Biochemistry : Anabolic Pathways and Synthesis

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #101 : Anabolic Pathways And Synthesis

The molecule repeatedly added to the growing carbon chain of an endogenously synthesized fatty acid contains how many carbons?

Possible Answers:

Correct answer:

Explanation:

The molecule which is repeatedly added to a growing fatty acid is malonyl-CoA. Malonyl-CoA is synthesized from acetyl-CoA (two carbons) and  (one carbon), and, thus, contains three carbons. Of course, it is important to remember that the  of malonyl-CoA leaves during the reaction with the acyl chain being synthesized. 

Example Question #1 : Lipid Synthesis Reactants, Intermediates, And Products

Which of the following is true about fatty tissue?

Possible Answers:

Adipocytes stock mainly glycerol

The lipoprotein lipase digests chylomicrons, yielding glycerol and triacyglycerides

Fatty tissue is composed of lipids, which originate mostly in the liver

Chylomicrons are secreted by enterocytes

Correct answer:

Chylomicrons are secreted by enterocytes

Explanation:

Adipocytes stock mainly fatty acids, not glycerol; glycerol produced during triacylglycerol degradation is shuttled through the blood to the liver. Lipase does not digest chylomicrons, but rather triacylglycerol, producing glycerol and fatty acids. The lipids (fatty acids) in fatty tissue mostly originate in our diet, not in our liver. Mammals specifically require certain polyunsaturated fatty acids which they are unable to synthesize, like linoleate; these are known as the essential fatty acids. Chylomicrons are proteins which carry triacylglycerols, cholesterol, and other lipids, obtained by the diet, away from the intestine. Chylomicrons are created in the endoplasmic reticulum of small intestine cells i.e., enterocytes and exoctyosed into lymphatic capillaries.

Example Question #1233 : Biochemistry

In what order does fatty acid synthesis occur?

Possible Answers:

Condensation, reduction, dehydration, reduction

Reduction, condensation, dehydration, reduction

Condensation, reduction, reduction, cleavage

Oxidation, hydration, oxidation, cleavage

Reduction, reduction, dehydration, condensation

Correct answer:

Condensation, reduction, dehydration, reduction

Explanation:

The proper order for fatty acid synthesis is condensation, reduction, dehydration, and reduction once again.  This creates an activated acyl group that has been lengthened by two carbons through this anabolic biosynthetic pathway.

Example Question #1234 : Biochemistry

Triglycerides (triacylglycerols) contain three fatty acid molecules and a glycerol molecule. Glycerol-3-phosphate is necessary for triglyceride synthesis. Which of these are the sources of glycerol-3 phosphate in the body?

I. Phosphorylation of glycerol by glycerol kinase in the liver forms glycerol-3 phosphate

II. Glycerol 3-phosphatase reduces dihydroacetone phosphate to glycerol 3-phosphate in the liver and adipose tissue

III. Phosphorylation of glycerol by glycerol kinase in the adipose tissue

Possible Answers:

II only

I and II

I only

I, II, and III

I and III

Correct answer:

I and II

Explanation:

The sources of glycerol-3 phosphate for triglyceride synthesis are glycerol in the liver, but not the adipose tissue (adipose tissue does not have glycerol kinase) and from the conversion of dihydroxyacetone phosphate (obtained in glycolysis) to glycerol-3 phosphatase in liver and adipose tissue. Triglycerides are one of the most important forms of storage of lipids in the body. 

Example Question #1 : Lipid Synthesis Reactants, Intermediates, And Products

Triglycerides and glycerophospholipids are lipids similar in structure. What are some structural and functional characteristics of triglycerides and glycerophospholipids?

Possible Answers:

Triglycerides are a form of storing lipids in the body

All of these are correct

Glycerophospholipids contain choline or inositol groups

Glycerophospholipids are part of the cellular membrane and are sources of second messengers such as diacylglycerol

Triglycerides are formed by three fatty acids and a glycerol molecule

Correct answer:

All of these are correct

Explanation:

Triglycerides are the major form of storing dietary lipids in the body.Triglycerides are composed of three fatty acids and a glycerol molecule. In glycerophospholipids the third fatty acid of a triglyceride particle is replaced by a phosphate group and a choline or inositol group. Choline groups are ammonium salt groups in neurotransmitters or phospholipids on cell membranes. Inositol groups are found in second messengers. Glycerophospholipids are part of the cellular membrane and are sources of second messengers such as diacylglycerol and inositol-3-phosphate. 

Example Question #11 : Lipid Synthesis Reactants, Intermediates, And Products

What is the role of cholesterol ester transfer protein in lipid metabolism?

