Biochemistry : Reactants and Products of Lipid Catabolism

Study concepts, example questions & explanations for Biochemistry

varsity tutors app store varsity tutors android store

Example Questions

Example Question #54 : Catabolic Pathways And Metabolism

How many rounds of beta oxidation will a fatty acid with a hydrocarbon tail that is 20 carbons long undergo? What are the products?

Possible Answers:

Nine rounds, pyruvate

Twenty rounds, acetyl-CoA

Ten rounds, pyruvate

Ten rounds, acetyl-CoA

Nine rounds, acetyl-CoA

Correct answer:

Nine rounds, acetyl-CoA

Explanation:

Each round of beta oxidation yields one molecule of acetyl-CoA (along with one molecule of FADH2 and one molecule of NADH). Both electron carriers feed into the electron transport chain, ultimately yielding ATP via chemiosmosis. Since the organic product of beta oxidation (acetyl-CoA) contains two carbons, nine rounds of beta oxidation are required to fully oxidize a fatty acid with a 20-carbon hydrocarbon tail. Recognize that it is NOT 10 rounds because the last round (9th) will cut the now 4-carbon hydrocarbon tail, yielding two acetyl-CoA molecules. As a rule, a fatty acid with a hydrocarbon tail of  of carbons undergoes  rounds of beta oxidation to be full oxidized.

Overall, for a fatty acid with 20 carbons in its hydrocarbon chain, 9 rounds of beta oxidation yielding 10 molecules of acetyl-CoA constitutes complete oxidation. 

Example Question #1 : Reactants And Products Of Lipid Catabolism

Which of the following is not a product of beta oxidation of fatty acids?

Possible Answers:

Acetyl-CoA

ATP

NADH

All are products of beta oxidation

Correct answer:

ATP

Explanation:

One round of beta oxidation of fatty acids removes 2 carbons off the fatty acid chain at a time, yielding acetyl-CoA as well as  and  (from the 2 oxidation steps). ATP is not a direct product of beta oxidation, however the acetyl-CoA and reduced coenzymes will provide ATP via Krebs cycle and electron transport.

Example Question #56 : Catabolic Pathways And Metabolism

In plants, a cycle known as the glyoxylate cycle exists, which allows plants to synthesize carbohydrates from fats. This is how fat-rich seeds are anabolized to produce carbohydrates in plants. 

Even though the glyoxylate cycle does not exist in humans, there is one molecule that is used in gluconeogenesis that comes from -oxidation of fatty acids. Which molecule is this? 

Possible Answers:

Pyruvate

Oxaloacetate

NADH

Glyceraldehyde-3-phosphate

Correct answer:

Oxaloacetate

Explanation:

-oxidation of fatty acids yields large quantities of acetyl-CoA, all of which enters the citric acid cycle. The only intermediate in the citric acid cycle that can form back into glucose is oxaloacetate. It can be turned into phosphoenolpyruvate via phosphoenolpyruvate carboxykinase. 

Example Question #57 : Catabolic Pathways And Metabolism

What is the first step of the beta oxidation of fatty acids?

Possible Answers:

Addition of a hydroxyl group

Formation of a carbon oxygen double bond

Formation of a trans double bond between two carbons

Formation of a cis double bond between two carbons

Reduction of a carbon carbon double bond

Correct answer:

Formation of a trans double bond between two carbons

Explanation:

In the first step of beta oxidation the enzyme acyl-CoA dehydrogenase forms a trans-double bond between the two carbons at the site that will eventually be cleaved.

Example Question #58 : Catabolic Pathways And Metabolism

On average, how many ATP and GTP molecules can be generated from an eighteen-carbon fatty acid?

Possible Answers:

106 ATP, no GTP

106 ATP, 8 GTP

159 ATP, 10 GTP

60 ATP, 4 GTP

120 ATP, 9 GTP

Correct answer:

120 ATP, 9 GTP

Explanation:

First, one must know what the products of one cycle of the beta oxidation pathway are. Each cycle yields , , 1 acetyl-CoA, and a fatty acyl-CoA molecule 2 carbons shorter than the original.

