-OXIDATION

Importance of fatty acid

Fatty acids are building blocks of phospholipids and glycolipids

Many proteins are modified by the covalent attachment of fatty acids, which targets them to membrane locations

Fatty acids are fuel molecules : They are stored as TAGs (uncharged esters of fatty acids with glycerol) Fatty acids mobilized from TAGs are oxidized to meet the

energy needs of a cell or organism

Fourth, fatty acid derivatives serve as hormones and intracellular messengers

Importance of fatty acid as fuel

TAGs are highly concentrated stores of metabolic energy because they are reduced and anhydrous

The yield from the complete oxidation of fatty acids is about 9 kcal g-1 (38 kJ g-1), in contrast with about 4 kcal g-1 (17 kJ g-1) for carbohydrates and proteins

Consider a typical 70-kg man, who has fuel reserves of 100,000 kcal in TAGs, 25,000 kcal in protein, 600 kcal in glycogen, and 40 kcal in glucose

TAGs constitute about 11 kg of his total body weight

Examples

Golden plover

The ruby-throated hummingbird

β-Oxidation of fatty acids

Fatty acid in body mostly oxidised by β-oxidation

Oxidation of fatty acid on the β carbon

Two-carbon fragments are successively removed from the carboxyl end of the fatty acyl CoA, producing acetyl CoA, NADH, and FADH2

Tissue location for oxidation : Most of the tissue in the body

Steps of -oxidation

Activation of fatty acid occuring in cytosol

Transport of Fatty acids into mitochondria

-Oxidation proper in mitochondrial matrix.

Activation of fatty acid Eugene Kennedy and Albert Lehninger showed in

1949 that fatty acids are oxidized in mitochondria

Subsequent work demonstrated that they are activated before they enter the mitochondrial matrix

ATP drives formation of a thioester linkage between the carboxyl group of FAs and the sulfhydryl group of CoA

Activation reaction : outer mitochondrial membrane, catalyzed by acyl CoA synthetase

Activation of fatty acid

Transport of long-chain fatty acids (LCFA) into the mitochondria

Special transport mechanism : Carnitine shuttle

Activated LCFA are transported across the membrane by conjugating them to carnitine, a zwitterionic alcohol

The acyl group is transferred from the sulfur atom of CoA to the hydroxyl group of carnitine to form acyl carnitine (carnitine acyltransferase I)

Second, the acylcarnitine is transported into the mitochondrial matrix in exchange for free carnitine by carnitine–acylcarnitine translocase (CPT-II, or CAT-II)

Transport of long-chain fatty acids (LCFA) into the mitochondria

Acyl Carnitine Translocase

The entry of acyl carnitine into the mitochondrial matrix is mediated by a translocase

Carnitine returns to the cytosolic side of the inner mitochondrial membrane in exchange for acyl carnitine

Inhibitor of the Carnitine shuttle Malonyl CoA inhibits CPT-I, thus preventing the entry of

long-chain acyl groups into the mitochondrial matrix

Therefore, newly made palmitate cannot be transferred into the mitochondria and degraded

The phosphorylation and inhibition of acetyl CoA carboxylase decreases malonyl CoA production, removing the break on fatty acid oxidation

Fatty acid oxidation is also regulated by the acetyl CoA to CoA ratio: As the ratio increases, the thiolase reaction decreases

Sources of Carnitine

Diet, found primarily in meat products

Also synthesized from the amino acids lysine and methionine

This enzymatic pathway found in the liver and kidney but not in skeletal or heart muscle

Skeletal muscle : 97% of all carnitine in the body

Carnitine deficiencies

Decreased ability of tissues to use LCFA as a metabolic fuel

Results in the accumulation of toxic amounts of free fatty acids and branched-chain acyl groups in cells.

Causes of Secondary carnitine deficiency

1. Patients with liver disease

2. Malnutrition or those on strictly vegetarian diets

3. Increased requirement for carnitine (for e.g, pregnancy, severe infections, burns, or trauma)

4. Patients undergoing hemodialysis

Congenital deficiencies of Carnitine palmitoyltransferase (CPT) system, cause primary carnitine deficiency

Genetic CPT-I deficiency affects the liver

CPT-II deficiency occurs primarily in cardiac and skeletal muscle

Symptoms of carnitine deficiency range from cardiomyopathy to muscle weakness with myoglobinemia following prolonged exercise

Primary carnitine deficiency

Entry of short- and medium-chain fatty acids into the mitochondria

Fatty acids shorter than 12-C can cross the inner mit. membrane without the aid of carnitine or the CPT system

Once inside the mitochondria, they are activated to their CoA derivatives by matrix enzymes, and are oxidized.

