Fatty Acid Oxidation & Peroxidation

“Activated” Fatty Acid

Has an S-CoA group

There are three types of fatty acid oxidation:

alpha (α), beta (β), and omega (ω)

α-oxidation

• Involves the breakdown of dietary branch-chained fatty acids (BCFA) that come from plants

• Most likely site is peroxisomes in liver – detoxification mechanism compared ⍵ and β- oxidation (production of energy generating molecules)

• Small membrane bound organelles – cytoplasm of all eukaryotic cells – contain oxidative enzymes- replicate themselves – high in liver as toxic by products accumulate in the liver and detoxify in the liver

• Does not involve NADH and NADPH

• Does not generate high energy Pi (ATP)

Β-oxidation

• Main energy generating molecule synthesizing pathway

• Β-oxidation is blocked – methyl group on 3 C position (β-C position)

• However, shorten the FAs by 1 carbon to allow β- oxidation to proceed –remove α-C (next to –COOH)

• Now this resulting FA undergo Β-oxidation

Dietary BCFAs

• Primarily saturated FAs with a methyl branch or more on the C chain.

• They can be mono, di, or multi-methyl BCFAs

• Unexplored class of diary-derived fatty acids

α-oxidation – BCFA s – Health Benefits

Neonatal rat study: replace 20% wt of fat as BCFA:

• alter GI microbial ecology,

• change was towards the bacteria that utilize BCFAs and bacteria containing BCFAs.

• Reduced incidence of Necrotizing enterocolitis (NEC), increased expression of anti-inflammatory markers: IL-10

• NEC: premature/low birth wt newborns at risk

• Histopathology- inflammation, tissue death and necrosis –bacterial invasion, cellular damage, cellular death, necrosis of colon and intestine. Bacteria and gut content leak to abdominal cavity – sepsis - life threatening.

• Early diagnosis is difficult- damage intestine removal is needed sometimes absence of early diagnosis

• IV feeding

• BCFA- induce apoptosis in human tumor cells

α-oxidation – BCFAs

• Ruminant milk and beef products: major dietary contributors. Not in poultry, not in fish

• They are important: modulate biophysical properties of membrane – similar in function to cis double bonds FAs: maintain phase transition temp of membranes

• Rare in internal human organs, high conc in skin and white waxy substance coating the skin of term newborns.

• Healthy term newborn gut – important bioactive lipid in early human development – but under studied.

Term Newborn Intestine & BCFAs

• Possible Source - Swallowed amniotic fluid during/end to the last trimester of healthy pregnancy

• Placental transfer ...

• Dietary BCFA fed to rat pups; appeared in serum, liver, incorporated to ileal PH fraction

• Found in colostrum and mature milk

• But not in formula milk

• Adults diet: American diet; more BCFA than DHA (LC-PUFAs)

Refsum’s Disease

• Rare genetic disease → deficiency in phytanoyl-coenzyme A hydroxylase

• Accumulation of an α-oxidation intermediate (phytanic acid intermediates – that leads to neurological damage)

• Absence of detoxification in hepatocytes – lead to accumulation in circulation and then must be crossing BBB resulting neurological defects

Β- oxidation vs α and ⍵- oxidation of FAs:

• Β- oxidation: mitochondria: major contributor for cellular energy

• α –oxidation of FAs : peroxisomes

• ⍵- oxidation of FAs – peroxisomes, ER

Transport of a fatty acid from cytosol to mitochondrion:

• An “activated” fatty acid has its CoA group replaced with carnitine, allowing transport of the acyl group into mitochondria

• The acyl-carnitine then has its carnitine group replaced with CoA

• The acyl-CoA in mitochondria can undergo β-oxidation to ultimately yield ATP

  • 108 gross ATP .....106 net ATP

Activation of FAs:

• Enzyme thiokinase or acyl-CoA synthetases convert the fatty acids to fatty acyl-CoA

• These enzymes are a family of three isoenzymes having activity against specific chain length fatty acids

• There are 3 thiokinases each specific for short, intermediate and long carbon chains

