Question 1

In studies with a particular cell line in culture you discover that the rate of mitochondrial fatty acid oxidation increases when the cells are stimulated to increase their rate of de novo fatty acid synthesis. This observation is quite unexpected since these 2 processes do not normally coincide. Which of the following enzymes involved in fatty acid synthesis is most likely to be altered thus, allowing for fat oxidation to be enhanced along with an increase in fat synthesis?
A) acetyl-CoA carboxylase 2
B) ATP-citrate lyase
C) fatty acid synthase
D) malic enzyme
E) steroyl-CoA desaturase

Accepted Answer

Acetyl-CoA carboxylase 2 (ACC2) is involved in the regulation of fatty acid oxidation. It produces malonyl-CoA, which inhibits carnitine palmitoyltransferase 1 (CPT1), the enzyme responsible for transporting fatty acids into mitochondria for oxidation. Alteration in ACC2 could reduce malonyl-CoA levels, thus enhancing fatty acid oxidation even when fatty acid synthesis is increased.  During fatty acid synthesis the malonyl-CoA produced from acetyl-CoA carboxylase 2 (ACC2) directly interacts with carnitine palmitoyltransferase I (CPT I) at the outer mitochondrial membrane to prevent fatty acyl-CoA movement into the mitochondria. A defect in ACC2 activity would therefore lead to reduced inhibition of regulated mitochondrial transport of fatty acyl-CoA resulting in increased fat oxidation from the newly synthesized fatty acids.

Question 2

You are studying fatty acid synthesis in a cell line isolated from a hepatic cancer. You discover that addition of glucose-6-phosphate does not lead to an increase in the rate of palmitic acid synthesis as is the situation in normal hepatic cells. Which of the following proteins is most likely to be altered in the hepatic cancer cells leading to the obtained results?
A) acetyl-CoA carboxylase
B) ATP-citrate lyase
C) ChREBP
D) fatty acid synthase
E) steroyl-CoA desaturase

Accepted Answer

Regulation of de novo fatty acid synthesis takes place primarily at the level of acetyl-CoA carboxylase (ACC) but also at the level of fatty acid synthase (FAS). FAS is regulated by both short-term and long-term mechanisms. Short-term regulation is primarily the result of the allosteric activation of FAS by phosphorylated sugars, such as glucose 6-phosphate.

Question 3

Gas chromatographic analysis of fatty acids present in cells isolated from a particular strain of mice shows little to no palmitoleic and oleic acid but expected levels of palmitic acid. A defect in which of the following proteins is most likely causing the absence of the 2 fatty acids?
A) acetyl-CoA carboxylase
B) ATP-citrate lyase
C) ChREBP
D) fatty acid synthase
E) steroyl-CoA desaturase

Accepted Answer

Steroyl-CoA desaturase is responsible for introducing double bonds into saturated fatty acids like palmitic acid to form unsaturated fatty acids like palmitoleic and oleic acids. A defect in this enzyme would result in low levels of these unsaturated fatty acids.  The desaturation of fatty acids in mammalian cells involves 3 broad specificity fatty acylCoA desaturases. Steroyl-CoA desaturase (SCD1) is the rate-limiting enzyme catalyzing the synthesis of monounsaturated fatty acids, primarily oleate (18:1) and palmitoleate (16:1). These 2 monounsaturated fatty acids represent the majority of monounsaturated fatty acids present in membrane phospholipids, triglycerides, and cholesterol esters.

