{i}$ ((\equiv 2) ATP). • β-Ox cycle per 2-C removal: $\text{FADH}</em>2 (1.5\,\text{ATP}) + \text{NADH} (2.5\,\text{ATP}) + \text{acetyl-CoA} (10\,\text{ATP via TCA})$ → $14\ \text{ATP}$ gross.
• Palmitate net: $106\ \text{ATP}$ + $\approx 23\ \text{H}_2\text{O}$ (metabolic water).