Biochemistry : Other Lipid Catabolism Concepts

Study concepts, example questions & explanations for Biochemistry

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Example Questions

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Example Question #1 : Other Lipid Catabolism Concepts

Where in a cell are fatty acids broken down via -oxidation?

Possible Answers:

Smooth endoplasmic reticulum

Cytoplasm

Mitochondria

Nucleus

Lysosome

Correct answer:

Mitochondria

Explanation:

Fatty acids are taken into the mitochondria to be broken down. This makes sense especially if you consider that the acetyl-CoA generated can be directly used in the citric acid cycle and oxidative phosphorylation immediately afterwards. Note that some beta-oxidation of fatty acids also occurs in the lysosome when the fatty acids chains are too long for the mitochondria. 

Example Question #2 : Other Lipid Catabolism Concepts

Which of the following is the general overview the process of beta-oxidation of saturated fatty acid?

Possible Answers:

Oxidation, hydration, oxidation, thiolysis

Reduction, hydration, oxidation, thiolysis

Reduction, dehydration, reduction, thioesterification

Oxidation, hydration, reduction, thiolysis

Oxidation, hydration, oxidation, decarboxylation

Correct answer:

Oxidation, hydration, oxidation, thiolysis

Explanation:

The basic pattern of saturated fatty acid catabolism is that the chain is broken down two carbons at a time by release of acetyl-CoA. So, the challenge for the cell is to turn a two-carbon alkyl group at the end of the chain into a thioester (remember, acetyl-CoA is . To do this, the cell first desaturates the chain (removes some hydrogens by oxidation) to form a double bond. Then, water is added across the double bond to form an alcohol. Thinking back to organic chemistry, remember we can oxidize an alcohol to get a carbonyl, which is exactly what the cell does. The result is a ketone which reacts with the thiol of CoA-SH to form the new thioester acetyl-CoA, which is removed from the chain in the process of its formation.

Example Question #3 : Other Lipid Catabolism Concepts

Long-chain fatty acids are broken down through beta-oxidation in the mitochondrial matrix. The result is an abundance of acetyl-CoA which can then go onto the Krebs cycle and oxidative phosphorylation. However, when plasma glucose is low, stores of oxaloacetate are depleted to form more glucose, and the Krebs cycle becomes unable to incorporate all of the acetyl-CoA from beta-oxidation. 

What is the fate of the resultant excess of acetyl-CoA? 

Possible Answers:

Further beta-oxidation

Citrate production

Gluconeogenesis

Ketogenesis

Fermentation to ethanol

Correct answer:

Ketogenesis

Explanation:

When plasma glucose is low (or when plasma glucose is inaccessible to cells as in diabetes) and glycogen stores have been depleted, the cells resort to oxidation of fatty acids for energy. Because the brain cannot utilize fat for energy, though, and because glucose is an important fuel source for other tissues as well, the liver begins to produce glucose from Krebs cycle intermediates like oxaloacetate. Once these begin to run low, Krebs cycle function slows, and acetyl-CoA from fatty acid oxidation builds up. The body's solution to this problem is to have the liver convert this excess acetyl-CoA into ketone bodies, which can be utilized by the brain and other tissues for energy. 

Example Question #4 : Other Lipid Catabolism Concepts

Which of the following characterizes the differences between chloroplasts and mitochondria as regards the way their relationship to lipids?

Possible Answers:

Neither mitochondria nor chloroplasts synthesize the lipids they require.

Mitochondria usually make the lipids they need, whereas chloroplasts get most of their lipids from an external source.

Both chloroplasts and mitochondria synthesize the lipids they require.

Neither mitochondria nor chloroplasts require lipids to function.

Chloroplasts usually make the lipids they need, whereas mitochondria get most of their lipids from an external source.

Correct answer:

Chloroplasts usually make the lipids they need, whereas mitochondria get most of their lipids from an external source.

Explanation:

Lipids tend to be created outside and brought into mitochondria. For example, the endoplasmic reticulum makes phosphatidylcholine and phosphatidylserine, which are then moved to the mitochondrial outer membrane. Chloroplasts, on the other hand, create lipids themselves, such as glycolipids. Both mitochondria and chloroplasts require lipids to function.

Example Question #5 : Other Lipid Catabolism Concepts

Which is not a chemical reaction fundamental to peroxisomes?

Possible Answers:

Oxidation of substrates to remove hydrogen atoms and generate hydrogen peroxide

Hydrolysis of peptidoglycans

Using hydrogen peroxide to detoxify molecules dangerous to the cell

Starting the process of synthesizing plasmalogens

Beta-oxidation of fatty acids, converting them into acetyl-CoA

Correct answer:

Hydrolysis of peptidoglycans

Explanation:

Peroxisomes have a wide variety of functions, including both the production and catabolism of hydrogen peroxide (hence "peroxisome"). These organelles also perform the first chemical steps in the synthesis of plasmalogens, which are used to make the myelin sheaths around nerve cells. Peroxisomes break down fatty acids into acetyl-CoA. This process also occurs in mitochondria. The hydrolysis and thus breakdown of peptidoglycans, which are found in bacteria, is performed by the lysozyme enzyme, not within peroxisomes. (It is easy to confuse peroxisomes and lysozymes, because they serve some similar catabolic purposes.)

