Question

Consider the complete oxidation of caprylic acid ${\mathrm{CH}}_3-{\left({\mathrm{CH}}_2\right)}_6-\mathrm{COOH},\mathrm{a}{\mathrm{C}}_8$ fatty acid. a. How many acetyl CoA units are produced? b. How many cycles of $\beta$ oxidation are needed? c. How many ATPs are generated from the oxidation of caprylic acid?

Short answer

4 acetyl CoA; 3 $\beta$-oxidation cycles; 40 ATP generated

Step-by-step solution

Identify the number of carbons in caprylic acid

Caprylic acid is a fatty acid with the formula ${\text{CH}}_3{\text{-(CH}}_2{)}_6\text{COOH}$. It contains 8 carbon atoms.

Determine the number of acetyl CoA units

Since each acetyl CoA unit contains 2 carbon atoms, the total number of acetyl CoA units produced is given by $\frac{8}{2}=4$. Therefore, 4 acetyl CoA units are produced.

Calculate the number of $\beta$-oxidation cycles

$\beta$-oxidation removes 2 carbons per cycle, producing one acetyl CoA. For a ${\text{C}}_8$ fatty acid, $\frac{8}{2}-1=3$ cycles are required to completely oxidize the fatty acid.

Calculate the ATP generated from $\beta$-oxidation

Each $\beta$-oxidation cycle generates 1 FADH\textsubscript{2} (1.5 ATP), 1 NADH (2.5 ATP), and 1 acetyl CoA (which produces 10 ATP in the citric acid cycle). Total ATP from $\beta$-oxidation and citric acid cycle: $(3cycles\times (1.5+2.5+10))+1acetylCoA(10ATP)=42ATP-2ATP(activationcost)=40ATP$.

Key concepts

acetyl CoA

Acetyl CoA is a vital molecule in cellular metabolism. It acts as a bridge between carbohydrate, fat, and protein metabolism. Each acetyl CoA molecule contains two carbon atoms. In the context of caprylic acid oxidation, which is a C8 fatty acid, it breaks down into four acetyl CoA units. This process involves breaking the long carbon chain of caprylic acid into smaller, two-carbon fragments, which are then used in various metabolic pathways.

beta-oxidation

Beta-oxidation refers to the process of fatty acid breakdown in mitochondria. During beta-oxidation, fatty acids like caprylic acid are broken down into acetyl CoA units. Each cycle of beta-oxidation removes two carbon atoms from the fatty acid chain. For caprylic acid, the beta-oxidation process involves three cycles, as each cycle produces one acetyl CoA and shortens the chain by two carbons. This enzymatic sequence ensures that long fatty acid chains are efficiently broken down into usable energy forms.

ATP generation

ATP (adenosine triphosphate) is the primary energy currency in cells. During caprylic acid oxidation, ATP is generated from both the beta-oxidation cycles and the subsequent Krebs cycle (citric acid cycle). In each beta-oxidation cycle, 1 FADH2 and 1 NADH are produced, generating approximately 1.5 and 2.5 ATPs respectively when going through the electron transport chain. Additionally, each acetyl CoA unit generates 10 ATPs through the Krebs cycle. For caprylic acid, this means a total of 42 ATPs produced. However, there is an activation cost of 2 ATPs, so the net ATP gain is 40.

fatty acid metabolism

Fatty acid metabolism encompasses the processes of fatty acid activation, transport, and breakdown. Fatty acids like caprylic acid are first activated by coupling with Coenzyme A to form fatty acyl-CoA. This activated fatty acid is transported into mitochondria where beta-oxidation occurs. Here, the fatty acid chains are progressively shortened, generating acetyl CoA, NADH, and FADH2. These products enter the citric acid cycle and electron transport chain to produce ATP. Efficient metabolism of fatty acids ensures that cells have a consistent and ample supply of energy, especially during periods when carbohydrates are scarce.