Biochemistry : Lipid Synthesis Enzymes

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #1 : Lipid Synthesis Enzymes

In mammals, the enzyme used to synthesize fatty acids is called mammalian fatty acid synthase. It has 7 components that all work together to synthesize a fatty acid chain. 

If the DH subunit of the enzyme is mutated and cannot catalyze its proper reaction, which functional group would you expect to see on the resulting carbon chain? 

Possible Answers:

Alcohol

Ketone

Alkane

Alkene

Hydrogen

Correct answer:

Alcohol

Explanation:

The DH subunit is a dehydratase, meaning it removes alcohol groups from carbon chains. If this subunit is mutated, the alcohol cannot be removed. 

Example Question #2 : Lipid Synthesis Enzymes

Which coenzyme is common to reactions catalyzed by pyruvate dehydrogenase and β-hydroxybutyrate dehydrogenase?

Possible Answers:

Thiamine pyrophosphate

coenzyme A

Lipoic acid

Correct answer:

Explanation:

-hydroxybutyrate dehydrogenase catalyzes the interconversion of the ketone bodies acetoacetate and -hydroxybutyrate, which are transported out of liver cells into the blood to be used as fuel by the rest of the body, particularly during times of starvation. -hydroxybutyrate dehydrogenase’s only coenzyme is . As for the other answers: the pyruvate dehydrogenase complex converts pyruvate into acetyl-CoA, the key connection between glycolysis and the citric acid cycle, and this process uses a number of co-factors, including , thiamine pyrophosphate, lipoamide (the protein-bound form of lipoic acid), and, of course, coenzyme A (to make acetyl-CoA). Pyruvate dehydrogenase uses  and  depending on the cell type.

Example Question #3 : Lipid Synthesis Enzymes

Malonyl-CoA is a potent inhibitor of carnitine palmitoyl transferase, which facilitates the movement of long-chain fatty acids into the mitochondria for beta-oxidation. This leads to decreased utilization of fatty acids for fuel. 

If we wanted to eliminate this effect in a targeted manner, which enzyme would we seek to inhibit?

Possible Answers:

Acyl-CoA dehydrogenase

Pyruvate dehydrogenase

Carnitine palmitoyl transferase

HMG-CoA-reductase

Acetyl-CoA carboxylase

Correct answer:

Acetyl-CoA carboxylase

Explanation:

Because acetyl-CoA Carboxylase (ACC) is directly responsible for synthesis of malonyl-CoA, inhibiting it would be the most targeted approach. Inhibiting acyl-CoA dehydrogenase or pyruvate dehydrogenase would decrease available acetyl-CoA to ACC by inhibiting beta-oxidation and conversion of pyruvate to acetyl-CoA, but this would have a large impact on other biosynthetic pathways as well, due to the ubiquity of acetyl-CoA.

Example Question #4 : Lipid Synthesis Enzymes

Which of the following is true about major lipoprotein metabolism pathways?

Possible Answers:

In the endogenous pathway, circulating chylomicrons are transformed into remnants by lipoprotein lipase (LPL)

High density lipoprotein, HDL, transports cholesterol in tissues to the liver, where it is recognized the apolipoprotein B100 in hepatocytes

Macrophages absorb oxidized low density lipoproteins (LDLs), thus transforming themselves into foam cells, an indicator for plaque build-up in atherosclerosis

The exogenous pathway involves progressive transformation of very low density lipoproteins (VLDLs), synthesized by the liver, into low density lipoproteins (LDLs)

Correct answer:

Macrophages absorb oxidized low density lipoproteins (LDLs), thus transforming themselves into foam cells, an indicator for plaque build-up in atherosclerosis

Explanation:

The endogenous, not the exogenous, pathway, involves the transformation of VDLDs into LDLs. Meanwhile, the exogenous, not the endogenous, pathway, involves transforming chylomicrons into remnants. The apolipoprotein in plasma (transporting cholesterol to the liver) which is the major component of HDL is A1, not B100. Macrophages do indeed oxidize LDLs and transform themselves into the foam cells which indicate atherosclerosis. This is one of the reasons that LDL cholesterol levels can indicate atherosclerosis (which is associated with increased risk of heart attack or stroke).

Example Question #5 : Lipid Synthesis Enzymes

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

Possible Answers:

Proteins

Nucleic Acids

Carbohydrates

Lipids

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 #6 : Lipid Synthesis Enzymes

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 is an enzyme that converts malonyl-CoA into palmitate

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

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 #7 : 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, II, and III

I, III, and IV

I and IV

I and II

I, II, III, and IV

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 #8 : Lipid Synthesis Enzymes

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

II, III, and IV

I only

I and II

I, 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 #9 : Lipid Synthesis Enzymes

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 IV

I, II, and III

I, II, III and IV

I and II

III and IV

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.

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