β-Oxidation proper A saturated acyl CoA is degraded by a recurring sequence

of four reactions: Oxidation by FAD, hydration, oxidation by NAD+, and

thiolysis by CoA

The fatty acyl chain is shortened by 2-C atoms as a result of these reactions

FADH2, NADH, and acetyl CoA are generated

Because oxidation is on the β-carbon, this series of reactions is called the β-oxidation pathway

Enzymes involved in the β-oxidation of Fatty acyl Coa

Oxidation: the acyl CoA undergoes dehydrogenation by acyl CoA dehydrogenase.

Hydration: the enoyl CoA hydratase brings about the hydration of double bond to form β-hydroxy acyl CoA.

Oxidation: β-hydroxy acyl CoA dehydrogenase catalyses the oxidation to form β-keto acyl CoA.

Thiolytic cleavage: the thiolase cleaves acetyl CoA from acyl CoA

Figure 3. Processing and -oxidation of palmitoyl CoA

matrix side

inner mitochondrialmembrane

2 ATP3 ATP

respiratory chain

recycle6 times

Carnitinetranslocase

Palmitoylcarnitine

Palmitoylcarnitine

Palmitoyl-CoA

+ Acetyl CoA

CH3-(CH)12-C-S-CoA

O

oxidationFAD

FADH2

hydration H2O

thiolase CoA

oxidationNAD+

NADH

Citricacid cycle 2 CO2

S UMMARY

Energy yield from fatty acid oxidation

For example, the oxidation of a molecule of palmitoyl CoA to CO2 and H2O produces 8 acetyl CoA, 7 NADH, and 7 FADH2

The Complete Oxidation of Palmitate Yields 129 Molecules of ATP

Comparision between FA synthesis and β-oxidation

Medium-Chain Fatty Acyl CoA Dehydrogenase (MCAD) deficiency

In mitochondria, there are 4 fatty acyl CoA dehydrogenase species (SCFA, MCFA, LCFA, VLCFA)

MCAD deficiency : autosomal recessive disorder, is one of the most common inborn errors of metabolism

Incidence - 1:12,000 births in the West, and 1:40,000 worldwide

It causes a decrease in fatty acid oxidation and severe hypoglycemia

Treatment includes a carbohydrate-rich diet

Oxidation of fatty acids with an odd number of carbons

Reactions proceeds by the same steps as that of fatty acids with an even number, until the final three carbons are reached

This compound, propionyl CoA, is metabolized by a three-step pathway

Only example of a glucogenic precursor generated from fatty acid oxidation

Oxidation of unsaturated fatty acids Provides less energy than that of saturated fatty acids

Oxidation of monounsaturated fatty acids, such as 18:1(9) (oleic acid) requires one additional enzyme, 3,2-enoyl CoA isomerase

Oxidation of PUFAs, such as 18:2(9,12) (linoleic acid), requires an NADPH-dependent 2,4-dienoyl CoA reductase in addition to the isomerase

So, an Isomerase and a Reductase Are Required for the Oxidation of Unsaturated Fatty Acids

β-oxidation in the peroxisome

VLCFAs, or those 20 carbons long or longer, undergo a preliminary β-oxidation in peroxisomes.

The shortened fatty acid is then transferred to a mitochondrion for further oxidation.

In contrast to mitochondrial β-oxidation, the initial dehydrogenation in peroxisomes is catalyzed by an FAD-containing acyl CoA oxidase.

Zellweger syndrome

X-linked adrenoleukodystrophy : Genetic defects in the ability to transport VLCFA across the peroxisomal membrane

A peroxisomal biogenesis disorder in all tissues resulting from the absence of functional peroxisomes

Characterized by liver, kidney, and muscle abnormalities and usually results in death by age six

The syndrome is caused by a defect in the import of enzymes into the peroxisomes

α-Oxidation of Fatty acids

Branched-chain fatty acid, phytanic acid: not a substrate for acyl CoA dehydrogenase because of the methyl group on its third (β) carbon

Instead, it is hydroxylated at the α-carbon by fatty acid α-hydroxylase

The product is decarboxylated and then activated to its CoA derivative, which is a substrate for the enzymes of β-oxidation.

Refsum’s disease

Rare, autosomal recessive disorder caused by a deficiency of α-hydroxylase

This results in the accumulation of phytanic acid in the plasma and tissues

Symptoms are primarily neurologic

Treatment involves dietary restriction to halt disease progression

ω-Oxidation

ω-Oxidation (at the methyl terminus) also is known, and generates dicarboxylic acids.

Normally a minor pathway of the ER

Its up-regulation is seen with conditions such as MCAD deficiency that limit fatty acid β-oxidation

REFERENCES

Biochemistry 5th edition by Jeremy M. Berg, JL Tymoczko, Lubert Stryer

Lippincots Illustrated Biochemistry 3rd edition

Harpers Illustrated Biochemistry 28th edition

Lehningers principles of Biochemistry, 5th edition