SATURATED FA

Short Chain

< 6C; product of bacterial metabolism

Middle Chain

6-12C; from diet

Long Chain

13-22C; depends on pancreatic lipase absorption

Very Long Chain

23-28C

16C Palmitic Acid & 18C Stearic Acid

most abundant in human & animals

2C Acetic Acid

considered as a universal precursor for fatty acids, lipids, & other organic plant products

7C monocarboxylic acid

Enanthic acid

8C monocarboxylic acid

Pelargonic acid

Butyric acid

Carbon Count : Double Bonds: 4:0

Systemic Name: Butanoic acid

Primary Source / Board Relevance: Butterfat; short-chain FA produced by gut microbiota.

Caproic acid

Carbon Count : Double Bonds: 6:0

Systemic Name: Hexanoic acid

Primary Source / Board Relevance: Goat milk / butterfat.

Caprylic acid

Carbon Count : Double Bonds: 8:0

Systemic Name: Octanoic acid

Primary Source / Board Relevance: Coconut oil / Medium-Chain Triglycerides (MCTs).

Capric acid

Carbon Count : Double Bonds: 10:0

Systemic Name: Decanoic acid

Primary Source / Board Relevance: Coconut oil / Palm kernel oil.

Lauric acid

Carbon Count : Double Bonds: 12:0

Systemic Name: Dodecanoic acid

Primary Source / Board Relevance: Coconut oil; major medium-chain SFA.

Myristic acid

Carbon Count : Double Bonds: 14:0

Systemic Name: Tetradecanoic acid

Primary Source / Board Relevance: Nutmeg butter, coconut oil.

Palmitic acid

Carbon Count : Double Bonds: 16:0

Systemic Name: Hexadecanoic acid

Primary Source / Board Relevance: Most abundant SFA in the human body; primary end-product of lipogenesis

Stearic acid

Carbon Count : Double Bonds: 18:0

Systemic Name: Octadecanoic acid

Primary Source / Board Relevance: Cocoa butter, animal fats; neutral effect on LDL cholesterol.

Arachidic acid

Carbon Count : Double Bonds: 20:0

Systemic Name: Eicosanoic acid

Primary Source / Board Relevance: Peanut oil

Lignoceric acid

Carbon Count : Double Bonds: 24:0

Systemic Name: Tetracosanoic acid

Primary Source / Board Relevance: Very Long-Chain Fatty Acid (VLCFA); component of brain sphingolipids.

Board Mnemonic (12C to 24C)

Lauric, Myristic, Palmitic, Stearic, Arachidic, Lignoceric → "Little Monkeys Prefer Sweet Apple Lollipops".

Biosynthesis of Saturated Fatty Acids (Lipogenesis)

• Location: Cytosol of liver, adipose tissue, and lactating mammary glands.

• End Product: Palmitate (16:0). Synthesis stops at 16 carbons; longer chains require separate elongation enzymes in the endoplasmic reticulum and mitochondria.

• Rate-Limiting Enzyme: Acetyl-CoA Carboxylase (ACC), which converts Acetyl-CoA into Malonyl-CoA.

  • Required Cofactor: Biotin (Vitamin B7), ATP, and CO2.

  • Hormonal Control: Activated by Insulin and Citrate; inhibited by Glucagon, Epinephrine, and Palmitoyl-CoA.

• Reducing Equivalent: Requires NADPH, supplied predominantly by the Pentose Phosphate Pathway (PPP / HMP Shunt).

beta-Oxidation of Saturated Fatty Acids (Degradation)

• Location: Mitochondrial Matrix.

• Transport Mechanism: Fatty acyl-CoA cannot freely cross the inner mitochondrial membrane and relies on the Carnitine Shuttle.

  • Carnitine Palmitoyltransferase-1 (CPT-1): Rate-limiting enzyme of $\beta$-oxidation. Inhibited by Malonyl-CoA to prevent simultaneous synthesis and breakdown.

• ATP Yield Calculation for Palmitic Acid (16:0)

ATP Yield Calculation for Palmitic Acid (16:0)

• Activation cost: Consumes 2 ATP equivalents (ATP $\rightarrow$ AMP + PPi).

• Number of $\beta$-oxidation cycles = carbons/2 - 1 = 7 cycles

• Yield per cycle: 1 NADH + 1 FADH2.

• Total Acetyl-CoA produced = carbons / 2 = 8 acetyl-CoA

• Modern ATP Yield: (7 \times 2.5 NADH) + (7 x 1.5 FADH2) + (8 x 10 Acetyl-CoA}) - 2 activation ATP = 106 Net ATP. (Note: Older PHLE textbooks referencing classic yields calculate this as 129 Net ATP).

Odd-Chain Saturated FAs

Rare in humans. beta-Oxidation proceeds normally until the final 3-carbon fragment remains: Propionyl-CoA. Propionyl-CoA is converted to Succinyl-CoA (a TCA cycle intermediate) using Biotin (B7) and Vitamin B12 (Cobalamin). A deficiency in B12 leads to the accumulation of Methylmalonic acid.

Cardiovascular Risk

Lauric, Myristic, and Palmitic acids downregulate hepatic LDL receptors, leading to increased plasma LDL cholesterol. Stearic acid (18:0) does not raise LDL because it is rapidly desaturated to oleic acid (18:1) in the liver.

Peroxisomal Oxidation (VLCFAs)

Very Long-Chain Fatty Acids (> 22 carbons, e.g., Lignoceric acid) cannot be oxidized in the mitochondria initially. They must undergo preliminary beta-oxidation in peroxisomes. Defective peroxisomal oxidation leads to Zellweger Syndrome and X-linked Adrenoleukodystrophy (X-ALD).