Explain beta oxidation of fatty acids

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beta oxidation of fatty acids pathway diagram

This pathophysiology diagram illustrates the role of oxidative stress in Metabolic Associated Fatty Liver Disease (MAFLD). The pathway begins with metabolic triggers: insulin resistance, high-fat diet, obesity, and hyperinsulinemia, leading to lipolysis and increased free fatty acids (FFAs). FFAs contribute to lipid droplet formation (steatosis) and altered beta-oxidation. A central red starburst represents Reactive Oxygen Species (ROS), which mediates several downstream pathological processes: lipid peroxidation (TBARS, MDA markers), mitochondrial dysfunction (CYPE1, CYP4A), endoplasmic reticulum (ER) stress (CHOP, BiP, PDI), and hepatic stellate cell (HSCs) activation. These processes culminate in liver inflammation (TNF-alpha, IL-8, IL-6) and ultimately fibrosis, marked by alpha-SMA and COL1A1. The diagram also identifies therapeutic action sites for Vitamin D, Vitamin E, Silybin, and Extra Virgin Olive Oil (EVOO). These antioxidants are shown to upregulate protective mechanisms like Nrf2 and antioxidant enzymes (CAT, SOD, GPx, GSH) while downregulating NF-kB, inflammation, and fibrotic markers. The visual focuses on the molecular transition from simple steatosis to advanced fibrosis driven by oxidative damage.

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beta oxidation steps acyl CoA mitochondria

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Beta Oxidation of Fatty Acids

Beta oxidation is the primary catabolic pathway for fatty acids, occurring mainly in the mitochondrial matrix. It systematically cleaves 2-carbon units from the fatty acyl chain, generating acetyl-CoA, FADH2, and NADH with each cycle. These products feed the TCA cycle and the electron transport chain to produce large amounts of ATP.

Overview

Beta oxidation occurs principally in mitochondria but also in peroxisomes (for very-long-chain fatty acids). The pathway is named because it is the beta-carbon (second carbon from the carboxylate end) that undergoes oxidation during each cycle.

Step 0: Activation - Formation of Fatty Acyl-CoA (Cytoplasm)

Before entering the pathway, a free fatty acid must be activated:

Fatty acid + CoA + ATP → Fatty acyl-CoA + AMP + Pyrophosphate

This reaction is catalyzed by fatty acyl-CoA synthetase (also called acyl-CoA synthetase or thiokinase), located on the outer mitochondrial membrane. ATP is consumed (equivalent to 2 high-energy bonds, since AMP + PPi are produced and pyrophosphate is hydrolyzed). The product is a fatty acyl-CoA thioester.

Guyton and Hall Textbook of Medical Physiology, p. 845

Step 1: Transport into Mitochondria - The Carnitine Shuttle

Long-chain fatty acyl-CoA (>12 carbons) cannot cross the inner mitochondrial membrane directly. A dedicated transport system is required:

Carnitine palmitoyltransferase I (CPT-I) (outer mitochondrial membrane): transfers the acyl group from CoA to carnitine, forming fatty acylcarnitine.
Acylcarnitine:carnitine translocase: shuttles fatty acylcarnitine into the matrix in exchange for free carnitine.
Carnitine palmitoyltransferase II (CPT-II) (inner mitochondrial membrane): reverses the reaction, regenerating fatty acyl-CoA inside the matrix.

CPT-I is the key regulatory checkpoint for beta oxidation. It is potently inhibited by malonyl-CoA (the committed intermediate in fatty acid synthesis), preventing both processes from occurring simultaneously.

Short- and medium-chain fatty acids can enter the mitochondrial matrix without carnitine.

Sleisenger and Fordtran's Gastrointestinal and Liver Disease

The Four Repeating Steps of Beta Oxidation (the "Spiral")

Here is the reaction diagram from Guyton & Hall:

Beta oxidation of fatty acids - 5 reaction steps showing enzymes and products

Figure 69.3 from Guyton & Hall - Beta oxidation of fatty acids to yield acetyl-CoA

Each cycle consists of four enzymatic reactions:

Step	Reaction	Enzyme	Coenzyme
1. Oxidation	Fatty acyl-CoA → trans-2-enoyl-CoA	Acyl-CoA dehydrogenase	FAD → FADH₂
2. Hydration	trans-2-enoyl-CoA + H₂O → L-3-hydroxyacyl-CoA	Enoyl-CoA hydratase	-
3. Oxidation	L-3-hydroxyacyl-CoA → 3-ketoacyl-CoA	3-Hydroxyacyl-CoA dehydrogenase	NAD⁺ → NADH
4. Thiolysis	3-ketoacyl-CoA + CoA → acetyl-CoA + acyl-CoA (2C shorter)	Thiolase (β-ketothiolase)	-

Step 1 - Acyl-CoA Dehydrogenase: Creates a trans double bond between carbons 2 and 3 (α and β carbons). There are three isoforms: VLCAD (very long-chain), MCAD (medium-chain), and SCAD (short-chain), each specific to fatty acid chain length. Two electrons are transferred to FAD, generating FADH₂.

Step 2 - Enoyl-CoA Hydratase: Adds water across the double bond, generating the L-stereoisomer of 3-hydroxyacyl-CoA.

Step 3 - 3-Hydroxyacyl-CoA Dehydrogenase: Oxidizes the hydroxyl group at carbon 3 to a keto group, reducing NAD⁺ to NADH.

