In this article we will discuss about the breakdown of fatty acids.
Fats are found in dynamic state in plants, i.e., at one time they are synthesized; while at other time they break down to meet specific requirement of the cells.
Active breakdown of fats (insoluble) takes place as follows:
(i) During germination of fatty seeds so that the decomposition products may enter into glycolysis and Kreb’s cycle to release energy and also to synthesise soluble sucrose through glyoxylic acid cycle which is then translocated to the growing regions of the young germinating seedling till it develops green leaves to manufacture its own food.
(ii) In plants, when carbohydrates reserve declines, the fats (and also proteins), may form the respiratory substrates which are broken down and oxidised to release energy.
The fats are first hydrolysed in the presence of the enzymes lipases to yield fatty acids and glycerol.
Oxidation of Glycerol:
The glycerol may react with ATP under the catalytic influence of glycerol kinase to form glycerol-3-phosphate which is then oxidised in the presence of glycerol-3-phasphate dehydrogenase and NAD+ to produce dihydroxyacetone phosphate and enters into glycolysis.
The conversion of glycerol into pyruvic acid that takes place in cytoplasm yields 2ATPs by substrate level phosphorylation and 2NADH which on reoxidation by terminal electron transport chain via the external NADH dehydrogenase (located on the outer surface of the inner mitochondrial membrane in plants) further generate 4 ATP molecules (2 mol./NADH oxidised).
If the pyruvic acid also undergoes complete oxidation into CO2 and H2O in Kreb’s cycle (or TCA cycle), it will produce another 15ATP molecules. Thus a total of 2 + 4 + 15 = 21ATPs are produced per glycerol moleule oxidised. However, 1 ATP molecule is consumed in the glycerol kinase catalysed reaction. Hence, the net gain is 21 – 1 = 20 ATPs per glycerol molecule oxidised.
Oxidation (Breakdown) of Fatty Acids:
Here, the long chain of fatty acid is gradually broken down until it is reduced to 12C-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, whereas α-C-atom, i.e., C-atom no., 2, adjacent to COOH is oxidised (α-oxidation).
Process of α–oxidation is as follows:
(i) The fatty acid is oxidatively decarboxylated in presence of enzyme fatty acid peroxidase and H2O2 to form an aldehyde. Here, CO2, comes out from the carboxylic group and oxidation takes place at a-C-atom that converts into aldehyde group.
(ii) Now, the aldehyde is further oxidised in the presence of enzyme 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 oxidise repeatedly, till it consists of 12-C atoms by the same process of a-oxidation.
β -oxidation is the main process of fatty acid degradation in plants. This mechanism is well established for saturated fatty acids, while obscure for unsaturated fatty acids.
β -oxidation takes place in mitochondria and also in glyoxysomes. It involves sequential removal of 2-C in the form of acetyI-CoA (CH3CO. SCoA) molecules from the caboxyl end of fatty acid. This is called β-C (i.e., C atom No.3) of the fatty oxidized during this process.
Various steps of this process are as follows:
(i) 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 enzyme adenylate kianes to form 2 ADP molecules.
(ii) In the second step of β-oxidation, two hydrogen atoms are removed between α and β-C atom and a trans α, β-unsaturated fatty acyl CoA is formed. This reaction is catalysed by FAD-containing enzyme acyl-CoA dehydrogenase.
(iii) The third step involves the addition of a water molecule across the double bond to form corresponding β-hydroxyacyle-CoA in the presenceof enzyme enoyl hydrase.
(iv) In the fourth step β-hydroxyacyle-CoA is dehydrogenated in the presence of NAD-specific β-hydroxyacyle-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 is formed.
(v) The fifth and last step involves the thioclastic cleavage of β-ketofatty 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 shoter by two-carbon atoms than when it entered the β-oxiadtion spiral.
The fatty acyl-CoA so produced again re-enters the P-oxidation spiral at step 2, by passing the first step as it is already activated, and losing a further 2-C unit. This sequence continues till whole molecule is degraded.
Each turn of the P-oxidation generates one FAD1I, (step 2), one NADH + H+ (step 4) and one acetyl-CoA molecule (step 5). However, in the last turn of the spiral two acetyl-CoA molecules will be produced. Reoxidation of FADH, 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 2 ATPs are consumed in the first step, thus 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 CO, and H,0 will result in the production of 12 ATP molecules.
Thus huge amount of energy is generated in the form of ATP molecules by the mitochondrial oxidation of fatty acids through the P-oxidation spiral and TCA cycle. For instance, one molecule of palmitic acid (with 16 C atoms) on complete oxidation will produce 129 ATP molecules.
Fate of Acetyl-CoA (CH3CO-CoA):
Acety1-CoA units are end products of β-oxidation of fatty acids. Which may enter (i) into Kreb’s cycle (TCA cycle) and are oxidised to release energy as mentioned in preceding paragraphs, or (ii) in case of germination of fatty acids, they are converted into soluble sucrose through the glyoxylic acid cycle.
Korenberg and Krebs (1957) framed a cycle which is known as Glyoxylic Acid Cycle or Glyoxylate Cycle through which the fats could be converted into sucrose (carbohydrates) during the germination of fatty seeds in plants.
The glyoxylate cycle is completed in glyoxysomes, mitochondria and cytosol. Various steps of this cycle occurring in higher plants especially during the germination of fatty seeds are shown in fig. 7.6 (Glyoxylate cycle) that occur in glyoxysome and mitochondrion.