Question 4

You are examining a 27-year-old female adult who presents with redness of the skin, swelling, itching, and skin lesions, several of which are oozing pus. In addition the patient complains that a bad scrape, suffered in a fall from a bike, has not properly healed even though the injury occurred several weeks ago. Patient history indicates that she started on a fat-free diet 10 months ago. Which of the following is most likely deficient in her diet leading to her current physical condition?
A) citric acid
B) linoleic acid
C) oleic acid
D) palmitic acid
E) palmitoleic acid

Accepted Answer

Linoleic acid is an essential fatty acid that must be obtained from the diet. A deficiency can lead to poor wound healing, skin issues, and other symptoms as described in the scenario.  Linoleic is an essential fatty acid in that it must be acquired in the diet. Linoleic acid is especially important in that it required for the synthesis of arachidonic acid. Arachidonate is a precursor for the eicosanoids (the prostaglandins, thromboxanes, and leukotrienes). It is this role of linoleic acid in eicosanoid synthesis that leads to poor growth, wound healing, and dermatitis in persons on fat-free diets

Question 5

You are studying the activity of acetyl-CoA carboxylase in an isolated buffered system containing the enzyme and its substrate. Addition of which of the following substances to this in vitro assay system would be expected to result in the largest decrease in malonyl-CoA production?
A) acetyl-CoA
B) citrate
C) glucose 6-phosphate
D) glutamate
E) oleic acid

Accepted Answer

Oleic acid is a fatty acid that can inhibit acetyl-CoA carboxylase, leading to a decrease in malonyl-CoA production.  Both isoforms of ACC are allosterically activated by citrate and inhibited by palmitoylCoA and other short- and long-chain fatty acyl-CoAs. Citrate triggers the polymerization of ACC1 which leads to significant increases in its activity. Although ACC2 does not undergo significant polymerization (presumably due to its mitochondrial association), it is allosterically activated by citrate. Glutamate and other dicarboxylic acids can also allosterically activate both ACC isoforms.

Question 6

Glucose is known to regulate the activity of ChREBP which in turn regulates the expression of numerous genes involved in glucose and lipid homeostasis. Which of the following proteins, whose activity rises as glucose levels rise, is involved in the signaling of glucose levels to ChREBP?
A) acetyl-CoA carboxylase
B) fatty acid synthase
C) malonyl-CoA decarboxylase
D) protein phosphatase 2Aδ
E) steroyl-CoA desaturase

Accepted Answer

Protein phosphatase 2Aδ is involved in the dephosphorylation and activation of ChREBP in response to rising glucose levels, facilitating its role in regulating gene expression related to glucose and lipid metabolism.  Under conditions of low (basal) glucose concentration, ChREBP is phosphorylated and resides in the cytosol. When glucose levels rise, protein phosphatase 2Aδ (PP2Aδ) is activated by xylulose 5-phosphate which is generated in the PPP. PP2Aδ dephosphorylates ChREBP allowing it to migrate into the nucleus. In the nucleus PP2Aδ carries out a second dephosphorylation which allows ChREBP to bind to specific sequence elements (ChoREs) in target genes.

Question 7

Which of the following enzymes is most responsive to changes in the level of circulating insulin such that as insulin levels rise fatty acid synthesis is increased?
A) acetyl-CoA carboxylase
B) ATP-citrate lyase
C) fatty acid synthase
D) malonyl-CoA decarboxylase
E) steroyl-CoA desaturase

Accepted Answer

Acetyl-CoA carboxylase is the rate-limiting enzyme in fatty acid synthesis and is activated by insulin. When insulin levels rise, it promotes the dephosphorylation and activation of acetyl-CoA carboxylase, leading to increased fatty acid synthesis.  ACC activity is inhibited by phosphorylation. Both ACC1 and ACC2 undergo phosphorylation. Glucagon-stimulated increases in cAMP and subsequently to increased PKA activity lead to phosphorylation and inhibition of ACC. The activating effects of insulin on ACC are complex but are, in part, related to reductions in cAMP levels via the insulinmediated activation of phosphodiesterase.