Step 4 - Thiolase: Cleaves the bond between carbons 2 and 3 by attacking with a second CoA molecule. This releases acetyl-CoA (2-carbon unit) and a new acyl-CoA that is 2 carbons shorter than the original. This shortened acyl-CoA re-enters the cycle.

Guyton and Hall Textbook of Medical Physiology, p. 845
Sleisenger and Fordtran's Gastrointestinal and Liver Disease

Fate of Products

Acetyl-CoA enters the TCA (citric acid) cycle, combining with oxaloacetate to form citrate. Each acetyl-CoA ultimately generates 10 ATP via the TCA cycle and oxidative phosphorylation.

FADH₂ and NADH enter the electron transport chain at complexes II and I respectively, generating ~1.5 ATP (FADH₂) and ~2.5 ATP (NADH) per molecule via oxidative phosphorylation.

In the liver specifically, excess acetyl-CoA can be directed into ketone body synthesis (acetoacetate, β-hydroxybutyrate, acetone) via the HMG-CoA pathway - this only occurs in liver mitochondria.

ATP Yield: Example with Palmitic Acid (C16:0)

Palmitate (16-carbon saturated fatty acid) undergoes 7 cycles of beta oxidation to produce 8 acetyl-CoA molecules:

Source	Quantity	ATP/molecule	ATP yield
FADH₂ from beta oxidation	7	1.5	10.5
NADH from beta oxidation	7	2.5	17.5
Acetyl-CoA → TCA cycle	8	10	80
Activation cost			-2
Net ATP			~106

For stearic acid (C18), Guyton & Hall cite a net yield of 146 ATP molecules.

Guyton and Hall Textbook of Medical Physiology, p. 846

Regulation of Beta Oxidation

Beta oxidation is regulated at several levels:

Malonyl-CoA inhibition of CPT-I - the primary control point. When carbohydrates are plentiful, malonyl-CoA (first committed intermediate in fatty acid synthesis) rises and blocks CPT-I, shutting off beta oxidation.
Product inhibition - high levels of FADH₂, NADH, and acetyl-CoA inhibit the pathway enzymes when energy status is high.
Hormonal control - glucagon and epinephrine activate hormone-sensitive lipase, releasing more free fatty acids for oxidation; insulin has the opposite effect.

Basic Medical Biochemistry - A Clinical Approach, 6e

Special Cases

Unsaturated Fatty Acids

Require additional enzymes:

Isomerase (for monounsaturated): converts cis-3-enoyl to trans-2-enoyl for normal beta oxidation.
Reductase (for polyunsaturated): additional NADPH-requiring step, yielding one fewer FADH₂ per double bond.

Odd-Chain Fatty Acids

Beta oxidation of odd-chain fatty acids eventually produces propionyl-CoA (3-carbon) as the final product. This is converted to succinyl-CoA via propionyl-CoA carboxylase (requires biotin) and methylmalonyl-CoA mutase (requires vitamin B12), entering the TCA cycle.

Peroxisomal Beta Oxidation

Handles very-long-chain fatty acids (>C22) that cannot be directly processed by mitochondria
The first oxidation step transfers electrons to FAD → FADH₂, which is then passed directly to O₂ → H₂O₂ (detoxified by catalase) - this means no ATP is generated from this step
Products (medium-chain acyl-CoA) are eventually transferred to mitochondria for completion
Defects in peroxisomal biogenesis cause Zellweger syndrome
Sleisenger and Fordtran's Gastrointestinal and Liver Disease
Basic Medical Biochemistry - A Clinical Approach, 6e

Clinically Relevant Disorders

Disorder	Defect	Consequence
MCAD deficiency	Medium-chain acyl-CoA dehydrogenase	Hypoglycemia, hypoketosis, sudden death in infants
Carnitine deficiency	Membrane carnitine transporter or CPT deficiency	Inability to oxidize long-chain FA; cardiomyopathy, muscle weakness
Zellweger syndrome	Peroxisome biogenesis	Accumulation of very-long-chain fatty acids; liver/brain disease
Trifunctional protein (TFP) deficiency	Mitochondrial trifunctional protein (steps 2-4)	Hypoglycemia, hypoketonemia, liver problems
Jamaican vomiting sickness	Hypoglycin from ackee fruit inhibits acyl-CoA dehydrogenase	Severe hypoglycemia, can be fatal
Refsum disease	Phytanoyl-CoA hydroxylase (peroxisomal α-oxidation)	Phytanic acid accumulation; retinitis pigmentosa, ataxia, neuropathy

Basic Medical Biochemistry - A Clinical Approach, 6e
Goldman-Cecil Medicine

Summary of the Full Pathway

Fatty acid (cytoplasm)
     ↓  [Acyl-CoA synthetase, -2 ATP equivalent]
Fatty acyl-CoA
     ↓  [Carnitine shuttle via CPT-I / translocase / CPT-II]
Fatty acyl-CoA (mitochondrial matrix)
     ↓  ↙ Repeat 7x (for palmitate)
FOUR STEPS PER CYCLE:
  1. Acyl-CoA dehydrogenase → FADH₂
  2. Enoyl-CoA hydratase
  3. 3-Hydroxyacyl-CoA dehydrogenase → NADH
  4. Thiolase → Acetyl-CoA released
     ↓
Acetyl-CoA → TCA cycle → CO₂ + H₂O + ATP
FADH₂/NADH → Electron transport chain → ATP

Beta oxidation is a highly efficient energy pathway - fatty acids yield roughly 2.5 times more ATP per gram than carbohydrates, making stored fat the body's primary long-term energy reserve.

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