Example Question #6 : Other Lipid Catabolism Concepts

How do fatty acids get into the mitochondrial matrix to be further catabolized as a source of energy?

Possible Answers:

The fatty acids do not need to move into the matrix - they are catabolized in the cytoplasm.

The fatty acids are converted back to triacylglycerols which can then move easily into the matrix.

The fatty acids are activated by attaching to carnitine.

The fatty acids are transported into the matrix by lipase.

The fatty acids diffuse into the mitochondrial matrix.

Correct answer:

The fatty acids are activated by attaching to carnitine.

Explanation:

Fatty acids must be transported into the mitochondrial matrix in order to go through beta oxidation. Before they can move into the matrix, they must be conjugated to carnitine. Only then can the newly conjugated compound (acyl carnitine) be taken into the matrix by a translocase enzyme.

Example Question #7 : Other Lipid Catabolism Concepts

After beta oxidation of a fatty acid with an odd number of carbons in its carbon chain, propionyl-CoA is produced. How does this molecule enter into the Krebs cycle?

Possible Answers:

Propionyl-CoA is converted to fumarate which is a Krebs Cycle intermediate molecule

Propionyl-CoA is converted to oxaloacetate which is a Krebs cycle intermediate molecule

Propionyl-CoA enters into the Krebs cycle directly similarly to acetyl-CoA's mechanism of action

Propionyl-CoA is converted to succinyl-CoA which is a Krebs cycle intermediate molecule

Propionyl-CoA does not enter into the Krebs cycle - it is converted into an intermediate molecule involved in glycolysis and enters during that stage of oxidative respiration

Correct answer:

Propionyl-CoA is converted to succinyl-CoA which is a Krebs cycle intermediate molecule

Explanation:

In order to enter into the Krebs Cycle, propionyl-CoA must be converted into a similar molecule, because it can not enter into the citric acid cycle as is. So, it becomes succinyl-CoA via a 3 step process.

Example Question #8 : Other Lipid Catabolism Concepts

Cholesterol and triglycerides are transported in the blood by lipoproteins. These lipoproteins are: very low density proteins (VLDL), high density proteins (HDL), intermediate density proteins (IDL), chylomicrons and low-density proteins (LDL). Which of the following is true regarding these lipoproteins?

Possible Answers:

IDLs have more protein content than LDLs

Chylomicrons have less protein content than VLDLs

VLDLs have more protein content than IDLs

IDLs have more protein content than HDLs

Chylomicrons have more protein content than HDLs

Correct answer:

Chylomicrons have less protein content than VLDLs

Explanation:

Chylomicrons, made up mostly of triglycerides, are the lipoproteins with the least amount of  protein (percentage of protein in the lipoprotein). Following, in the order of increasing protein amount are: VLDLs, IDLs, LDLs and HDLs. 

Example Question #9 : Other Lipid Catabolism Concepts

A deficiency of an enzyme in lipid metabolism leads to high levels of triglycerides in the blood. What is the name of the deficient enzyme most likely involved?

Possible Answers:

Acetyl-CoA carboxylase

Fatty acid synthase

Lipoprotein lipase

3-hydroxy-3-methylglutaryl-coenzyme A synthase (HMG-CoA synthase)

3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMG-CoA reductase)

Correct answer:

Lipoprotein lipase

Explanation:

Lipoprotein lipase is responsible for degrading triglycerides into two fatty acids and a monoacylglycerol molecule. It is attached to the endothelial lumen of the blood vessel. It removes triglycerides from very-low density lipoproteins and chylomicrons in the blood. HMG-CoA reductase and synthase are important enzymes in de novo cholesterol synthesis, but do not cause hypertriglyceridemia (high levels of triglycerides in the blood). Acetyl-CoA carboxylase and fatty acid synthase are part of lipid anabolism, form fatty acids and do not cause hypertriglyceridemia.

Example Question #10 : Other Lipid Catabolism Concepts

How do high-density lipoproteins (HDL) remove cholesterol from the periphery?

I. HDL transfer cholesterol to liver cells through the scavenger receptor SR-B1

II. HDL transfer cholesterol to intermediate-density lipoproteins using the cholesterol transfer protein

III. HDL removes cholesterol accumulating in blood vessels

IV. HDL is taken up by macrophages in atherosclerotic plaques

Possible Answers:

II and IV

I and II

II and III

I, II, and III

I and IV

Correct answer:

I, II, and III

Explanation:

Cholesterol and lipids are carried in the blood by lipoproteins. HDL are lipoproteins that remove cholesterol accumulated in blood vessels and take it to the liver for excretion in the bile or further processing by steroidogenic tissues. HDL delivers cholesterol to liver cells through the scavenger receptor SR-B1. HDL can also transfer cholesterol to intermediate-density lipoproteins (IDL) using the cholesterol transfer protein. Incorporation of LDL (low-density lipoproteins), not HDL by macrophages leads to formation of fatty streaks in atherosclerotic plaques. This does not remove cholesterol from periphery, but rather contributes to it. 

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