Beta-Oxidation of Fatty acids

MUNMUN CHATTERJEE

Definition

• Beta-Oxidation may be defined as the oxidation of fatty acids on the beta-carbon atom.
• This results in the sequential removal of a two carbon fragment, acetyl CoA.

• Three stages
• Activation of fatty acids occurring in the cytosol
• Transport of fatty acids into mitochondria
• Beta-Oxidation proper in the mitochondrial matrix
• Fatty acids are oxidized by most of the tissues in the body.
• Brain, erythrocytes and adrenal medulla cannot utilize fatty acids for energy requirement.

• Fatty acids are activated to acyl CoA by thiokinases or acyl CoA synthetases
• The reaction occurs in two steps and requires ATP,

coenzyme A and Mg²⁺

• Fatty acid reacts with ATP to form acyladenylate which then combines with coenzyme A to produce acyl CoA.
• Two high energy phosphates are utilized, since ATP is

converted to pyrophosphate (PPi).

• The enzyme inorganic pyrophosphafase hydrolyses PPi to phosphate.
• The immediate elimination of PPi makes this reaction

totally irreversible.

O

R-CH₂-CH₂-C-CoA

Acyl CoA

R-CH₂-CH₂-COO^-

Fatty Acid

ATP

PPi

O

Thiokinase

Pyrophosphatase _PPi

R-CH₂-CH₂-C-AMP

Acyladenylate

CoASH

AMP

• The inner mitochondrial membrane is impermeable to fatty acids.
• A specialized carnitine carrier system (carnitine shuttle) operates to transport activated fatty acids from cytosol to the mitochondria.
• This occurs in four steps

1. Acyl group of acyl CoA is transferred to carnitine (β-hydroxy γ-trimethyl aminobutyrate)

catalyzed by carnitine acyltransferasIe (CAT) (present on the outer surface of inner mitochondrial membrane).

2. The acyl-carnitine is transported across the membrane to mitochondrial matrix by a specific carrier protein.
3. Carnitine acyl transferase ll (found on the inner surface of inner mitochondrial membrane) converts acyl-carnitine to acyl CoA.
4. The carnitine released returns to cytosol for reuse.

Cytosol

Carrier

Protein

Acyl CoA

Carnitine

CoASH

Acyl Carnitine

Acyl Carnitine

Carnitine

CoASH

Acyl CoA

CAT-I

CAT-II

Mitochondrial Matrix

Inner Mitochondrial membrane

• Each cycle of β -oxidation, liberating a two carbon unit-acetyl CoA, occurs in a sequence of four reactions

​

• Oxidation
• Hydration
• Oxidation
• Cleavage

1.Oxidation

• Acyl CoA undergoes dehydrogenation by an FAD-dependent flavoenzyme, acyl CoA dehydrogenase.
• A double bond is formed between α and β

carbons (i.e., 2 and 3 carbons)

​

2.Hydration:

• Enoyl CoA hydratase brings
• about the hydration of the double bond to form β -hydroxyacyl CoA.

3.Oxidation

• β-Hydroxyacyl CoA dehydrogenase

catalyses the second oxidation and generates NADH.

• The product formed is β-ketoacyl CoA.

​

4.Cleavage

• The final reaction in β -oxidation is the liberation of a 2 carbon fragment, acetyl CoA from acyl CoA.
• This occurs by a thiolytic cleavage catalysed by β-ketoacyl CoA thiolase (or thiolase).

• The new acyl CoA, containing two carbons less than the original, reenters the β-oxidation cycle.

• The process continues till the fatty acid is completely oxidized.

β-Oxidation of fatty acids

R – CH₂ – CH₂ – CH₂ – C –^SCoA

Acyl CoA

Cytosol

​

Carnitine Transport system

​

Mitochondria

O

Thiokinase

O

_Mg+2

ADP + PPi

R – CH₂ – CH₂ – CH₂ – C – O

Fatty acid

ATP

CoASH

O

R – CH₂ – CH₂ – CH₂ – C – SCoA

Acyl CoA

FAD

2ATP ----- ETC FADH₂

R – CH₂ – CH₂ CH₂ – C – SCoA

Trans-enoyl CoA

Acyl CoA

Dehydrogenase

O

R – CH₂ – CH – CH₂ – C – SCoA

β - Hydroxyacyl CoA

OH

Enoyl CoA Hydratase

O

H₂O

SIDS

OH O

R – CH₂ – CH – CH₂ – C – SCoA

β - Hydroxyacyl CoA

3ATP ----- ETC

^NAD β-Hydroxy Acyl CoA Dehydrogenase

NADH + H⁺

O O

R – CH₂ – C – CH₂ – C – SCoA

β - Ketoacyl CoA

O

R – CH₂ – C – SCoA

Acyl CoA

Thiolase

O

CH₃ – C – SCoA

Acetyl CoA

TCA

Cycle

Acyl CoA

Oxidation of palmitoyl CoA

​

• Palmitoyl CoA + 7 CoASH + 7 FAD ⁺

7 NAD⁺ + 7 H₂O 8 Acetyl CoA + 7 FADH₂ + 7 NADH + 7H⁺

• Palmitoyl CoA undergoes 7 cycles of β - oxidation to yield 8 acetyl CoA.

