ALPHA OXIDATION by This Process, the Long Chain Fatty Acid Is Gradually Broken Down Until It Is Reduced to 12 C-Atoms

COMPILED AND CIRCULATED BY PROF. NANDITA BHAKAT, ASSISTANT PROFESSOR, DEPARTMENT OF BOTANY, NARAJOLE RAJ COLLEGE.

ALPHA OXIDATION By this process, the long chain fatty acid is gradually broken down until it is reduced to 12 C-atoms. Fatty acids with less than 13C-atoms are not affected by this process. One complete α oxidation results in the elimination of one carbon atom in the form of CO2 from the -COOH group of the fatty acid, while α carbon atom i.e., C no. 2 (which is adjacent to -COOH is oxidized: α oxidation).

Αlpha - oxidation takes place as follows:

1) The fatty acid is oxidatively decarboxylated in the presence of fatty acid

peroxidase and H2o2 to form an aldehyde. In this reaction CO2 comes from the carboxylic group and oxidation takes place at α-carbon atom which becomes converted in to the aldehyde group.

Fatty acid Peroxidase RCH2CH2CH2COOH RCH2CH2CHO + CO2 + H2O Fatty acid 2(H2O2) Aldehyde

2) The aldehyde is further oxidized in the presence of aldehyde dehydrogenase to form the new fatty acid containing one carbon atom less than in the original fatty acid. NAD+ is reduced in the reaction.

Aldehyde dehydrogenase + + RCH2CH2CHO + NAD RCH2CH2COOH + NADH + H

Aldehyde H2O2 New fatty acid

BOTANY: SEM-VI, PAPER-C13T: PLANT METABOLISM, UNIT-5: ATP SYNTHESIS.

COMPILED AND CIRCULATED BY PROF. NANDITA BHAKAT, ASSISTANT PROFESSOR, DEPARTMENT OF BOTANY, NARAJOLE RAJ COLLEGE.

The new fatty acid will be oxidized again and again, till it consists of 12-C atoms by the same process of α- oxidation.

SIGNIFICANCE OF ALPHA-OXIDATION

 α- oxidation is important in the catabolism of branched-chain fatty acids.

 Oxidation of methylated fatty acid.

 Production of cerebronic acid which synthesizes cerebroside and sulfatides.

 Production of odd chain fatty acids.

 The reaction is also a route for the synthesis of hydroxy fatty acids. The α-hydroxy fatty acid can be further oxidized and decarboxylated to a fatty acid one carbon shorter than the original. Thus, if an odd- chain-length compound is used initially, an even-chain-length acid is produced that can be further oxidized by β-oxidation.

BOTANY: SEM-VI, PAPER-C13T: PLANT METABOLISM, UNIT-5: ATP SYNTHESIS.

COMPILED AND CIRCULATED BY PROF. NANDITA BHAKAT, ASSISTANT PROFESSOR, DEPARTMENT OF BOTANY, NARAJOLE RAJ COLLEGE.

BETA -OXIDATION

β- oxidation is the chief process of fatty acids degradation in plants. While this mechanism is well established for saturated fatty acids, it remains obscure for unsaturated fatty acids. Β oxidation takes place in mitochondria and also in glyoxysomes and involves sequential

removal of 2C in the form of Acetyl-CoA (CH3CO.SCoA) molecules from the carboxyl end of the fatty acid. This is called as β-oxidation as the β-C (i.e., C no. 3) of the fatty acid is oxidised during this process. Various steps of β- oxidation are as follows: 1) The first step involves the activation of fatty acid in the presence of ATP and enzyme thiokinase. CoASH is consumed and CoA derivative of fatty acid is produced.

Mg++

RCH2CH2COOH + CoASH + ATP

RCH2CH2CSCoA + AMP + P-Pi Pyrophosphate O Fatty acyl- CoA

The AMP molecule thus produced reacts with another ATP molecule under the catalytic influence of the enzyme adenylate kinase to form 2ADP molecules.

BOTANY: SEM-VI, PAPER-C13T: PLANT METABOLISM, UNIT-5: ATP SYNTHESIS.

COMPILED AND CIRCULATED BY PROF. NANDITA BHAKAT, ASSISTANT PROFESSOR, DEPARTMENT OF BOTANY, NARAJOLE RAJ COLLEGE.

AMP + ATP ADP + ADP

Fig: β- oxidation of fatty acid

BOTANY: SEM-VI, PAPER-C13T: PLANT METABOLISM, UNIT-5: ATP SYNTHESIS.

COMPILED AND CIRCULATED BY PROF. NANDITA BHAKAT, ASSISTANT PROFESSOR, DEPARTMENT OF BOTANY, NARAJOLE RAJ COLLEGE.

2) In the second step, two hydrogen atoms are removed between α and β carbon atoms. And a trans α, β- unsaturated fatty acyl CoA is formed. This is catalysed by FAD- containing enzyme acyl-CoA dehydrogenase. H O RCH2CH2CSCoA + FAD RC=CCSCoA + FADH2 o H

Fatty acyl CoA Trans α, β unsaturated fatty acyl CoA 3) The third step involves the addition of a water molecule across the double bond to form corresponding β- hydroxyacyl-CoA in the presence of enoyl hydrase.

H O RC=CCSCoA + H2O RCHCH2CSCoA H OH O β hydroxy acyl-CoA 4) In the fourth step, β- hydroxyacyl-CoA is dehydrogenates in the presence of NAD-specific β- hydroxyacyl-CoA dehydrogenase. Two hydrogen atoms are removed from the β- carbon atom (β- oxidation) which now bears a carbonyl function and β- keto fatty acyl CoA is formed.

BOTANY: SEM-VI, PAPER-C13T: PLANT METABOLISM, UNIT-5: ATP SYNTHESIS.

COMPILED AND CIRCULATED BY PROF. NANDITA BHAKAT, ASSISTANT PROFESSOR, DEPARTMENT OF BOTANY, NARAJOLE RAJ COLLEGE.

+ + RCH.CH2CSCoA + NAD RCCH2CSCoA + NADH +H O O O β keto fatty acylCoA

5) The fifth and the last step involves the thioclastic cleavage of β- keto fatty acyl CoA and results in the formation of an active 2-C unit acetyl- CoA and a fatty acyl-CoA molecule which is shorter by two carbon atoms than when it entered the β oxidation spiral.

RCCH2CSCoA + CoASH RCSCoA + CH3COSCoA

O O O Acetyl CoA Fatty acyl CoA The fatty acyl-CoA so produced again re-enters the β-oxidation spiral at step 2 losing a further 2-C unit. This sequence continues until whole molecule is degraded.

SIGNIFICANCE OF β - OXDATION

+ Each turn of the β-oxidation generates one FADH2, one NADH+H and one acetyl CoA molecule will be produced which on reoxidation through electron transport chain will yield 2 and 3 ATP molecules respectively. Thus each turn of β oxidation generates 5 ATP molecules. However in the first turn there is consumption of 2 ATP molecules in the first step hence, in this turn there will be a net gain of only 3 ATP molecules.

BOTANY: SEM-VI, PAPER-C13T: PLANT METABOLISM, UNIT-5: ATP SYNTHESIS.