Biochemistry : Anabolic Pathways and Synthesis

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #91 : Anabolic Pathways And Synthesis

The enzyme Acyl-Carrier Protein (ACP) plays an important role in the biosynthesis of which of the following?

Possible Answers:

Carbohydrates

Lipids

Nucleic Acids

Proteins

Correct answer:

Lipids

Explanation:

Acyl-Carrier Protein (ACP) is a protein that is important to the generation of lipids. Specifically, it aids in the production of fatty acids. Furthermore, ACP is just one component of the Fatty Acid Synthase enzyme, which is devoted to the synthesis of fatty acids.

To begin the process, ACP is first activated by having an acetyl-CoA molecule attached to it. Next, a compound called malonyl-CoA is attached to the bound acetyl-CoA. Malonyl-CoA is a three carbon compound, but upon being added to the acetyl-CoA, the malonyl-CoA becomes decarboxylated. The importance of this is that by producing carbon dioxide as a product, this helps to greatly drive the reaction forward.

Keep in mind that there are other chemical transformations happening when these malonyl-CoA molecules are being "stitched" together. Every time a malonyl-CoA is added, the carbon chain becomes increased by two more carbons. This keeps happening until, finally, a fatty acid is generated.

Example Question #92 : Anabolic Pathways And Synthesis

What is the role of lecithin-cholesterol acyltransferase (LCAT) in cholesterol metabolism and transport in the body?

Possible Answers:

LCAT is an enzyme that incorporates acetyl-CoA into fatty acids

LCAT produces cholesterol esters from cholesterol, which are transported from the peripheral tissues to the liver

LCAT is an enzyme that hydrolyzes fatty acids from triglycerides and cholesterol

LCAT does not impact cholesterol transport

LCAT is an enzyme that converts malonyl-CoA into palmitate

Correct answer:

LCAT produces cholesterol esters from cholesterol, which are transported from the peripheral tissues to the liver

Explanation:

Lecithin-cholesterol acyltransferase-LCAT adds a fatty acid to cholesterol, which can then be loaded onto high-density lipoproteins. Without the enzyme, cholesterol does not get to be transported by high density lipoproteins to the liver. Cholesterol then accumulates in tissue such as the eye and renal tissue. LCAT does impact cholesterol transport. Lipoprotein lipase is the enzyme that hydrolyzes fatty acids from triglycerides and cholesterol. Fatty acid synthase converts malonyl-CoA into palmitate. Acetyl-CoA carboxylase is the enzyme that incorporates acetyl-CoA into fatty acids.

Example Question #1 : Lipid Synthesis Enzymes

Which of the following are true about fatty acid desaturase and lipid metabolism?

I. The enzyme is located in the endoplasmic reticulum.

II. The enzyme converts saturated fatty acids to unsaturated fatty acids.

III. The enzyme creates double bonds in fatty acid chains.

IV. The enzyme is important in production of arachidonic acid, an inflammatory pathway and cellular signal intermediate.

Possible Answers:

I, III, and IV

I, II, and III

I and IV

I, II, III, and IV

I and II

Correct answer:

I, II, III, and IV

Explanation:

Fatty acid desaturases are located on the endoplasmic reticulum and convert saturated fatty acids to unsaturated fatty acids by producing double bonds. The enzymes have a N-terminal cytochrome b5-like domain. Arachidonic acid is a highly unsaturated fatty acid.

Example Question #4 : Lipid Synthesis

What is the role of citrate lyase in fatty acid synthesis? 

I. It catalyzes the formation of acetyl-CoA and oxaloacetate from citrate and coenzyme A.

II. It converts ATP to ADP and phosphate. 

III. It converts ADP to ATP.

IV. It catalyzes the formation of citrate and coenzyme A from acetyl-CoA and oxaloacetate.

Possible Answers:

I and IV

I and II

I, III, and IV

I only

II, III, and IV

Correct answer:

I and II

Explanation:

Citrate crosses the mitochondrial matrix into the cytosol and is converted into acetyl-CoA and oxaloacetate by citrate lyase during fatty acid synthesis, as part of the citrate shuttle. The process requires hydrolysis of energy-rich ATP bonds.

Example Question #1224 : Biochemistry

Which of the following are fatty acid synthase components?

I. Malonyl CoA-ACP transacetylase.

II. Beta-ketoacyl synthase.

III. Beta-ketoacyl reductase.

IV. Acetyl CoA-ACP transacetylase.

Note: acyl carrier protein = ACP

Possible Answers:

I and II

I, II, III and IV

I and IV

III and IV

I, II, and III

Correct answer:

I, II, III and IV

Explanation:

The role of fatty acid synthase is to synthesize fatty acids,more specifically to convert acetyl-CoA, malonyl-CoA, and NADPH to palmitate (a fatty acid) and NADP. It is a multienzyme complex consisting of 7 components: acetyl CoA-ACP transacetylase, malonyl-CoA-ACP transacetylase, Beta-ketoacyl synthase, Beta-ketoacyl reductase, Beta-hydroxyacyl dehydratase, enoyl reductase, thioesterase.

