β-OXIDATION SIMPLIFIED
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)
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
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:
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)
Comments