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β-OXIDATION SIMPLIFIED

Updated: Nov 8, 2023

β-OXIDATION SIMPLIFIED

source: PHGY170

We discussed that there are three major macromolecules we use to create ATP in the cells, namely carbohydrates, fats (=lipids), and proteins. Whereas glycolysis involved breakdown of glucose to create high-energy electron carriers (NADH, FADH2) and ATP, this section will discuss the breakdown of fatty acid (component of a fat molecule) to create the same energy molecules.


1st step: Fatty acid in the cytosol of the cell is activated as Acyl-CoA in order to enter the mitochondria. This process uses up 2 ATP.

Fatty acid --> Acyl-CoA (-2ATP)
source: Wikipedia


















2nd step: Acyl-CoA then undergoes β-oxidation within the mitochondrial matrix, where the fatty acids are broken down 2 carbons at a time to release acetyl-CoA. Each β-oxidation cycle gives rise to 1 NADH & 1 FADH2 and this cycle continues until the chain is reduced all the way to a 2-carbon acetyl-CoA.

Acyl-CoA --> Acyl-CoA (2 C shorter), acetyl-CoA, 1NADH & 1 FADH2
source: Wikipedia


















The diagram shows β-oxidation cleaving off 2 carbons from the Acyl-CoA. The 2 carbons go off to form acetyl-CoA, which enters the Krebs Cycle


3rd step: Acetyl-CoA from the β-oxidation will enter Krebs Cycle to give rise to 3 NADH, 1 FADH2, and 1 GTP (=ATP) per cycle.

[See Krebs Cycle SIMPLIFIED]


 

Now that we know the general mechanisms for fat metabolism and β-oxidation, let's have a look at how we can calculate the number of NADH, FADH2, and ATP generated from Acyl-CoA.

1. β-oxidation n number of times:

n refers to number of times fatty acid can undergo β-oxidation

β-oxidation n number of times:

  • n NADH

  • n FADH2

  • n+1 Acetyl CoA

Meaning, if β-oxidation occurs 8 times, it will give rise to 8 NADH, 8 FADH2, and 9 Acetyl-CoA


2. Krebs Cycle:

We know this cycle runs for every 1 acetyl-CoA, yielding 3 NADH, 1 FADH2, and 1 ATP. How many will enter Krebs Cycle is determined by n +1 as previously shown (number of times Acyl-CoA undergoes β-oxidation to release acetyl-CoA AND 2-carbon acetyl-CoA that is left at the end of all the β-oxidation cycles - there is always one molecule of acetyl-CoA that is left for any Acyl-CoA)

Krebs Cycle

  • 3(n+1) NADH

  • n+1 FADH2

  • n+1 ATP


3. ATP we used up for fatty acid activation in the cytosol:

Fatty Acid Activation

  • -2ATP


EXAMPLE:


source: PHGY170

Above is a 16-carbon palmitic acid.

It will undergo β-oxidation 7 times and leave behind the last 2-carbon acetyl-CoA. For this specific example:

β-oxidation gives rise to 7 NADH, 7 FADH2, and 8 acetyl-CoA
Krebs cycle gives rise to 24 NADH, 8 FADH2, and 8 ATP
Fatty acid activation used up 2 ATP (represented by -2ATP)

source: PHGY170


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