Let us make an in-depth study of the process of α – oxidation and β – oxidation in plants.
Long chain fatty acids are now broken down by the processes of a-oxidation and β-oxidation. The later ultimately produces the active 2-C units, the acetyl-CoA (CH3CO.CoA).
α-Oxidation:
By this process, the long chain of fatty acid is gradually broken down until it is reduced to 12 C-atoms. Fatty acids with less than 13-C 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-C-atom i.e., C atom no. 2 (which is adjacent to — COOH) is oxidised (α-oxidation).
α-oxidation takes place as follows:
(1) The fatty acid is oxidatively decarboxylated in 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 a-C-atom which becomes converted into the aldehyde group.
(2) The aldehyde is further oxidised 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.
The new fatty acid will be oxidised again and again, till it consist of 12-C atoms by the same process of a-oxidation (Fig. 14.1).
(Significance of α-oxidations in plants is disputed. One NADH produced during α-oxidation might be expected to be re-oxidised by terminal electron transport chain to yield ATPs, but this has not been demonstrated. Recent findings suggest that α-oxidation is not linked to production of ATP molecules.
It is considered that α-oxidations are a means to produce fatty acids with odd no. of C atoms. Long chain aldehydes produced during α-oxidations may give rise to long chain alcohols after reduction. The latter are then consumed in wax formation. It is also probable that α-D-hydroxy fatty acids are formed by offshoot of the α-oxidation spiral. The former are chief acyl components of cerebrosides in higher plants.)
β-Oxidation:
β-Oxidation is the chief process of fatty acids degradations 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 2-C in the form of acetyl-CoA (CH3CO.SCoA) molecules from the carboxyl end of the fatty acid. This is called as β-oxidation because β-C (i.e., C atom No. 3) of the fatty acid is oxidised during this process.
Various steps of β-oxidation which is also shown in Fig. 14.2 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.
The AMP (Adenosine Mono Phosphate) molecule thus produced reacts with another ATP molecule under the catalytic influence of the enzyme adenylate kinase to form 2ADP molecules.
(2) In the second step two hydrogen atoms are removed between a and β-C atoms and a trans α, β-unsaturated fatty acyl CoA is formed. This is catalysed by FAD-containing enzyme acyl-CoA dehydrogenase.
(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.
(4) In the fourth step β-hydroxyacyl-CoA is dehydrogenated in the presence of NAD- specific 18-hydroxyacyl-CoA dehydrogenase. Two hydrogen atoms are removed from the β-C atom (β-oxidation) which now bears a carbonyl function and β-keto fatty acyl Co A is formed.
(5) The fifth and the last step involves the thioclastic cleavage of β-keto fatty acyl-CoA in the presence of the enzyme β-ketoacyl thiolase 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 P-oxidation spiral.
The fatty acyl-CoA so produced again re-enters the β-oxidation spiral at step 2 (bypassing the first step as it is already activated) losing a further 2-C unit. This sequence continues until whole molecule is degraded.
Each turn of the β-oxidation generates one FADH2 (step 2), one NADH + H+ (step 4) and one acetyl-CoA molecule (5th step). However, in the last turn of the spiral two acetyl-CoA molecules will be produced. Re-oxidation of FADH2 and NADH + H+ by the electron transport chain will yield 2 and 3 ATP molecules respectively. Thus each turn of P-oxidation generates 5 ATP molecules. However, in the first turn there is consumption of 2 ATPs in the first step hence, in this turn there will be a net gain of only 3 ATP molecules.
Complete oxidation of one acetyl-CoA molecule in TCA cycle to CO2 and H2O will result in the production of 12 ATP molecules. It is quite obvious from the above account that huge amount of energy is generated in the form of ATP molecules by the mitochondrial oxidation of fatty acids through the β-oxidation spiral and TCA cycle.
For instance, one molecule of palmitic acid (with 16 C atoms) on complete oxidation will produce 129 ATP molecules as follows:
(Complete oxidation of stearic acid (with 18-C) through p-oxidation spiral and TCA cycle will result in production of 146 ATP molecules).
Fate of Acetyl-CoA (CH3CO.CoA):
Acetyl-CoA units which are end-products of P-oxidation of fatty acids may enter:
(i) Into Krebs’ Cycle (TCA Cycle) and are oxidised to release energy as mentioned earlier, or
(ii) In case of germination of fatty seeds, they are converted into soluble sucrose through the glyoxylic acid cycle.