Example Question #93 : Anabolic Pathways And Synthesis

Which statement about fatty acids is false?

Possible Answers:

Carnitine transports fatty acids into the mitochondrial matrix

Fatty acids are aliphatic

None of the other answers is false

Fatty acid synthesis takes place in the mitochondria

Beta-oxidation takes place in the mitochondria

Correct answer:

Fatty acid synthesis takes place in the mitochondria

Explanation:

Beta-oxidation is the process by which fatty acid molecules are broken down in the mitochondria to generate acetyl-CoA, which then enters the Krebs cycle. Fatty acids are not aromatic (they do not have aromatic rings), rather they are organized in straight chains of hydrocarbons and are therefore aliphatic. Carnitine transports long-chain acyl groups from fatty acids into the mitochondria (so that they can undergo beta-oxidation). Fatty acid synthesis, however, takes place in the cytosol.

Example Question #91 : Anabolic Pathways And Synthesis

Which of the following are true?

Possible Answers:

The pentose pathway produces a coenzyme that is needed for fatty acid degradation

Only one malonyl-CoA molecule is required for palmitate synthesis from acetyl-CoA

Palmitic acid is the precursor of stearic acid

 is synthesized during ketogenesis

None of the other answers are true

Correct answer:

Palmitic acid is the precursor of stearic acid

Explanation:

During palmitate synthesis, malonyl-CoA molecules keep on being added to the fatty acid chain, seven malonyl CoA molecules total. Ketogenesis does not involve the synthesis of ; rather, NADH is oxidized to  as -hydroxybutyrate is formed from acetoacetate. The cofactor produced by the pentose phosphate pathway is NADPH, whereas the -oxidation-mediated degradation of fatty acids requires  and FAD. Palmitic acid is indeed the precursor of stearic acid, as well as of many other fatty acids.

Example Question #1231 : Biochemistry

Statins are a class of drugs that act to decrease the levels of LDL in blood. Which of the following could be the mechanism of action of statins?

Possible Answers:

Decreasing the production of mevalonate

Decreasing the production of acetyl-CoA

Increasing the production of malonyl-CoA

Decreasing the production of HMG-CoA

Correct answer:

Decreasing the production of mevalonate

Explanation:

Statins function to decrease the activity of HMG-CoA reductase, an important enzyme in the cholesterol synthesis pathway. This enzyme converts HMG-CoA to mevalonate. This step is the rate-limiting (and irreversible) step in this pathway. Statins inhibit this enzyme; therefore, statins prevent the production of mevalonate and cause an accumulation of HMG-CoA. The HMG-CoA can be converted into acetyl-CoA, which can now be used for many other processes.

Example Question #3 : Lipid Synthesis Reactants, Intermediates, And Products

A researcher is analyzing HMG-CoA. He isolates a region of the cell and observes that the HMG-CoA found in this region participates in the synthesis of ketone bodies. What can you conclude about this location in the cell?

Possible Answers:

Ribosomes found here to synthesize cytosolic proteins

It is bound by an inner and an outer membrane

It has histones

It has enzymes for degradation of cell debris

Correct answer:

It is bound by an inner and an outer membrane

Explanation:

HMG-CoA can be found in two locations: cytosol and mitochondria. In the cytosol, HMG-CoA participates in the production of cholesterol. In mitochondria, it participates in the production of ketone bodies. The question states that ketone bodies are produced; therefore, the researcher must be analyzing the mitochondria. Recall that mitochondria has two membranes: an inner and an outer membrane. Ribosomes in cytosol synthesize cytosolic proteins. The nucleus contains histones, which are proteins that facilitate packaging of DNA molecules. Degradative enzymes are found in organelles such as lysosomes and peroxisomes. These organelles clean the cell by removing unwanted cell debris.

Example Question #1232 : Biochemistry

What molecule is the source for all of the carbons in cholesterol?

Possible Answers:

Malonyl-CoA

Acetoacetic acid

Pyruvate

Acetyl-CoA

Correct answer:

Acetyl-CoA

Explanation:

Cholesterol is synthesized from acetyl-CoA. A cholesterol molecule contains 27 carbons and an acetyl-CoA molecule contains 2 carbons. Cholesterol is synthesized from a total of 18 acetyl-CoA molecules. These 18 molecules undergo reactions that yield a 30 carbon molecule and 6 carbon dioxide molecules (total of 36 carbons). The 30 carbon molecule loses 3 methyl groups and becomes the 27-carbon cholesterol molecule. Malonyl-CoA, acetoacetic acid, and pyruvate are not involved in this pathway.

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