clinical biochemistry/ second stage dr.khawla a. shemran · web view6- thus during de nova...

Clinical biochemistry/ Second stage Dr.Khawla A. Shemran

Reactions of fatty acid β-oxidationFatty acids stored in adipose tissue, in form of neutral TAG, serve as the body's major fuel storage. TAGs provide concentrated stores of metabolic energy because they are highly reduced and largely anhydrous. Stored triacylglycerols are first hydrolyzed by lipases to release glycerol and free fatty acids. The yield from complete oxidation of fatty acids is nine kcal/g as compared to four kcal/g of protein or carbohydrate

The main oxidation of fatty acid

β Oxidation

α Oxidation

Quantitatively β oxidation of fatty acids is the most important pathway which occurs in the mitochondria.

β oxidation of fatty acids occur on the matrix side of the inner mitochondrial membrane. It’s the most important pathway for fatty acid oxidation. In β oxidation 2 c atoms are cleaved at a time from fatty Acyl CoA molecules starting at the COOH end.

Several enzymes collectively known as fatty acid oxidase are found in the mitochondrial matrix catalyze the oxidation of fatty acyl CoA to acetyl CoA. The initial event in the utilization of fat as an energy source is the hydrolysis of triacylglycerol This process is initiated by hormone-sensitive lipase (HSL) , which removes a fatty acid from carbon 1 and/or carbon 3 of the TAG.

Hormone-mediated regulation of adipocyte lipolysis:

Epinephrine (as well as norepinephrine) and glucagon stimulate fatty acid release from triglycerides stored in adipocyte fat droplets, whereas insulin action is to counter the responses to these two hormones, and conversely, to induce fat storage. Epinephrine and glucagon binding to their respective receptors triggers activation of adenylate cyclase which lead to activation of This enzyme is 'activated when phosphorylated by 3',5'-cyclic AMP-dependent protein kinase while the presence of high plasma levels of insulin and glucose, HSL is dephosphorylated, and becomes inactive.

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Fate of glycerol: The glycerol released during TAG degradation cannot be metabolized by adipocytes because they lack glycerol kinase. Rather, glycerol is transported through the blood to the liver, where it can be phosphorylated. The resulting glycerol phosphate can be used to form TAG in the liver, or can be converted to DHAP by reversal of the glycerol phosphate dehydrogenase reaction DHAP can participate in glycolysis or gluconeogenesis.

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Stages of fatty acid oxidation

)1 (Activation of fatty acids takes place on the outer mitochondrial membrane

(2) Transport into the mitochondria

(3) Degradation to two-carbon fragments (as acetyl CoA) in the mitochondrial matrix (B-oxidation pathway)

(1) Activation:

1- Fatty acids are activated to acyl –CoA by thiokinases or acyl-CoA

synthetases.

2- The reaction occurs in two steps and requires ATP, coenzyme A and

Mg2+.

3- Fatty acid reacts with ATP to form acyladenylate which then combines

with coenzyme A to produce acyl CoA.

4- Two high energy phosphates are utilized, since ATP is converted to

pyrophosphate (PPi).

5- The enzyme inorganic pyrophosphatase hydrolyses PPi to phosphate.

6- The immediate elimination of PPi makes this reaction totally irreversible.

(2) Transport into the mitochondria

Fatty acyl-CoA derivatives are carried across the inner

mitochondrial membrane by conjugation to carnitine.

The carrier process requires

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1-carnitine acyltransferase I (outside)

2- carnitine acyltransferase II (inside).

The two carnitine acyltransferase enzymes (I and II) function on either side of the inner mitochondrial membrane.

Once inside the mitochondrial matrix, fatty acids are oxidized 2 carbons ata time, releasing acetyl-CoA, NADH and FADH2.

(3) (B-oxidation pathway)

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There are four steps in fatty acid β-oxidation pathway:1) oxidation

2) hydration

3) oxidation

4) thiolysis.

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Taken together with acyl-CoA synthetase and carnitine acyltransferase

reactions, 6 steps are actually required to release the first acetyl-CoA

from a fatty acid starting in the cytosol.

After this initial set of reactions, each round of FA oxidation releases 2

carbons as acetyl-CoA, and produces 1 NADH and 1 FADH2. It requires

7 rounds of FA oxidation to metabolize.

