Biochemistry: Lipid Metabolism Quiz

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Questions and Answers

What is produced alongside ATP during the complete oxidation of palmitoyl-CoA?

Glucose

Insulin

CoA (correct)

Lactate

How many molecules of CO2 are produced from the complete oxidation of palmitoyl-CoA?

16 (correct)

Which enzyme catalyzes the step yielding 10.5 ATP during β-oxidation?

Malate dehydrogenase

Succinate dehydrogenase

Isocitrate dehydrogenase

Acyl-CoA dehydrogenase (correct)

What is the total ATP yield during the complete oxidation of one molecule of palmitoyl-CoA?

108 Signup and view all the answers

From the overall reaction, how many molecules of O2 are consumed during the complete oxidation of palmitoyl-CoA?

23 Signup and view all the answers

What is the primary function of gluconeogenesis in the liver?

Synthesize glucose from non-carbohydrate sources Signup and view all the answers

Which of the following is a unique characteristic of ketone bodies?

They can cross the blood-brain barrier Signup and view all the answers

Which of these is NOT a significant source of fatty acid fuels in vertebrates?

Conversion of excess amino acids to fats Signup and view all the answers

How do chylomicrons primarily transport lipids?

As aggregates of triacylglycerols and apolipoproteins Signup and view all the answers

What activates lipoprotein lipase?

Apolipoprotein C-II Signup and view all the answers

Which statement best describes lipoprotein particles?

They have varying densities based on lipid and protein composition Signup and view all the answers

What role does apolipoprotein B-48 play in chylomicrons?

It is the primary structural protein Signup and view all the answers

Which of the following represents a way cells obtain fatty acids?

From the breakdown of chylomicrons in muscle tissue Signup and view all the answers

What role does apolipoprotein C-II play in the body?

It activates lipoprotein lipase activity on chylomicrons. Signup and view all the answers

Which process occurs in the intestinal mucosa regarding dietary lipids?

Water-soluble lipases break down triacylglycerols into smaller components. Signup and view all the answers

What role does inorganic pyrophosphatase play in the formation of fatty acyl–CoA?

It hydrolyzes pyrophosphate, making the activation reaction more favorable. Signup and view all the answers

What is the main function of lipoprotein lipase activated by apoC-II?

It hydrolyzes triacylglycerols into free fatty acids and monoacylglycerols. Signup and view all the answers

Which enzyme catalyzes the attachment of a fatty acyl-CoA to carnitine?

Carnitine acyltransferase 1 (CAT1) Signup and view all the answers

Which statement regarding dietary lipid processing in vertebrates is true?

Chylomicrons transport dietary lipids to the lymph system. Signup and view all the answers

Which statement is false concerning lipoprotein aggregation and chylomicrons?

Chylomicrons are primarily composed of water-soluble proteins. Signup and view all the answers

How does fatty acyl-carnitine enter the mitochondrial matrix?

Through the acyl-carnitine/carnitine cotransporter by passive transport. Signup and view all the answers

What is the primary function of carnitine in fatty acid metabolism?

To transport fatty acyl-CoA across mitochondrial membranes. Signup and view all the answers

In which location does the activation of lipoprotein lipase by apolipoprotein C-II primarily take place?

In the capillaries of muscle and adipose tissues. Signup and view all the answers

What happens to the fatty acyl group once it enters the mitochondrial matrix?

It is reattached to coenzyme A by carnitine acyltransferase 2 (CAT2). Signup and view all the answers

What is a key function of bile salts in the processing of dietary lipids?

They emulsify dietary lipids in the intestine. Signup and view all the answers

How do hormones and growth factors affect metabolic processes in the body?

They coordinate activities among various tissues and organs. Signup and view all the answers

Which statement correctly describes the transport mechanism for fatty acyl-carnitine?

It relies on a mechanism that exchanges it with carnitine in the intermembrane space. Signup and view all the answers

What occurs immediately after the activation of a fatty acid to form fatty acyl-CoA?

Inorganic pyrophosphatase hydrolyzes the produced pyrophosphate. Signup and view all the answers

What is the result of the action of carnitine acyltransferase 2 (CAT2) within the mitochondria?

Regeneration of free carnitine and fatty acyl-CoA. Signup and view all the answers

What is the primary function of the β-hydroxyacyl-CoA dehydrogenase enzyme?

To convert L-β-hydroxyacyl-CoA to β-ketoacyl-CoA Signup and view all the answers

Which enzyme is specifically associated with the cleavage of β-ketoacyl-CoA?

Acyl-CoA acetyl-transferase (thiolase) Signup and view all the answers

How many enzymes are involved in the mitochondrial β oxidation of fatty acids?

Four enzymes Signup and view all the answers

What is the role of NADH dehydrogenase in the β-oxidation pathway?

It accepts electrons from NADH Signup and view all the answers

What is the composition of the trifunctional protein (TFP) involved in β-oxidation?