Possible Answers:

It transfers cholesterol esters between different types of lipoproteins

It is part of of high density lipoproteins

It is a receptor on liver cells

It is part of chylomicrons

It adds fatty acid groups to cholesterol

Correct answer:

It transfers cholesterol esters between different types of lipoproteins

Explanation:

Cholesterol ester transfer protein's role in lipid metabolism involves transferring cholesterol esters or triglycerides between different types of lipoproteins in the blood. It is not part of the lipoprotein particle and is not a receptor but, rather, a protein in the blood. Cholesterol and triglycerides are carried in the blood by lipoproteins, which depending on the amount of protein contained are: chylomicrons, very low density proteins, low-density proteins, intermediate density lipoproteins and high density lipoproteins. 

Example Question #12 : Lipid Synthesis Reactants, Intermediates, And Products

Cholesterol is an important lipid required in membranes and for steroid synthesis. Cholesterol can be synthesized from acetyl-coenzyme A. Which of the following is correct about this process?

Possible Answers:

3-hydroxy-3 methylglutaryl-CoA reductase (HMG-CoA reductase) converts HMG-CoA to mevalonate

3-hydroxy-3 methylglutaryl-CoA synthase (HMG-CoA synthase) converts  acetyl-CoA to HMG-CoA

All of these

3-hydroxy-3 methylglutaryl-CoA reductase (HMG-CoA reductase) is the rate-limiting enzyme of the de novo cholesterol synthesis

HMG-CoA reductase is localized in the smooth endoplasmic reticulum and HMG-CoA synthase is in the cytoplasm

Correct answer:

All of these

Explanation:

All the answers are correct and show reactions that are necessary in the process of cholesterol synthesis.Cells receive cholesterol from dietary lipoproteins such as low-density lipoproteins and high-density lipoproteins. However, cholesterol can be synthesized " de novo" by the liver directly from acetyl CoA thru a series of reactions described above. HMG-CoA reductase, the rate-limiting enzyme in the process is stimulated by insulin and inhibited by stain drugs.

Example Question #1242 : Biochemistry

Where are triglycerides produced in the body and what hormone regulates their production?

I. In the adipose tissue and liver. Their production is regulated by insulin and glucagon.

II. In the blood and liver. Their production is regulated by epinephrine and antidiuretic hormone.

III. In the blood as very-low density lipoproteins and chylomicrons. Their production is regulated by epinephrine and growth hormone.

IV. In the muscle. Their production is regulated by growth hormone.

Possible Answers:

I and IV

II and III

I only

I and II

III and IV

Correct answer:

I only

Explanation:

Triglycerides are produced in the adipose tissue and liver. Their production is regulated by insulin and glucagon. They are not directly regulated by growth factors or antidiuretic hormone (ADH). Also, they are transported in the blood as lipoproteins, but are not produced in the blood.

Example Question #1243 : Biochemistry

How can fatty acids be created from glucose?

Possible Answers:

Glucose can be redirected through the pentose phosphate pathways to create fatty acids.

Fatty acids can not be created from glucose, but glucose can be created from fatty acids.

The ATP created at the end of the electron transport chain can be used directly to synthesize fatty acids chains.

Fatty acids can be created from several of the citric acid cycle's intermediate molecules.

Acetyl-CoA created from pyruvate can be utilized to create fatty acids.

Correct answer:

Acetyl-CoA created from pyruvate can be utilized to create fatty acids.

Explanation:

Fatty acid synthesis occurs via the addition of acetyl-CoA carbons to a growing fatty acid chain. And so, in order to create fatty acids from glucose, there must be a link between the two molecules. That connection is the acetyl-CoA that is formed from pyruvate at the end of glycolysis and the pyruvate dehydrogenase complex. However, fatty acids can not create pyruvate, as there are no enzymes capable of this reverse reaction.

Example Question #1244 : Biochemistry

Which of the following are true regarding acetyl-CoA carboxylase in fatty acid synthesis? 

I. The active form of the enzyme is dephosphorylated.

II. Acetyl-CoA carboxylase converts acetyl-CoA to malonyl-CoA, necessary for fatty acid synthesis.

III. Acetyl-CoA carboxylase is highly expressed in adipose tissue and lactating mammary glands, where fatty acid synthesis is important.

IV. Acetyl-CoA carboxylase is highly expressed in the liver.

Possible Answers:

I and IV

I only

II, III, and IV

I and II

I, II, III, and IV

Correct answer:

I, II, III, and IV

Explanation:

Acetyl-CoA carboxylase is high in adipose tissue, lactating mammary glands and liver where fatty acid synthesis is important. It has two catalytic activities as a biotin carboxylase and carboxytransferase. Acetyl-CoA carboxylase converts acetyl-CoA to malonyl-CoA. Compared to other enzymes that are phosphorylated when active, acetyl-CoA carboxylase needs to be dephosphorylated in order to be active.

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