Fatty Acid Oxidation Quiz

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Questions and Answers

What is a common clinical condition associated with carnitine deficiency?

Preterm infants (correct)

Severe dehydration

Hypertension

Diabetes mellitus

What is the primary symptom of carnitine deficiency?

Episodic hypoglycemia (correct)

Weight loss

Chronic pain

Constant fatigue

Which enzyme deficiency leads to dicarboxylic aciduria?

Carnitine palmitoyltransferase

Long-chain fatty acyl-CoA synthetase

Mitochondrial medium chain acyl-CoA dehydrogenase (correct)

Short-chain acyl-CoA dehydrogenase

What is the consequence of the enzyme deficiency in Refsum disease?

Accumulation of phytanic acid Signup and view all the answers

Which condition is characterized by the inherited absence of peroxisomes?

Zelleweger’s syndrome Signup and view all the answers

What is the role of ATP in the activation of fatty acids?

ATP is converted to AMP during the activation process. Signup and view all the answers

How do long-chain fatty acids penetrate the inner mitochondrial membrane?

They are converted to acyl-carnitine derivatives. Signup and view all the answers

Which coenzymes are utilized during fatty acid oxidation?

NAD and FAD Signup and view all the answers

What is the end product of β-oxidation of palmitoyl-CoA?

Eight acetyl-CoA molecules Signup and view all the answers

Where does fatty acid oxidation predominantly take place within the cell?

Mitochondria Signup and view all the answers

Which amino acids can be synthesized to produce carnitine?

Lysine and methionine Signup and view all the answers

In what form are fatty acids transported in the blood?

As free fatty acids (FFAs) Signup and view all the answers

What distinguishes fatty acid oxidation from fatty acid biosynthesis?

Fatty acid oxidation is an aerobic process. Signup and view all the answers

What happens to odd-chain fatty acids during β-oxidation?

They end with propionyl-CoA. Signup and view all the answers

Which of the following statements is true regarding peroxisomal β-oxidation?

It is involved in the synthesis of dolichol. Signup and view all the answers

What is a characteristic feature of α-oxidation of fatty acids?

It is specifically for branched-chain fatty acids. Signup and view all the answers

How does the oxidation of unsaturated fatty acids differ from saturated fatty acids?

Unsaturated fatty acids provide less energy due to being less highly reduced. Signup and view all the answers

Which enzyme is essential for the oxidation of monounsaturated fatty acids?

2,3-enoylCoA isomerase Signup and view all the answers

What is the role of cytochrome P450 in ω-oxidation of fatty acids?

It helps convert the -CH3 group into a –CH2OH group. Signup and view all the answers

What byproduct is generated during the initial dehydrogenation of β-oxidation in peroxisomes?

H2O2 Signup and view all the answers

What is the relationship between propionyl-CoA and glucose metabolism?

Propionyl-CoA is a glucogenic precursor. Signup and view all the answers

Study Notes

Oxidation of Fatty Acids

Fatty acids (FAs) are oxidized to acetyl-CoA and synthesized from acetyl-CoA.
FA oxidation is a different process than FA biosynthesis, occurring in a separate cellular compartment.
FA oxidation takes place in the mitochondria.
FA oxidation is an aerobic process, requiring oxygen.
NAD and FAD are used as co-enzymes, generating ATP.
FAs are transported in the blood as free FAs.
Longer-chain FAs are bound to albumin and then to a binding protein in cells.

Objectives

Describe the processes of transporting FAs in the blood, activating them, and transporting them into mitochondria for breakdown.
Outline the β-oxidation pathway for metabolizing FAs into acetyl-CoA.
Explain how this pathway leads to ATP production.

Steps of FA Oxidation

Activation: This is the only energy-requiring step in complete FA degradation, using ATP. Fatty acid + ATP + CoA → Acyl-CoA + AMP + PP_i. Several acyl-CoA synthetases (thiokinases) exist, each specific to FA chain lengths.
Transport of long-chain FAs into mitochondria: Short-chain FAs can be oxidized directly inside the mitochondria, independent of carnitine. Long-chain acyl-CoA cannot penetrate the inner mitochondrial membrane unless it forms acyl carnitine.
Long-chain FAs: These penetrate the inner mitochondrial membrane as carnitine derivatives. Carnitine (β-hydroxy-γ-trimethyl ammonium butyrate) is abundant in muscle and can be obtained from the diet or synthesized from lysine and methionine in the liver and kidney.