Heterooctamer of α4β4 subunits Signup and view all the answers

Which of the following options does NOT represent an enzyme in mitochondrial β oxidation?

NADH oxidase Signup and view all the answers

What mechanism does evolution select for in chemical reactions during the breakdown of fatty acids?

Energetically favorable reactions Signup and view all the answers

What reaction can be described as a reverse Claisen condensation in the context of β-oxidation?

Cleavage of β-ketoacyl-CoA Signup and view all the answers

Which enzyme catalyzes the condensation of two acetyl-CoA molecules to form acetoacetyl-CoA?

Thiolase Signup and view all the answers

What is produced during the cleavage of HMG-CoA?

Acetoacetate and acetyl-CoA Signup and view all the answers

Which process is catalyzed by D-β-hydroxybutyrate dehydrogenase?

Oxidation of D-β-hydroxybutyrate to acetoacetate Signup and view all the answers

What is the role of β-ketoacyl-CoA transferase in ketone body metabolism?

It activates acetoacetate. Signup and view all the answers

Which of the following statements is true regarding the metabolism of D-β-hydroxybutyrate?

It produces NADH during its catabolism. Signup and view all the answers

What happens to acetyl-CoA after it enters the citric acid cycle?

It is oxidized for energy production. Signup and view all the answers

How do ketone bodies differ from fatty acids in their function?

Ketone bodies serve as an energy source for the brain. Signup and view all the answers

What is the initial substrate used to form D-β-hydroxybutyrate during ketone body metabolism?

Acetoacetate Signup and view all the answers

Study Notes

Fatty Acid Catabolism

Fatty acid catabolism is a central energy-yielding pathway in many organisms and tissues
Long-chain fatty acids provide up to 80% of energetic needs in mammalian heart and liver
Fatty acid oxidation contributes to over 40% of daily energy requirements in mammals
Electrons released during fatty acid oxidation travel through the respiratory chain, driving ATP synthesis
Acetyl-CoA produced from fatty acids can be completely oxidized to CO2 in the citric acid cycle

β Oxidation

β oxidation is the oxidation of the fatty acyl group at the C-3 position (hence the name)
It occurs after the carboxyl group at C-1 is activated by attachment to coenzyme A

Principles of Fatty Acid Catabolism

Principle 1: Diverse metabolites funnel into few central pathways, whereby fatty acid catabolism and glycolysis convert distinct starting materials into a common product (acetyl-CoA). Associated electrons from oxidative pathways and the citric acid cycle are then carried by cofactors (NAD and FAD) to the mitochondrial respiratory chain, which leads to oxygen, ultimately producing ATP through oxidative phosphorylation.
Principle 2: Evolution prioritizes chemical mechanisms that enhance energy efficiency. In fatty acid breakdown, the conversion of carboxylic acids into thioesters, such as acetyl-CoA, within the citric acid cycle is a key mechanism. This process breaks the relatively inert C-C bonds in long fatty acid chains by creating a carbonyl group adjacent to the CH2 group.
Principle 3: Allosteric mechanisms and post-translational regulation (e.g., protein phosphorylation) coordinate metabolic processes within a cell. Hormones and growth factors control metabolic activity among tissues and organs. Reciprocal control of catabolic and anabolic pathways prevent futile cycling.
Principle 4: Loss of essential components like enzymes, cofactors, or regulatory agents disrupt homeostasis and manifest across a spectrum of severity, leading to diseases. Fatty acid breakdown defects exemplify this principle.
Principle 5: The liver plays a critical role in whole-body metabolism. When glucose is not available the liver produces glucose via gluconeogenesis and distributes it to tissues (including the brain). During starvation, the liver converts fatty acids to ketone bodies, which, unlike fatty acids, can cross the blood-brain barrier and fuel the brain.

Digestion, Mobilization, and Transport of Fats

Dietary fats are absorbed in the small intestine, where bile salts emulsify them, forming mixed micelles.
Intestinal lipases then degrade triacylglycerols (TAGs) into fatty acids (FAs) and other breakdown products.
These products are taken up by the intestinal mucosa, reconverted to triacylglycerols, and packaged into chylomicrons.
Chylomicrons are lipoprotein aggregates that transport lipids and lipoprotein lipase converts the TAGs in the chylomicrons into fatty acids and monoacylglycerols. These are then used by myocytes or adipocytes for oxidation as fuel or reesterfication for storage.

Sources of Fatty Acid Fuels

Fats originate from four sources: dietary fats, stored fats in lipid droplets, fats synthesized in one organ for transport to another, and fats obtained via autophagy

Chylomicron Formation

Apolipoproteins are lipid-free proteins that bind lipids to form lipoproteins
Chylomicrons = particles consisting of triacylglycerols, cholesterol, and apolipoproteins
These are essential for the transport of lipids between organs.