*One round of B oxidation: 4 enzyme steps produce acetyl CoA from fatty acyl CoA

Each round generates one molecule each of ;

FADH2 , NADH, Acetyl CoA , Fatty acyl CoA (2 carbons shorter each round)

Fates of the products of B-oxidation

NADH and FADH2 - are used in electron respiratory chain acetyl CoA - enters the citric acid cycle acyl CoA – undergoes the next cycle of oxidation

Regulation of B-oxidation

1-the availability of free fatty acid (AFF) regulates the net utilization through B-oxidation.

2- The level of FFA is controlled by glucagon: insulin ratio.

3- Glucagon increases FFA level and insulin has the opposite effect.

4-CAT-1 is the regulator of entry fatty acid into mitochondria.

5- Malonyl CoA inhibiter CAT -1 activity.

6- Thus during de nova synthesis of fatty acid, the beta oxidation is inhibited.

Oxidation of fatty acids with an odd number of carbons: The β-oxidation of a saturated fatty acid with an odd number of carbon atoms proceeds by the same reaction 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 [Note: Propionyl CoA is also produced during the metabolism of certain amino acids

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Propionyl CoA I s Converted into Succinyl CoA

1. Propionyl CoA is carboxylated to yield the D isomer of methylmalonyl CoA. The hydrolysis of an ATP is required.Enzyme: propionyl CoA carboxylaseCoenzyme: biotin

2. The D isomer of methylmalonyl CoA is racemized to the L isomerEnzyme: methylmalonyl- CoA racemase

3. L isomer of methylmalonyl CoA is converted into succinyl CoA by an intramolecularrearrangementEnzyme: methylmalonyl CoA mutaseCoenzyme: vitamin B12 (cobalamin)

Oxidation of fatty acids produces a large quantity of ATP:* Palmitoyl CoA + 7 FAD + 7 NAD+ + 7 CoA + 7H2O *8 acetyl CoA + 7 FADH2 + 7 NADH + 7 H+

Oxidation of NADH - 3ATP '' FADH2 - 2 ATP

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'' Acetyl-CoA - 12 ATP7 FADH2 yields 147 NADH yields 218 acetyl-CoA yields 96Total 131 ATP

2 high energy phosphate bonds are consumed in the activation of palmitate Net yield is 129 ATP or 129 X 51.6 = 6656 kJ

Peroxisomes oxidize very long chain fatty acids β-oxidation in the peroxisome: Very-long-chain fatty acids(VLCFA),

twenty carbons long or longer, undergo a preliminary β-oxidation in peroxisomes organelles containing enzyme catalase, which catalyzes the dismutation of hydrogen peroxide into water and molecular oxygen The shortened fatty acid is then transferred to a mitochondrion for further oxidation. contrast to mitochondria/ β-oxidation, the initial dehydrogenation peroxisomes is catalyzed by an FAD-containing CoA oxidase. The produced is oxidized by molecular oxygen, which is reduced to H2O2. The H2O2 is reduced to H2O by catalase

α - oxidation of fatty acids are specialized pathways The branched-chain fatty acid, phytanic acid, is not a substrate

for acyl CoA dehydrogenase due to the methyl group on its third carbon Instead, it is hydroxylated at the α-carbon by fatty acid α-hydroxylase

β Oxidation of Unsaturated Fatty Acids The oxidation of unsaturated fatty acids provides less energy than

that of saturated fatty acids because they are less highly reduced and, therefore, fewer reducing equivalents can be produced from these Oxidation of Degradation requires two other enzymes in addition to the β oxidation pathway enzymes:(1) Enoyl CoA isomerase(2) 2,4 Dienoyl CoA reductase

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Inherited defects in the enzymes of -oxidation may lead to non-ketotic hypoglycemia, coma, and fatty liver.

*Fatty acid biosynthesis is different from the β-oxidation

degradation pathway.

1- Fatty acid biosynthesis occurs in the cytosol, β-oxidation occurs in

the mitochondrial matrix.

2- All of the carbon atoms of fatty acid come from acetyl CoA.

3- Intermediates of fatty acid biosynthesis are covalently liked to an

acyl carrier protein. In β-oxidation the fatty acids are attached to

Coenzyme A.

4- The oxidation / reduction reagents of fatty acid biosynthesis are

NADP +/ NADPH whereas the redox reagents of β-oxidation are

NAD +/ NADH and FAD / FADH2.

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clinical biochemistry/ second stage dr.khawla a. shemran · web view6- thus during de nova...

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