​

• Acetyl CoA can enter citric acid cycle and get completely oxidized to CO₂ and H₂O.

Energetics of β -oxidation

Sudden infant death syndrome (SIDS)

• Unexpected death of healthy infants, usually overnight
• Due to deficiency of medium chain acyl CoA dehydrogenase.
• Glucose is the principal source of energy, soon after eating or feeding babies.
• After a few hours, the glucose level and its utilization decrease and the rate of fatty acid oxidation must simultaneously increase to meet the energy needs.
• The sudden death in infants is due to a blockade in β -oxidation caused by a deficiency in medium chain acyl CoA dehydrogenase (MCAD)

Jamaican vomiting sickness

• This disease is characterized by severe hypoglycemia, vomiting, convulsions, coma and death.
• lt is caused by eating unriped ackee fruit which contains an unusual toxic amino acid, hypoglycin A.
• This inhibits the enzyme acyl CoA dehydrogenase and thus β -oxidation of fatty acids is blocked, leading to various complications

• Abnormalities in transport of fatty acids into mitochondria & defects in oxidation leads to deficient energy production by oxidation of long chain fatty acids.
• Features:
• Hypoketotic hypoglycemia, hyperammonemia, skeletal muscle weakness & liver diseases.
• Acyl carnitine accumulates when the transferases or translocase is deficient.
• Dietary supplementation of carnitine improve the condition.

• Oxidation of odd chain fatty acids is similar to that of even chain fatty acids.
• At the end 3 carbon unit, propionyl CoA is produced.
• Propionyl CoA is converted into succinyl CoA.
• Succinyl CoA is an intermediate in TCA cycle
• So, propionyl CoA is gluconeogenic.

• Propionyl CoA is carboxylated to D-methyl malonyl CoA by a biotin dependent carboxylase.
• Biotin is B7 vitamin & ATP is utilized in this step.
• Recemase:
• Recemase acts upon D-methyl malonyl CoA to give L-methyl malonyl CoA.
• This reaction is essential for the entry of this compound into metabolic reactions of body.

• Mutase:
• Mutase catalyzes the conversion of L-methyl malonyl CoA (a branched chain compound) to succinyl CoA (a straight chain compound).
• Mutase is an vitamin B₁₂ dependent enzyme.
• Succinyl CoA enters the TCA cycle, & converted into oxaloacetate, it is used for gluconeogenesis.
• Propionyl CoA is also derived from metabolism of valine & isoleucine.

CH₃ I CH₂ I

CO-S-CoA

Propionyl CoA

CH₃ I

H - C- COO^-

I

CO-S-CoA

D-methyl malonyl CoA

CH

3

I

^-OOC – C - H

I

CO-S-CoA

L - methyl malonyl CoA

COO^-

I CH₂ I CH₂ I

CO-S-CoA

Succinyl CoA

ATP

CO₂

Methyl malonyl CoA recemase

Methyl malonyl CoA mutase

Vitamin B

12

TCA

Propionyl CoA carboxylase

Biotin

ADP + Pi

• Propionyl CoA carboxylase deficiency:
• Characterized by propionic acidemia, ketoacidosis & developmental abnormalities.
• Methyl malonic aciduria:
• Two types of methyl malonic acidemias
• Due to deficiency of vitamin B₁₂
• Due to defect in the enzyme methyl malonyl CoA mutase or recemase.
• Accumulation of methyl malonic acid in the body.

• Methyl malonic acid is excreted into urine.
• Symptoms:
• Severe metabolic acidosis, damages the central nervous system & growth retardation.
• Fetal in early life.
• Treatment:
• Some patients respond to treatment with pharmacological doses of B₁₂.

• Oxidation of fatty acids on α-carbon atom is known as α-oxidation.
• In this, removal of one carbon unit from the carboxyl end.
• Energy is not produced.
• No need of fatty acid activation & coenzyme A
• Hydroxylation occurs at α-carbon atom.
• It is then oxidized to α-keto acid.
• This, keto acid undergoes decarboxylation, yielding a molecule of CO₂ & FA with one carbon atom less.

• Occurs in endoplasmic reticulum.
• Some FA undergo α - oxidation in peroxisomes.
• α- oxidation is mainly used for fatty acids that have a methyl group at the beta-carbon, which blocks beta- oxidation.
• Major dietary methylated fatty acid is phytanic acid.
• It is derived from phytol present in chlorophyll, milk & animal fats.

Refsum’s disease

• Due to deficiency of the enzyme α-hydroxylase (phytanic acid oxidase)
• α – oxidation does not occur.
• Phytanic acid does not converted into compound that can be degraded by beta –oxidation.
• Phytanic acid accumulates in tissues.
• Symptoms:
• Severe neurological symptoms, polyneuropathy, retinitis pigmentosa, nerve deafness & cerebellar ataxia.
• Restricted dietary intake of phytanic acid

(including milk-is a good source of phytanic acid)

Omega- oxidation

• Minor pathway, takes place in microsomes.
• Catalyzed by hydroxylase enzymes involving NADPH & cytochrome P-450.
• Methyl (CH₃) group is hydroxylated to CH₂OH & subsequently oxidized with the help of NAD⁺ to COOH group to produce dicarboxylic acids.
• When β-oxidation is defective & dicarboxylic acids are excreted in urine causing dicarboxylic aciduria.