Question

b-oxidation follows a reaction sequence closely resembling a sequence in the citric acid cycle. Show

the analogous reaction sequences in the two pathways

Answer 1

Ans:-) Beta oxidation is a catabolic process in which fatty acid molecules are broken down in acetyl-coA which enters citric acid cycle. This occurs in cytosol in prokaryotes and in mitochondria in eukaryotes.

Citric acid cycle is also called as TCA cycle or krebs cycle. This is a series of enzyme catalysed reactions that form a key part of aerobic respiration in cells.

The analogous reaction between oxidation of fatty acids and citric acid cycle are :-

(i) Step 1 of fatty acid oxidation = Step 6 of citric acid cycle

The first step of fatty acid oxidation is dehydrogenation. It creates a double bond between alpha and beta carbons of fatty acid molecules. It results in formation of trans alpha beta fatty acyl CoA. Acyl dehydrogenase catalyse this reaction. In it two hydrogen atoms are given off to reduce FAD to FADH2.

fatty acyl CoA + FAD - - - - - - - - > Trans alpha beta unsaturated fatty acyl CoA + FADH2

This reaction is similar to conversion of succinate to fumarate in citric acid cycle where hydrogen are accepted by FAD.

Succinate +FAD - - - - - - - >Fumarate +FADH2

This is catalysed by enzyme succinic dehydrogenase.

(ii) Step 2 of fatty acid oxidation = step 7 of citric acid cycle.

The 2nd step in oxidation fatty acids cycle involves conversion of Trans alpha beta unsaturated fatty acyl CoA into beta hydroxy acyl CoA. Water is uptaken in this. This is catalysed by enoyl hydrase.

Trans alpha beta unsaturated fatty acyl CoA + H20 - - - - - - - >beta hydroxy acyl CoA

This reaction is similar to conversion of fumarate to malate in citric acid cycle where uptake of water is observed.

Fumarate +H2O - - - - - - - >Malate

This reaction is catalysed by enzyme fumarase.

(iii) step 3 of fatty acid oxidation = step 8 of citric acid cycle.

In 3rd step of beta oxidation of fatty acids :-

Beta hydroxyacyl CoA + NAD + - - - - - >betaketo fatty acyl CoA + NADH + H +

This is catalysed by enzyme beta hydroxyacyl dehydrogenase. Here acceptor of hydrogen is NAD.

This step is analogous to conversion of malate to oxaloaccetate in citric acid cycle. Here acceptor of hydrogen is also NAD.

Malate + NAD+ - - - - - - >Oxaloaccetate +NADH + H+

This reaction is catalysed by malate dehydrogenase.