Fatty Acid Synthesis 2

FA Synthesis

Energetically favorable:

• only a 1 ATP used during each FAS via CO2 release

• ¹³C-bicarbonate used

• no ¹³C was incorporated into palmitate FA

• ¹⁴C-bicarbonate (HCO3-) used (ACC step)

• no/undetectable amount of ¹⁴C was incorporated into palmitate FA

• 8 Acetyl CoA (2C) + 14 NADPH + 13H+ + 7 ATP→ Palmitate (16C) + 8 CoA-SH + 6 H2O + 14 NADP+ + 7 ADP + 7 Pi

FA Measure:

HPLC

Radioactivity Measure:

Liquid scintillation Counter

ACC Step

• Biotin-enzyme + ATP + HCO3- <—> CO2-biotin-enzyme + ADP + Pi

• CO2-biotin-enzyme + acetyl-CoA → Malonyl-CoA + biotin-enzyme

FAS Steps

Acetyl-CoA + Malonyl-CoA → Acetoacetyl ACP + ACP + CO2

ACC and FAS

• Both ACC and FAS are regulated

• FAS does not always release palmitate → It occasionally can instead release the 14-carbon saturated fatty acid myristate

In healthy adults:

FAS primarily expressed in hormone sensitive cells and cells with high lipid metabolism

Well-nourished individuals:

• Preferentially use circulating free FAs from diet

• FAs de novo synthesis → only minor contribution /rarely needed

CHO ingestion:

upregulates FAS

Glucagon

down regulates FAS

FAS activity is upregulated in tumors:

• Tumor cells rely on FAs de novo synthesis

• Accommodates more FA for membrane synthesis

• Regulate tumor DNA damage repair and apoptosis and used for tumor signaling

• Support rapid diving cell: more membrane phospholipids required for cell membranes

• Energy production

• Tumor control can be achieved via FAS control:

  • Inhibitors of Beta-ketoacyl ACP: condensation reaction

  • Inhibited phospholipid synthesis

    • Interestingly, these inhibitors showed weight loss – anti-tumor effect and anti-obesity effect

Lactating mammary glands:

Need high activity/expression of FAS:

• Fatty acid synthase is required for mammary gland development and milk production during lactation

Synthesis of long-chain fatty acids from palmitate:

Fatty acids of up to 36 carbons in length can be synthesized in vivo, but they all originate from the same precursor – the C16 fatty acid palmitate:

• Key to the process are elongases and desaturases

• Occurs in the endoplasmic reticulum and mitochondria

Elongation and Desaturation

Synthesize a variety of saturated, monounsaturated (MUFA) & polyunsaturated FAs (PUFA)

Essential FAs

Linoleic acid (18:2n-6) & α-linolenic acid (18:3n-3):

• must get from diet

Elongases

Now known as elongation of very long-chained fatty acids (aka, Elovl), add two carbons onto a fatty acyl-CoA

Desaturases

Introduce an enoyl group (or, carbon-carbon double bond) into a fatty acyl Co-A

Most of FA in the body:

• Dietary origin

  • But Palmitic acid (C16:0) can be de novo synthesized from excess CHOs

  • Myristic acid (C14:0) & stearic acid (C18:0): much lesser extent

Lipogenic Tissues

• liver

• adipose

• lactating mammary glands

The synthesis of the saturated 18-carbon fatty acid stearate from palmitate requires:

• palmitoyl-CoA (aka, “activated” palmitate)

• malonyl-CoA (endoplasmic reticulum) or acetyl-CoA (mitochondria)

• NADH or NADPH, plus H+

• 5 individual enzymes (rather than one enzyme complex as with palmitate synthesis)

• Coenzyme A as acyl carrier – rather than ACP

  • Rest of all in the ER is identical to palmitate synthesis; elongation, reduction, dehydration and reduction

Summary of Fatty Acyl-CoA Elongation

1. Condensation

• C donor: Malonyl-CoA activated form with CO2.

• Primer: Fatty acyl-CoA: Derived from Palmitate /palmitic acid or stearic acid/stearate

• Occurs in the smooth ER

Final step:

• involves either the action of thioesterase on the elongated fatty acyl-CoA (liberating the new fatty acid and CoA-SH), or the further elongation or desaturation of the elongated fatty acyl- CoA

De Novo Synthesis of FA s in Mitochondria

• Major function of this is to production of Lipoic acid

• Prosthetic group multienzyme complexes: PDH and Krebs cycle enzyme (α-ketoglutarate dehydrogenase) – critical for mitochondrial energy generation.

• How malonyl CoA synthesize in Mit. → not clear

• Both Malonyl-CoA and Acetyl-CoA membrane impermeable to the mitochondrial membrane

• Suggesting that there are isoforms of ACC: ACC1 vs ACC2

  • ACC1 → cytoplasmic

  • ACC2 may be attached cytoplasmic side of mitochondrial outer membrane → evidence lacks

Key to the addition of two carbon units are the Elovl enzymes:

Eight isoforms exist:

• Seven (Elovl1... Elovl7) are in present in mammals

• The eighth (Elovl8) to date is only identified in aquatic species

• Each isoform will act on specific classes of fatty acids

Whether with FAS, or individual enzymes to expand the length of a fatty acid beyond 16 carbons, the steps are identical:

1. Condensation

2. Reduction of ketone to alcohol

3. Dehydration to form enoyl group

4. Reduction (or hydrogenation) of enoyl group

Mammalian fatty acids derived from palmitate:

Starting with palmitate, we can make saturated, monounsaturated fatty acids (MUFA), & polyunsaturated fatty acids (PUFA):

• The MUFAs include omega-7, omega-9, and omega-10 species

• The PUFAs are only omega-9

Synthesis of unsaturated long-chained fatty acids:

The unsaturation of fatty acids requires the use of fatty acyl-CoA desaturases:

• In mammals, there are three fatty acyl-CoA desaturases:

  • Δ5 desaturase

  • Δ6 desaturase

  • Δ9 desaturase or stearoyl-CoA desaturase (SCD)

    • 5 isoforms exist: SCD1, SCD2... SCD5

• With very few exceptions, the initial desaturation (or the introduction of a carbon-carbon double bond by reduction) of a saturated fatty acyl-CoA is introduced at the 9th carbon from the carboxylic acid (with the carbon at C=O being #1)

Oleoyl-CoA (Oleate) from stearoyl-CoA by SCD

• Stearoyl-CoA desaturase → SCD – oleoyl-CoA is major pdc of SAD-1

• Fe²⁺ to Fe³⁺ = lost one electron

  • So need two of cytochrome b5

  • FADH2 – release 2 H+ (protons) and 2 e during one FADH2 reduction

Essential long-chained fatty acids from plants:

• Plants have Δ12 and Δ15 desaturases, which generate the “omega-3” and “omega-6” fatty acids from oleate

• Aquatic species and mammal intake these through diet, to anabolize them into other fatty acids (i.e., the omega-3 FAs DHA and EPA, and the omega-6 FA arachidonate)

Aside from providing energy via beta oxidation, other roles for fatty acids exist:

• Incorporation into phospholipids to influence curvature of cell membranes and lipoproteins

• Cell signaling (lipid rafts)

• Transcriptional regulation (Beta carotene –retinoic acid in heart tissues is a transcriptional regulator (alter gene expression) of lipid metabolizing enzymes – alter cardiac lipid metabolism)

• Others... hormones, steroids