Question

Identify each of the following reactions a to e in the \(\beta\) oxida tion of palmitic acid, a \(\mathrm{C}_{16}\) fatty acid, as (1) activation (2) dehydrogenation (3) hydration (4) oxidation (5) cleavage a. Palmityl CoA and FAD form \(\alpha, \beta\) -unsaturated palmityl CoA and \(\mathrm{FADH}_{2}\). b. \(\beta\) -Keto palmityl CoA forms myristyl CoA and acetyl CoA. c. Palmitic acid, CoA, and ATP form palmityl CoA. d. \(\alpha, \beta\) -Unsaturated palmityl \(\mathrm{CoA}\) and \(\mathrm{H}_{2} \mathrm{O}\) form \(\beta\) -hydroxy palmityl CoA. e. \(\beta\) -Hydroxy palmityl CoA and NAD \(^{+}\) form \(\beta\) -keto palmityl \(\mathrm{CoA}\) and \(\mathrm{NADH}+\mathrm{H}^{+}\)

Short answer

a: dehydrogenation, b: cleavage, c: activation, d: hydration, e: oxidation

Step-by-step solution

Identify the reaction types

Go through each reaction (a to e) and identify the type of reaction it represents in the \(\beta\) oxidation of palmitic acid.

Reaction a Analysis

Reaction a: Palmityl CoA and FAD form \(\alpha, \beta\)-unsaturated palmityl CoA and FADH\( _2\). This involves removal of hydrogen (dehydrogenation). Hence, it is a dehydrogenation reaction.

Reaction b Analysis

Reaction b: \(\beta\)-Keto palmityl CoA forms myristyl CoA and acetyl CoA. This involves the breaking down into smaller molecules. Hence, it is a cleavage reaction.

Reaction c Analysis

Reaction c: Palmitic acid, CoA, and ATP form palmityl CoA. This involves the formation of an activated form of fatty acid. Hence, it is an activation reaction.

Reaction d Analysis

Reaction d: \(\alpha, \beta\)-Unsaturated palmityl CoA and \(\mathrm{H}_{2}\mathrm{O}\) form \(\beta\)-hydroxy palmityl CoA. This involves the addition of water. Hence, it is a hydration reaction.

Reaction e Analysis

Reaction e: \(\beta\)-Hydroxy palmityl CoA and NAD\textsuperscript{+} form \(\beta\)-keto palmityl CoA and NADH + \(\mathrm{H}^{+}\). This involves the removal of hydrogen with NAD\textsuperscript{+}. Hence, it is an oxidation reaction.

Key concepts

Activation Reaction

Before fatty acids can undergo \( \beta \) oxidation, they must be activated in the cytoplasm. This process involves the conversion of free fatty acids into fatty acyl CoA molecules, making them ready for subsequent breakdown.
In the case of palmitic acid, the activation reaction is as follows: Palmitic acid (\(\text{C}_{16}[\text{H}_{31}\text{COOH}], \)), Coenzyme A (CoA), and ATP react to form palmityl-CoA, AMP, and two inorganic phosphates (\(P_{i},\).
This reaction can be summarized as:

\[\mathrm{Palmitic \ acid + CoA + ATP \rightarrow Palmityl \ CoA + AMP + 2 P_{i} }\]Activation is vital as it attaches CoA, marking the fatty acid for transport into the mitochondria. This process requires energy in the form of ATP.

Dehydrogenation Reaction

The first step in the cyclic process of \( \beta \) oxidation is dehydrogenation. Here, two hydrogen atoms are removed from the \( \alpha \) and \( \beta \) carbon atoms of the fatty acyl CoA, forming a double bond between these carbons.
This reaction involves the enzyme acyl CoA dehydrogenase and uses FAD (Flavin Adenine Dinucleotide) as a cofactor.
The reaction can be summarized as:

\[ \mathrm{Palmityl \ CoA \ + \ FAD \rightarrow \ \alpha, \beta-unsaturated \ palmityl \ CoA \ + \ FADH_{2}} \]
The end products are \( \alpha,\beta \)-unsaturated fatty acyl CoA and the electron carrier FADH\(_2\). FADH\(_2\) will then contribute to the electron transport chain for ATP production.

Hydration Reaction

Following dehydrogenation, the next step is the hydration reaction. Water (\(\text{H}_{2}O\) is added to the double bond formed during dehydrogenation. This step converts the double bond into a single bond, introducing a hydroxyl group (-OH) on the \( \beta \) carbon.
The reaction utilizes the enzyme enoyl-CoA hydratase.\[\mathrm{\alpha, \beta-unsaturated \ palmityl \ CoA + H_{2}O \rightarrow \beta-hydroxy \ palmityl \ CoA} \]This results in \( \beta- \)hydroxy palmityl CoA. Hydration is a preparatory step for further oxidation by making the molecule more reactive.

Oxidation Reaction

The \( \beta \) hydroxy group introduced during the hydration step is further oxidized to form a keto (-CO) group. This is achieved through the action of the enzyme \( \beta \)-hydroxyacyl CoA dehydrogenase and uses NAD+ (Nicotinamide Adenine Dinucleotide) as a cofactor.
Here is the reaction:

\[\mathrm{\beta-hydroxy \ palmityl \ CoA + NAD^{+} \rightarrow \beta-keto \ palmityl \ CoA + NADH + H^{+}} \]
The result is \( \beta \)-keto palmityl CoA and NADH, which will later participate in ATP synthesis through the electron transport chain. This step prepares the fatty acid for the final cleavage.

Cleavage Reaction

The final reaction in the \( \beta \) oxidation cycle is the cleavage reaction. This reaction uses the enzyme thiolase to break the \( \beta \)-keto fatty acyl CoA into two molecules: acetyl CoA and a fatty acyl CoA shortened by two carbon atoms.\[\mathrm{\beta - keto \ palmityl \ CoA \rightarrow myristyl \ CoA \ (C_{14} \text{-acyl-CoA}) + acetyl \ CoA } \]
This shortens the original fatty acyl chain and releases acetyl CoA, which can enter the citric acid cycle to produce more ATP. The remaining myristyl CoA (C14) goes back into the \( \beta \) oxidation cycle until entirely converted into acetyl CoA units.