Lipoprotein Particles

Spherical aggregates of apolipoproteins and lipids
Hydrophobic lipids are at the core, while hydrophilic lipids and protein components reside on the surface
Vary in density based on the mix of lipids and protein
Range from chylomicrons to HDL

Apolipoprotein B-48 (apoB-48) and Apolipoprotein C-II (apoC-II)

apoB-48 = major protein component of chylomicrons
apoC-II = protein found in the blood and is picked up by chylomicrons from HDL (high-density lipoprotein) particles

Lipoprotein Lipase

Extracellular enzyme in capillaries of muscle and adipose tissue.
Hydrolyzes triacylglycerols into free fatty acids and monoacylglycerols.
The enzyme is activated by apoC-II

Fatty Acyl-CoA Synthetase

Present in the outer mitochondrial membrane
Activates fatty acids through conversion into fatty acyl-CoA thioesters
Fatty acid + CoA + ATP produce fatty acyl-CoA + AMP + PP;

Carnitine

A compound that transports fatty acyl-CoAs across the inner mitochondrial membrane for oxidation.

Carnitine Acyltransferase 1 (CAT1/CPT1)

Catalyzes the transesterification reaction to attach fatty acyl-CoA to the hydroxyl group of carnitine creating fatty acyl carnitine, a necessary step for fatty acid entry into the inner mitochondrial membrane.

Acyl-carnitine/carnitine cotransporter

Allows passive transport of fatty acyl-carnitine esters across the inner mitochondrial membrane
Moves one carnitine into the intermembrane space while the other fatty acyl-carnitine moves into the matrix.

Carnitine Acyltransferase 2 (CAT2/CPT2)

Transfers the fatty acyl group from carnitine back to coenzyme A, then regenerates fatty acyl-CoA and free carnitine
Located on the inner face of the mitochondrial membrane.

Stage 1 of Fatty Acid Oxidation

Fatty acids undergo oxidative removal of successive two-carbon units in the form of acetyl-CoA

Stage 2 of Fatty Acid Oxidation

Acetyl-CoA groups are oxidized to carbon dioxide (CO2) in the mitochondrial matrix's citric acid cycle, producing NADH, FADH2 and GTP

Stage 3 of Fatty Acid Oxidation

Electron transfer chain and oxidative phosphorylation generate ATP from NADH and FADH2.

Fatty Acid Oxidation - Additional Information

The four enzymes of mitochondrial β oxidation are acyl-CoA dehydrogenase, enoyl-CoA hydratase, β-hydroxyacyl-CoA dehydrogenase, and acyl-CoA acetyltransferase (thiolase).
The trifunctional protein (TFP) is a multienzyme complex associated with the inner mitochondrial membrane that catalyzes steps 2–4 of the β-oxidation pathway for fatty acyl chains of 12+ carbons; this allows efficient substrate channeling through the process.
Oxidation of unsaturated fatty acids requires two additional enzymes: enoyl-CoA isomerase (converts cis double bonds to trans bonds) and 2,4-dienoyl-CoA reductase (reduces cis double bonds).
Oxidation of odd-number fatty acids requires three further reactions: propionyl-CoA carboxylase, methylmalonyl-CoA epimerase, and methylmalonyl-CoA mutase.
The liver lacks ß-ketoacyl-CoA transferase
The enzymes that catalyze the synthesis of ketone bodies are predominantly found in the cytosol of hepatocytes.
NADH is produced during the catabolism of D-β-hydroxybutyrate.
Conversion of 2 acetyl-CoAs to acetoacetyl-CoA is accompanied by the hydrolysis of ATP to AMP and PPi.

Regulation and Defects

Malonyl-CoA, the first intermediate in fatty acid synthesis, inhibits carnitine acyltransferase 1 (CAT1), preventing simultaneous synthesis and degradation of fatty acids.
PPARα (peroxisome proliferator-activated receptor alpha) is a nuclear transcription factor that enhances the expression of genes for β-oxidation enzymes which aids regulation of fatty acid oxidation/metabolism when energetic demand is high.
Genetic defects in fatty acyl-CoA dehydrogenases or other enzymes involved can lead to serious diseases like medium-chain acyl-CoA dehydrogenase (MCAD) deficiency, causing problems with fatty acid oxidation and potentially life-threatening consequences.
Zellweger and X-linked adrenoleukodystrophy also result from defects in peroxisome function/pathways and result in impaired fatty acid metabolism.

Peroxisomal β-oxidation

Peroxisomes contain the flavoprotein acyl-CoA oxidase which produces H2O2
The enzyme catalase cleaves H2O2.
Peroxisomal oxidation is more active for very-long-chain fatty acids and branched-chain fatty acids.
Phytanic acid undergoes a-oxidation and not β-oxidation due to its methyl group on the β carbon.

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Description

Test your knowledge on lipid metabolism with this quiz focused on the oxidation of palmitoyl-CoA and related metabolic pathways. Explore concepts such as ATP yield, enzyme functions, and the role of lipoproteins and ketone bodies. Perfect for biochemistry students looking to reinforce their understanding of fat metabolism.