• Fatty acyl-CoA is formed by the formation of thioester bond between the carboxyl group of the fatty acid and the thiol group of coenzyme A

• This reaction also involves use of energy, by breakdown of ATP to AMP + PPi and is an irreversible reaction – Cost 2 ATP

β-oxidation of fatty acids yields acetyl-CoA:

• Add new Coenzyme: activation of FAs

• Dehydration

• Hydration

• Oxidation

• Add new coenzyme

• Thiol attack: Remove 2-C units (Acetyl –CoA)

• Repeat 7 cycles

β-oxidation of palmitic acid – a C16 saturated fatty acid:

1. Dehydrogenation between the alpha- and beta-carbon

2. Hydration of the beta-carbon

3. Oxidation of the hydroxyl group on the beta-carbon

4. Thiol attack of the bond between the alpha- and beta-carbon

• Palmitoyl-CoA (C16) + H2O + FAD + NAD+ + CoA-SH → Myristoyl-CoA (C14) + Acetyl-CoA + FADH2 + NADH + H+

• TOTAL ATP: 28 + 8 + 12 + 60 = 108

• NET ATP: 108 - 2 (for “activation”) = 106

• Acetyl-CoA goes to the Krebs Cycle:

  • Leads to 3NADH+H+ + 1FADH2 + GTP(ATP)

• FADH2 and NADH + H+ go to the electron transport chain

  • FADH2 →→ 1.5ATP; NADH+H+ →→ 2.5ATP

From Cx fatty acyl-CoA to C(x-2) fatty acyl-CoA:

One cycle β-oxidation (4 ATP):

• 1NADH+H+ →→ 2.5ATP

• 1FADH2 →→ 1.5ATP

One Acetyl-CoA Via TCA cycle:

• 1 Acetyl-CoA →→ 3NADH+H+ + 1FADH2 + GTP(ATP)

  • 7.5ATP + 1.5ATP + 1GTP(ATP) = 10 ATP

TOTAL #ATP = 14

Fatty acid synthesis versus β-oxidation:

Fatty acid synthesis:

• Condensation

• Reduction of ketone to alcohol

• Dehydration to form enoyl group

• Reduction (or hydrogenation) of enoyl group

Beta-oxidation:

• Dehydrogenation between alpha- and beta- carbon

• Hydration of beta-carbon to introduce –OH

• Oxidation of alcohol to ketone

• Cleavage between alpha- and beta- carbon

ω-oxidation

• Involves the oxidation of the terminal methyl group of a fatty acid- C far away from –COOH

• Unlike β-oxidation, which occurs in mitochondria, ω-oxidation occurs in the smooth ER of hepatocytes (enzyme is in ER)

• Though usually a minor process,

• Typically ω-oxidation results in the formation of dicarboxylic acids (DCA)

• From medium-chain fatty acids (10-12 carbons in length)

• Increases β-oxidation rate as both –COOH ends can be “activated”

• Final product of DCA oxidation is succinate

• Essential for the synthesis of select eicosanoids (from 20 C PUFA- signaling molecules)

Process of ω-oxidation:

1. Hydroxylation of the methyl carbon at terminal end (away from –COOH)

2. Oxidation of the methyl hydroxyl into an aldehyde

3. Oxidation of the methyl aldehyde into a carboxyl group

4. Then β-oxidation

ω-oxidation: Step 1

• Hydroxylation of the methyl carbon is carried out by a mixed function oxidase (MFO – a class of cytochrome P450 monooxygenase)

  • At least 5 different MFOs are responsible, each using NADPH

    • CYP4Ax and CYP4Fx

An MFO:

is a cytochrome P450 monooxygenase

ω-oxidation: Steps 2 & 3

• Fatty omega-hydroxyacid → fatty omega-aldoacid

  • via alcohol dehydrogenase

  • NAD+ → NADH + H+

• Fatty omega-aldoacid → fatty dicarboxilic acid

  • via aldehyde dehydrogenase

  • NAD+ → NADH + H+

The DCA undergoes β-oxidation:

• The DCA can undergo β-oxidation following the “activation” of both ends with CoA

• The final product is succinate (which can feed back into the Krebs cycle)