Question

What are the four types of chemical reactions needed in the fatty acid synthesis pathway to add a \(\mathrm{C}_{2}\) acetyl group to the growing hydrocarbon chain? Include the names of the cytosolic enzyme subunits that catalyze each reaction.

Short answer

Answer: The four types of chemical reactions involved in the fatty acid synthesis pathway are (1) condensation, catalyzed by beta-ketoacyl-ACP synthase, (2) reduction, catalyzed by beta-ketoacyl-ACP reductase, (3) dehydration, catalyzed by beta-hydroxyacyl-ACP dehydratase, and (4) further reduction, catalyzed by enoyl-ACP reductase.

Step-by-step solution

Identify the First Reaction

The first chemical reaction in the fatty acid synthesis pathway is the condensation of acetyl-CoA and malonyl-CoA. This reaction forms a C4 hydrocarbon chain while releasing carbon dioxide. The cytosolic enzyme subunit that catalyzes this reaction is called beta-ketoacyl-ACP synthase (also known as fatty acid synthase subunit).

Identify the Second Reaction

The second chemical reaction in the fatty acid synthesis pathway is the reduction of the beta-keto group on the hydrocarbon chain. In this step, a hydrogen atom is added to the beta-carbon, creating a hydroxyl group. The cytosolic enzyme subunit that catalyzes this reaction is called beta-ketoacyl-ACP reductase.

Identify the Third Reaction

In the third reaction, the hydroxyl group on the beta-carbon is dehydrated. This step forms a double bond between the alpha and beta carbon atoms. The cytosolic enzyme subunit responsible for catalyzing this reaction is called beta-hydroxyacyl-ACP dehydratase.

Identify the Fourth Reaction

The fourth and final reaction of the fatty acid synthesis pathway is the reduction of the double bond between the alpha and beta carbon atoms. Two hydrogen atoms are added to the double bond, converting it to a single bond and forming a saturated C4 hydrocarbon chain. The cytosolic enzyme subunit responsible for this reaction is called enoyl-ACP reductase. In summary, the four types of chemical reactions involved in the fatty acid synthesis pathway to add a C2 acetyl group to the growing hydrocarbon chain are: (1) condensation, catalyzed by beta-ketoacyl-ACP synthase, (2) reduction, catalyzed by beta-ketoacyl-ACP reductase, (3) dehydration, catalyzed by beta-hydroxyacyl-ACP dehydratase, and (4) further reduction, catalyzed by enoyl-ACP reductase.

Key concepts

Beta-Ketoacyl-ACP Synthase

The enzyme beta-ketoacyl-ACP synthase plays a crucial role at the start of the fatty acid synthesis process. It catalyses the condensation of an acetyl-CoA molecule with a malonyl-ACP, resulting in the formation of a beta-ketoacyl-ACP and releasing a carbon dioxide molecule. This step efficiently extends the carbon chain by two carbons each cycle, a principle aspect of fatty acid elongation.

This reaction is remarkable because it not only links two molecules together but also establishes the foundation for the subsequent reactions in the fatty acid synthesis pathway. It's a prime example of anabolic chemistry, where simple building blocks are used to construct more complex structures, fundamental for life.

Beta-Ketoacyl-ACP Reductase

The enzyme beta-ketoacyl-ACP reductase takes the spotlight in the second reaction of fatty acid synthesis. It is tasked with reducing the keto group on the nascent fatty acid chain to a hydroxyl group. This reduction involves the addition of hydrogen atoms, which is facilitated by the coenzyme NADPH, reducing the keto group to an alcohol group (also known as a hydroxyl group).

This transformation underscores the importance of electron carriers such as NADPH in biosynthetic processes, thereby linking the metabolic pathways that generate these carriers to the broader biochemical landscape of cell function.

Beta-Hydroxyacyl-ACP Dehydratase

In the third step, the enzyme beta-hydroxyacyl-ACP dehydratase comes into action. It orchestrates a dehydration reaction, where the hydroxyl group that was added in the previous step is removed along with a hydrogen atom. This results in the formation of a double bond between the alpha and beta carbons, essentially creating an unsaturated bond within the hydrocarbon chain.

The precision of this enzyme in selectively removing molecules of water is quite remarkable, and it significantly alters the chemical property of the growing fatty acid chain by introducing a degree of unsaturation. This unsaturated intermediate is crucial for the next step in the pathway.

Enoyl-ACP Reductase

The final stage in the elongation cycle of fatty acid synthesis features the enzyme enoyl-ACP reductase. This enzyme is responsible for the reduction of the double bond between the alpha and beta carbon atoms in the fatty acyl chain, thus saturating the previously unsaturated bond. During this process, two hydrogen atoms are added across the double bond using NAD(P)H as the hydrogen donor, finalizing the creation of a saturated fatty acyl chain.

This not only highlights the interplay between various reducing agents within the cell, but also shows the versatility of reductase enzymes. Upon completion of this step, the elongated and now saturated fatty acyl chain is ready for further extension or to undergo other modifications needed for biological functions.