What is the Difference Between Acetyl CoA and Acyl CoA

The main difference between acetyl CoA and acyl CoA is that acetyl CoA is the major end-product of the oxidative decarboxylation of pyruvate in the carbohydrate metabolism, later entering into the citric acid cycle whereas, at low glucose levels, acyl CoA is produced by the fatty acid activation, later entering into the β-oxidation of fatty acids. Furthermore, the main function of acetyl CoA is to deliver the acetyl group to the citric acid cycle to be oxidized for energy production. On the other hand, by a four-step cycle of β-oxidation, the acyl CoA degrades into acetyl CoA and a new fatty acid with two or fewer carbon molecules.

In brief, acetyl CoA and acyl CoA are two types of coenzymes that mainly involve in the metabolism of fatty acids. And, both have a similar chemical structure. 

Key Areas Covered 

1. What is Acetyl CoA
     – Definition, Formation, Role
2. What is Acyl CoA
     – Definition, Formation, Role
3. What are the Similarities Between Acetyl CoA and Acyl CoA
     – Outline of Common Features
4. What is the Difference Between Acetyl CoA and Acyl CoA
     – Comparison of Key Differences

Key Terms 

Acetyl CoA, Acyl CoA, Citric Acid Cycle, Coenzymes, Fatty Acid Metabolism, β-oxidation

What is Acetyl CoA 

Acetyl CoA is a type of coenzyme of the acyl CoA group, containing a methyl group as the R group. Generally, oxidative decarboxylation of pyruvate is the main type of biochemical reaction that produces acetyl CoA.

Formation

In the cytosol, pyruvate undergoes oxidative decarboxylation in the presence of high concentrations of glucose. Here, pyruvate is the end-product of glycolysis, which is the initial reaction of cellular respiration occurring in the cytosol. At low levels of glucose in the cytosol, three types of reactions are responsible for the production of acetyl CoA. They include:  

1. acetylation of CoA with acetate by the action of the enzyme, acetyl CoA synthetase (ACS) coupling with  ATP hydrolysis, 
2. acetylation of CoA using the carbon in ethanol by the action of the enzyme ethanol dehydrogenase
3. catabolism of branched-chain ketogenic amino acids such as valine, leucine,  and isoleucine. 
   

   Figure 1: Oxidative Decarboxylation

Moreover, a number of reactions produce acetyl CoA inside the mitochondria; some of these include the oxidative decarboxylation of pyruvate at high glucose levels, catabolism of amino acids such as tyrosine, phenylalanine, lysine, leucine, and tryptophan, and the β-oxidation of fatty acids at low glucose levels.

Role

The main function of acetyl CoA inside the mitochondria is the transfer of acetyl groups to the citric acid cycle, which then undergoes oxidation to produce energy. Here, the citric acid cycle occurs in the matrix of mitochondria in eukaryotes.

Figure 2: Citric Acid Cycle

Furthermore, at high glucose levels, acetyl CoA forms citrate inside the mitochondria in the presence of insulin, which is then transported into the cytosol by tricarboxylate anion carrier system and cleaved to yield acetyl-CoA and oxaloacetate. Ultimately, the excess acetyl CoA in the cytosol is used in the synthesis of fatty acids. It is also used in the synthesis of cholesterol, steroids, acetylcholine, and melatonin.

Besides, cytosolic acetyl CoA takes part in acetylation in posttranslational modifications of proteins while serving as allosteric regulators. However, at low glucose levels, acetyl CoA in the cytosol forms ketone bodies released by the liver into the blood. These ketone bodies can even cross the blood-brain barrier to serve as fuel to the cells in the central nervous system. Therefore, high levels of ketone bodies in the blood indicate starvation, prolonged heavy exercises, low carbohydrate diet, ketosis or even ketoacidosis.  

What is Acyl CoA 

Acyl CoA is a group of coenzymes important in fatty acid metabolism. It is susceptible to β-oxidation to form acetyl CoA, which later enters into the citric acid cycle. Therefore, acyl CoA is a key biochemical component in converting fat into energy.

Formation

Acyl CoA basically forms in a two-step reaction known as fatty acid activation. Acyl CoA synthetase is the enzyme responsible for the above reaction. Moreover, the three structural components of the acyl CoA are the R group, carbonyl group, and the coenzyme A. Here, the R group is variable and is essentially a side chain of fatty acids. However, the length of the R group depends on the type of acyl CoA synthetase enzyme that takes part in the reaction.

