The Four Steps of Beta-Oxidation
Recall: Beta Oxidation (BO) is the catabolic process by which fatty acids are broken down to generate Acetyl-CoA (Citric Acid Cycle) and FADH2/NADH (Electron Transport Chain). This happens in the mitochondria.
Compare the four steps of beta oxidation to the four steps of fatty acid synthesis.
Beta Oxidation (BO)
Fatty Acid (FA) synthesis
Think of it as mapping to the opposite number. That is, 1->4 (Oxidation->Reduction), 2->3 (Hydration->Dehydration), 3->2 (Oxidation->Reduction), 4->1(Thiolysis->Condensation).
Also note that each step of BO reduces the FA chain by two carbons.
Step 1: Oxidation (Acyl CoA dehydrogenase)
Acyl COA —(FAD->FADH2)—> trans-delta2-Enoyl CoA
- FADH2 routes off to the electron transport chain
- The reaction product has a double bond at the second carbon (delta2)
Step 2: Hydration (Enoyl CoA hydratase)
trans-delta2-Enoyl CoA —(H20)—> L-3-hydroxyacyl-CoA
- hydration reaction
- eliminates the double bond
Step 3: Oxidation (L-3-hydroxyacyl-CoA dehydrogenase)
L-3-hydroxyacyl-CoA —(NAD+->NADH)—> 3-Ketoacyl-CoA
- coupled with NAD+ reduction;
- NADH feeds the ETC too
Step 4: Thiolysis (beta-ketothiolase)
3-Ketoacyl-CoA—(CoA-SH)—> Acetyl-CoA + Acyl-CoA (shortened by two carbons)
- The thiol group of the incoming Acetyl-CoA is involved in the cleavage
- The resulting Acyl-CoA is shortened by two carbons
- The shortened Acyl-CoA can now undergo a new cycle of beta-oxidation
Example: 16-carbon palmitoyl-CoA. Seven rounds of oxidation will be needed to release 8 the following:
Palmitoyl-CoA + 7 FAD + 7 NAD+ + 7CoA + 7H2O —> 8 acetyl CoA + 7 FADH2 + 7 NADH + 7 H+
Each Acetyl-CoA entering the CAC will yield 10 ATP. Similarly, each FADH2 yields 1.5 and each NADH yields 2.5.
ATP yield for palmitoyl-CoA:
8*10 + 7*1.5 + 7*2.5 = 108 ATP – 2 ATP (forming acyl-CoA) = 106 ATP
n-carbon needs n/2 -1 cycles
ATP Yield of C(2n) is (n-1)*(2.5 + 1.5) + 10*n = 14n – 4
Example above: 14*8 -4 = 140 – 28 – 4 = 112 – 4 = 108
minus startup cost (2) for acyl to acyl-CoA.
Odd numbered and unsaturated chains
The beta-oxidation for odd length chains is mostly the same except for the final round where the yield is Acetyl-CoA and instead of Acyl-CoA, a three-carbon propionyl-CoA. This gets converted to succinyl-CoA used by the CAC.
What about unsaturated fatty acids?
Example: Palmitoleoyl-CoA (delta9)
The first three rounds are the same as saturated. The next round can’t proceed because the resulting cis-∆3-enoyl CoA is not a substrate for the dehydrogenase. The normal substrate is Acyl-CoA, yielding trans-delta2-enoyl-CoA during step 1 of the BA cycle.
For this to proceed, the Cis-∆3-enoyl CoA isomerase converts this double bond into a trans-∆2-double bond. This trans-∆2-enoyl CoA is a substrate for acyl CoA dehydrogenase and proceeds through the β-oxidation pathway from Step 2.
Example: double bond at Carbon 4 instead (first round of oxidation)
If the double bond is at carbon 4, the acyl-CoA dehydrogenase (from Step 1) will insert a second double bond at carbon 2. Then a reductase uses NADPH will reduce the number of double bonds to one. Finally an isomerase will shift the DB to form a trans-delta2-enoyl-CoA allowing the cycle to resume from step 2.
Another example: two double bonds.
Oxidation vs Synthesis
Location, Role of Malonyl-CoA, Reactions
Synthesis: cytosol, activator: used as building blocks, multiple rounds of four-step processes: C.R.D.R.
Oxidation: mitochondrial matrix, inhibitor: inhibits CPT1, multiple rounds of four-step processes: O.H.O.T.
Fate of Acetyl-CoA produced by BO
Normally, Acetyl-CoA joins CAC by reacting with oxaloacetate to form citrate
If the supply of oxaloacetate is low (starvation, diabetes), acetyl-CoA accumulates and is converted to ketone bodies. Presence of ketone bodies indicates an active breakdown of fat and a depleted supply of carbs.
Ketone bodies are formed in the mitochondria in the liver. Three molecule of Acetyl-CoA react to form the ketone, acetoacetate, while regenerating one Acetyl-CoA used to form more ketone bodies.
When the [NADH] is high enough, acetoacetate is reduced to form D-3-Hydroxy-butyrate. They can also spontaneously (and slowly) be decarboxylated to release acetone. Acetone can be detected on the breath of a person producing ketone bodies.
Purpose of Ketone Bodies
Ketone bodies are distributed to tissues such as heart and kidney where they serve as a source of fuel molecules. Though the brain prefers glucose, it can metabolize ketone bodies during starvation and in diabetic patients.
Meanwhile, the D-3-Hydroxy-butyrate is converted back to acetoacetate producing NADH, which feeds the ETC (and thus contribute to the formation of ATP). The acetoacetate is then converted to 2 Acetyl-CoA and 1 succinyl-CoA. Both can feed the CAC as long as tissues have enough oxaloacetate.