Question

Calculate the number of ATPs produced if hexanoic acid is completely oxidized to form CO₂. Briefly describe where each NADH or FADH₂ are formed and how each of those is converted into ATP.

Answer 1

Hexanoic acid is a fatty acid with 6 carbon atoms. Fatty acids are oxidised by the process of beta-oxidation.

A single cycle of beta-oxidation involves three stages:

1. Activation of fattyacids to acyl co A in cytosol which involves utilization of 2 ATP.

2. Transport of acyl co A to mitochondria.

3. Beta oxidation proper where 1 FADH2, 1 NADH and 1 acetyl co A are produced.

Hexanoic acid is 6 carbon compound. For one cycle of beta-oxidation, 2 carbon atoms are removed as acetyl co A. But at last step when 4 carbon atoms are present with the 2 carbon compound remaining 1 acetyl co A released it makes it equivalent to 2 acetyl co A.

i.e.,

6 carbons gets converted to 1 acetyl co A (2C) for one cycle of beta-oxidation.

4 carbons gets converted to 1 acetyl co A (2C) for one cycle.

2 carbons makes equal to 1 acetyl co A (2C).

Hence beta-oxidation of hexanoic acid (6C) requires 2 cycles of beta-oxidation to give 3 acetyl co A, 2 FADH2, and 2 NADH.

The FADH2 and NADH produced here will produce ATP through electron transport chain.

The acetyl co A enters into the Tricarboxylic Acid (TCA) cycle and gets completely oxidised into carbon dioxide and water.

In this process, 1 acetyl coA undergoes 1 cycle of TCA and produce 3 NADH and 1 FADH2 which is equal to 12 ATP.

ENERGITICS:

For 1 NADH, 3 ATP are produced.

For 1 FADH2, 2 ATP are produced.

According to this,

S.No.	Mechanism:	ATP produced:
1.	Beta-oxidation (2cycles) 2 FADH2  (BY ETC) 2 NADH (BY ETC)	4 ATP 6 ATP
2.	From 3 acetyl co A (TCA Cycle)	3×12=36 ATP

A total of 46 ATP are produced.

But the net ATP will be 46-2= 44 ATP as 2 ATP are utilised for activation of fatty acid.

But according to recent studies,

1 NADH gives 2.5 ATP.

2 FADH2 gives 1.5 ATP.

Thus the total production will be 38 ATP ( 8 ATP are produced from beta-oxidation and 30 ATP are produced from 3 Acetyl coA) and the net ATP will be 38-2= 36 ATP.

Hence 1 Hexanoic acid molecule yields 44 ATP (36 ATP according to recent evidences).