Therefore, an eighteen-carbon fatty acyl-CoA input can go through the beta oxidation cycle 8 times (the last round only a 2 carbon acetyl CoA remains, which cannot enter the beta oxidation pathway again). That means there are  molecules, 9 acetyl-CoA molecules, and  molecules that can be generated from an eighteen-carbon fatty acyl-CoA. 

It may be confusing to understand why there are only  and  molecules and 9 acetyl-CoA, but if one is solving a problem such as this for the first time, go through and draw out the products following each round of the beta oxidation cycle.

2.5 ATP on average can be formed from each  molecule, and 1.5 ATP from each   molecule through the electron transport chain. 10 ATP can be formed in total from one acetyl-CoA entering into the Krebs cycle (by the products of  and  ), along with one GTP molecule.

Therefore:

 

  

If taken into consideration, 2 ATP are needed to activate a fatty acid and allow it to enter into the mitochondria via the enzyme acyl-CoA synthetase.

Therefore, the toTal net yield is

For a quick reference, the following equations can be used:

Where  is number of carbons of an even numbered fatty acid chain.

Example Question #59 : Catabolic Pathways And Metabolism

How many molecules of acetyl-CoA can be expected to be produced from the beta-oxidation of palmitic acid?

Note: Palmitic acid is a saturated fatty acid containing sixteen carbon atoms.

Possible Answers:

Correct answer:

Explanation:

For each round of beta-oxidation, two carbon atoms are removed from the fatty acid chain. These two carbon atoms come off of the chain in the form of acetyl-CoA. Additionally, high energy electron carriers in the form of NADH and  are also produced, thus making beta-oxidation a process that liberates a great deal of energy.

Since the fatty acid that we're starting with has a total of  carbon atoms, we can expect there to be a total of  acetyl-CoA molecules produced, since each of these contains two carbons from the fatty acid.

Example Question #60 : Catabolic Pathways And Metabolism

Which of the following molecules is/are the product(s) at the end of normal beta oxidation of a fatty acid?

Possible Answers:

2 propionyl-CoA molecules

2 acetyl-CoA molecules

2 acetyl-CoA molecules and 2 propionyl-CoA molecules

2 keto-acyl-CoA molecules

1 acetyl-CoA molecule and 1 propionyl-CoA molecule

Correct answer:

2 acetyl-CoA molecules

Explanation:

During beta oxidation of fatty acids, carbons are removed from the fatty acid chain two at a time. So when a fatty acid is composed of an even number of carbons (as most are) 4 carbons will be left at the end. This will be cleaved into two separate 2 carbon molecules - two acetyl-CoA molecules.

Example Question #2 : Reactants And Products Of Lipid Catabolism

Which of the following molecules is/are produced at the end of beta oxidation of a fatty acid with an odd number of carbons in its carbon chain?

Possible Answers:

2 acetyl-CoA molecules

2 propionyl-CoA molecules and 2 acetyl-CoA molecules

2 propionyl-CoA molecules

2 Ketoacyl molecules

1 propionyl-CoA molecule and 1 acetyl-CoA molecule

Correct answer:

1 propionyl-CoA molecule and 1 acetyl-CoA molecule

Explanation:

During beta-oxidation of fatty acids, carbons are removed from the fatty acid chain two at a time. So when a fatty acid is composed of an odd number of carbons, 5 carbons will be left at the end. This will be cleaved into two separate molecules - one with 3 carbons and one with 2 carbons. This is one acetyl-CoA molecule (2 carbons) and one propionyl-CoA molecule (3 carbons).

Example Question #3 : Reactants And Products Of Lipid Catabolism

During beta oxidation, what is the end product of a fatty acid with an odd number of carbons?

Possible Answers:

Succinyl-CoA

Acetyl-CoA

Nothing; these molecules do not exist.

Propionyl-CoA

Correct answer:

Propionyl-CoA

Explanation:

Fatty acids with an odd number of carbons are more common in plants and marine organisms than they are in mammals. When humans consume these organisms in their diets, they must metabolize them through beta oxidation. The end product of this reaction is propionyl-CoA rather than acetyl-CoA, which is formed during cleavage of two-carbon segments (fatty acids with an even number of carbons). Before entering the Krebs cycle, the propionyl-CoA must be transformed into succinyl-CoA.

Learning Tools by Varsity Tutors

Incompatible Browser

Please upgrade or download one of the following browsers to use Instant Tutoring: