Biochemistry : Regulating Lipid Synthesis

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #1 : Regulating Lipid Synthesis

Which of the following best characterizes the series of functional groups which are formed during fatty acid synthesis?

Possible Answers:

Aldehyde, alcohol, alkene, alkane

Ketone, alkene, alcohol, alkane

Ketone, diol, alkene, alkane

Hemiketal, alcohol, alkene, alkane

Ketone, alcohol, alkene, alkane

Correct answer:

Ketone, alcohol, alkene, alkane

Explanation:

The two carbons that remain after the addition of malonyl-CoA are added as an acetyl group with the carbonyl carbon on the interior of the chain, which is to say a ketone. Then, the carbonyl is reduced to form an alcohol. Next, the alcohol is dehydrated to form an alkene. Finally, the alkene is reduced to saturate the chain, forming an alkyl group.

Example Question #2 : Regulating Lipid Synthesis

During fatty acid synthesis, or lipogenesis, acetyl-CoA is transported from the mitochondria to the cytosol as which of the following?

Possible Answers:

Glutamate 

Alanine

Carnitine

Citrate

Glycerol 

Correct answer:

Citrate

Explanation:

Carnitine transports fatty acids from the cytosol to the mitochondria. Acetyl-CoA is converted to citrate as it exits the mitochondria and enters the cytosol.

Example Question #1 : Regulating Lipid Synthesis

Insulin regulates both carbohydrate and lipid metabolism. Which of the following enzymes are regulated by insulin?

I. Acetyl-CoA carboxylase

II. Fatty acid synthase

III. Pyruvate dehydrogenase

IV. Glucokinase

Possible Answers:

I, II, and IV

I, III, and IV

I and IV

I, II, III, and IV

II, III, and IV

Correct answer:

I, II, III, and IV

Explanation:

In fatty acid synthesis, all of the enzymes listed are are regulated by insulin. Pyruvate dehydrogenase transforms pyruvate into acetyl-CoA. Glucokinase transforms glucose in glucose 6-phosphate. Acetyl-CoA carboxylase converts acetyl-CoA to malonyl-CoA. Fatty acid synthase converts malonyl-CoA to fatty acid palmitate. Insulin regulation is essential for proper utilization of dietary carbohydrates and lipids after meals. 

Example Question #4 : Regulating Lipid Synthesis

What is the importance of the citrate shuttle in lipid biosynthesis?

I. It requires the activity of citrate lyase

II. Acetyl-CoA is converted to citrate in the mitochondria, which is then moved across the mitochondrial membrane

III. The process makes acetyl-CoA from the mitochondria available for fatty acid synthesis in the cytosol

IV. Oxaloacetate in the cytoplasm is moved directly back in the mitochondria

Possible Answers:

I and II

I, II, and III

II, III, and IV

I, III, and IV

II and III

Correct answer:

I, II, and III

Explanation:

The citrate shuttle moves acetyl-coenzyme A (CoA) from the mitochondria to the cytosol to make it available for fatty acid synthesis. The process involves multiple reactions and enzymes such as citrate lyase. Citrate acts as as a carrier agent for acetyl-CoA molecules from the mitochondria to the cytoplasm and in reverse. Oxaloacetate, a product of citrate lysis in the cytoplasm is not moved directly back in the mitochondria, but rather is converted back to malate and pyruvate.

Example Question #5 : Regulating Lipid Synthesis

Acetyl-CoA carboxylase is essential for fatty acid synthesis. Which of the following factors regulate acetyl-CoA carboxylase?

I. Glucagon

II. Citrate

III. Palmitoyl-CoA

IV. Insulin

Possible Answers:

I, II, and III

I and II

I and IV

II and III

I, II, III, and IV

Correct answer:

I, II, III, and IV

Explanation:

Acetyl-CoA carboxylase is essential for fatty acid synthesis, it provides malonyl-CoA, necessary for production of palmitate, a fatty acid. The enzyme is regulated via phosphorylation and dephosphorylation. Insulin activates the enzyme by dephosphorylation. Glucagon and epinephrine deactivate on the other hand the enzyme by phosphorylation (adding a phosphate group to the molecule). Citrate activates the enzyme while palmitoyl-CoA, the end product of fatty acid synthesis, inhibits it.

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