Figure 3: Acyl CoA

Moreover, acyl CoA can have three fates depending on the type of enzyme. They are the formation of 4-11 carbon acyl-CoA by the enzyme, medium-chain acyl-CoA synthase, the formation of 11-20 Acyl-CoA by a different acyl-CoA synthetase enzyme, and the formation of free fatty acid and coenzyme A by the enzyme, acyl-CoA thioesterase, which posses the opposite enzymatic activity to acyl CoA synthetase. 

Figure 4: Beta-Oxidation

Role

Although the production of acyl CoA occurs in the cytosol, β-oxidation occurs inside the mitochondria. Cytosolic acyl CoA is transported into mitochondria by means of carnitine participants. Moreover, this carnitine-dependent transport system is responsible for the transport of long-chain acyl-CoA into the mitochondria while it has a minor role in acetyl-CoA transport out of mitochondria. Furthermore, after the transportation into the mitochondria, acyl CoA undergoes β-oxidation in a four-step cycle of oxidation, hydration, oxidation, and thiolysis catalyzed by four respective enzymes: acyl-CoA dehydrogenase, enoyl-CoA hydratase, 3-hydroxyacyl-CoA dehydrogenase, and thiolase. 

Similarities Between Acetyl CoA and Acyl CoA  

• Acetyl CoA and acyl CoA are two types of coenzymes important in fatty acid metabolism.  
• Both of these have a similar structure and their structural components include carbonyl group, R group, and a coenzyme A group; the R group can be variable.  
• Furthermore, they play an important role in the energy production of the body through catabolism. 

Difference Between Acetyl CoA and Acyl CoA 

Definition 

Acetyl CoA refers to the acetylated form of coenzyme A formed as an intermediate in the oxidation of carbohydrates, fats, and proteins during metabolism, while acyl CoA refers to a product of fatty acid activation subsequently carried by carnitine into the mitochondria for β-oxidation. 

Correspondence 

Acetyl CoA is a type of acyl CoA while acyl CoA is a group of coenzymes that take part in the metabolism of fatty acids. 

R Group 

The R group of acetyl CoA is essentially a methyl group while the R group of acyl CoA is a side chain of fatty acids. 

Formation 

Moreover, acetyl CoA is the major end-product of the oxidative decarboxylation of pyruvate in the carbohydrate metabolism while at low glucose levels, acyl CoA is produced during the fatty acid activation, later entering into the β-oxidation of fatty acids.  

Function 

The main function of acetyl CoA is to deliver the acetyl group to the citric acid cycle to be oxidized for energy production while acyl CoA degrades into acetyl CoA and a new fatty acid with two or fewer carbon molecules by a four-step cycle.  

Conclusion 

In brief, acetyl CoA is a type of acyl CoA containing a methyl group as the R group. It also forms as the major end-product of the oxidative decarboxylation of pyruvate during carbohydrate catabolism. It is also responsible for transferring acetyl groups to the citric acid cycle for energy production. In contrast, acyl CoA is a group of coenzymes that involve in the metabolism of fatty acids. Generally, at low glucose levels, fatty acids undergo activation, producing acyl CoA, which undergoes β-oxidation in order to produce acetyl CoA. Finally, this acetyl CoA also enters the citric acid cycle. Therefore, the main difference between acetyl CoA and acyl CoA is their structure and function. 

References:

1. “Acetyl-CoA.” Acetyl-CoA – an Overview | ScienceDirect Topics, Available Here.
2. “Acetyl-CoA.” Wikipedia, Wikimedia Foundation, 17 Dec. 2019, Available Here.
3. “Acyl-CoA.” Wikipedia, Wikimedia Foundation, 22 Jan. 2020, Available Here.

Image Courtesy:

1. “Pyruvate dehydrogenase complex reaction” By akane700 – Own work (CC BY-SA 3.0) via Commons Wikimedia   
2. “Citric acid cycle with aconitate 2” By Narayanese, WikiUserPedia, YassineMrabet, TotoBaggins (CC BY-SA 3.0) via Commons Wikimedia   
3. “Acyl-CoA2” By NEUROtiker – Own work (Public Domain) via Commons Wikimedia   
4. “Metabolism4” By Cruithne9 – Own work (CC BY-SA 4.0) via Commons Wikimedia