β-oxidation of Fatty Acids

In β-oxidation, two carbons are activated at a time, starting from the carboxyl end of the acyl-CoA molecule.
The chain is broken between α(2) and β(3) carbons.
The resulting two-carbon units are acetyl-CoA molecules. For example, palmitoyl-CoA (16 carbons) forms 8 acetyl-CoA molecules.
Several enzymes (collectively known as FA oxidase) catalyze the reaction in the mitochondrial matrix or inner membrane adjacent to the respiratory chain.
These steps couple acyl-CoA oxidation with ADP phosphorylation to ATP.

Oxidation of Odd-Number FAs

Odd-number FAs are also oxidized via β-oxidation, creating acetyl-CoA until a 3-carbon propionyl-CoA residue remains.
Propionyl-CoA is converted to succinyl-CoA (a citric acid cycle constituent).
The propionyl residue is the only glucogenic portion of the odd-number FA.

β-oxidation in Peroxisomes

Very long-chain FAs (20 carbons or more) undergo preliminary β-oxidation in peroxisomes, yielding acetyl-CoA and H₂O₂.
The initial step of dehydrogenation is catalyzed by an FAD-containing acyl-CoA oxidase, unique to peroxisomal β-oxidation.
The produced FADH₂ is oxidized by molecular oxygen, generating H₂O₂. This H₂O₂ is then reduced to H₂O by catalase.
Peroxisomal β-oxidation isn't directly linked to ATP production. However, its role includes shortening the side chain of cholesterol during bile acid formation and participation in plasmalogen, cholesterol, and dolichol synthesis.

α-oxidation of FAs

α-oxidation involves removing one carbon from the carboxyl end of the FA molecule, primarily occurring in brain tissue.
Branched-chain FAs (e.g., phytanic acid), due to their methyl group on the β-carbon, are not substrates for acyl-CoA dehydrogenase.
Instead, these FAs are hydroxylated at the α-carbon by an α-hydroxylase and subsequently decarboxylated to generate a CoA derivative that enters the β-oxidation pathway.
α-oxidation does not lead to high-energy phosphate production.

ω-oxidation of FAs

ω-oxidation is a minor pathway that occurs in the endoplasmic reticulum (ER), involving cytochrome P450-dependent hydroxylases.
The ω-carbon (the last carbon) of the FA is hydroxylated, followed by oxidation to form a carboxyl group. The final product is a dicarboxylic acid.

Oxidation of Unsaturated FAs

Unsaturated FAs yield less energy compared to saturated FAs because they are less reduced and produce fewer reducing equivalents (NADH and FADH2).

Oxidation of Monounsaturated FAs

Monounsaturated fatty acids (e.g. oleic acid), require 2,3-enoyl-CoA isomerase to convert a 3-cis derivative after several β-oxidation cycles to a 2-trans derivative to be used by the hydroxylase.

Oxidation of Polyunsaturated FAs

The oxidation of polyunsaturated FAs (e.g., linoleic acid) necessitates NADPH-dependent reductase in addition to the isomerase.

Clinical Conditions

Carnitine deficiency: Characterized by episodic hypoglycemia and muscle weakness. Causes include preterm infants, liver disease, malnutrition, vegetarianism, pregnancy, severe infections, burns, trauma, or hemodialysis, or organic aciduria. Treatment involves oral carnitine supplementation.
Inherited defects in β-oxidation enzymes: lead to nonketotic hypoglycemia, coma, and fatty liver.
Dicarboxylic aciduria: Excretion of C6-C10 ω-dicarboxylic acids and nonketotic hypoglycemia. Due to a lack of mitochondrial medium-chain acyl-CoA dehydrogenase, the oxidation of long and medium-chain FAs is impaired, leading to the buildup and excretion of dicarboxylic acids.
Refsum disease: A rare, autosomal recessive disorder caused by a α-hydroxylase deficiency, resulting in phytanic acid accumulation. Symptoms are primarily neurological.
Zellweger's syndrome: An inherited condition characterized by the absence of peroxisomes in all tissues. It leads to the accumulation of very long-chain FAs in the blood and tissues.

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Description

Test your knowledge on the oxidation of fatty acids and the biochemical processes involved. This quiz covers the transport of fatty acids in the blood, their activation, and the β-oxidation pathway that converts them into acetyl-CoA for ATP production. Dive into the details of this essential metabolic process!