Question 8

Acetyl-CoA is transported out of the mitochondria in order to serve as a substrate for fatty acid or cholesterol synthesis. Which of the following enzymes used in this transport process provides a cofactor required for these reductive biosynthesis reactions?
A) ATP-citrate lyase
B) citrate synthase
C) malate dehydrogenase
D) malic enzyme
E) pyruvate carboxylase

Accepted Answer

Malic enzyme generates NADPH, a cofactor required for reductive biosynthesis reactions such as fatty acid and cholesterol synthesis.  Acetyl-CoA enters the cytoplasm in the form of citrate via the tricarboxylate transport system (see Figure 19-3). In the cytoplasm, citrate is converted to oxaloacetate and acetyl-CoA by the ATPdriven ATP-citrate lyase (ACL) reaction. This reaction is essentially the reverse of that catalyzed by the TCA enzyme citrate synthase except it requires the energy of ATP hydrolysis to drive it forward. The resultant oxaloacetate is converted to malate by malate dehydrogenase (MDH). The malate produced by this pathway can undergo oxidative decarboxylation by malic enzyme. The coenzyme for this reaction is NADP+-generating NADPH. The advantage of this series of reactions for converting mitochondrial acetyl-CoA into cytoplasmic acetyl-CoA is that the NADPH produced by the malic enzyme reaction can be a major source of reducing cofactor for the FAS activities

Question 9

Which of the following exerts a positive allosteric effect on de novo fatty acid synthesis at the level of acetyl-CoA carboxylase?
A) acetyl-CoA
B) AMPK
C) ATP
D) biotin
E) citrate

Accepted Answer

Citrate acts as a positive allosteric effector of acetyl-CoA carboxylase, the enzyme that catalyzes the rate-limiting step in fatty acid synthesis. It facilitates the polymerization of the enzyme, enhancing its activity.  Both isoforms of ACC are allosterically activated by citrate and inhibited by palmitoylCoA and other short- and long-chain fatty acyl-CoAs. Citrate triggers the polymerization of ACC1 which leads to significant increases in its activity. Although ACC2 does not undergo significant polymerization (presumably due to its mitochondrial association), it is allosterically activated by citrate. Glutamate and other dicarboxylic acids can also allosterically activate both ACC isoforms.

Question 10

ChREBP regulates the expression of numerous genes involved in lipid and carbohydrate homeostasis. The activity of ChREBP can be modulated by AMPKmediated phosphorylation. A defect in which of the following enzymes would most likely be apparent if ChREBP was found to be constitutively phosphorylated and inhibited?
A) AMPK
B) glucose-6-phosphate dehydrogenase
C) protein phosphatase 2Aδ
D) protein phosphatase inhibitor 1
E) SREBP

Accepted Answer

Protein phosphatase 2Aδ is responsible for dephosphorylating ChREBP, allowing it to become active. If ChREBP is constitutively phosphorylated and inhibited, it suggests a defect in the enzyme responsible for its dephosphorylation, which is protein phosphatase 2Aδ.  Under conditions of low (basal) glucose concentration, ChREBP is phosphorylated and resides in the cytosol. When glucose levels rise, protein phosphatase 2Aδ (PP2Aδ) is activated by xylulose 5-phosphate which is generated in the pentose phosphate pathway. PP2Aδ dephosphorylates ChREBP allowing it to migrate into the nucleus. In the nucleus PP2Aδ carries out a second dephosphorylation which allows ChREBP to bind to specific sequence elements (ChoREs) in target genes. Therefore, a defect in the activity of PP2Ad would lead to constitutive phosphorylation and inhibition of ChREBP

Question 11

When cells acquire sufficient energy such that the rate of flux through the TCA cycle declines, excess acetyl-CoA that cannot be oxidized is predominantly converted into fat. In order for the carbons in mitochondrial acetyl-CoA to serve as a precursor for fat synthesis, they must be delivered to the cytosol. Which of the following represents the molecule used to transport acetyl-CoA to the cytosol?
A) acetyl-CoA
B) carnitine
C) citrate
D) β-Hydroxybutyrate
E) pyruvate

Accepted Answer

Citrate is the molecule that transports acetyl-CoA from the mitochondria to the cytosol for fatty acid synthesis.  Acetyl-CoA cannot freely diffuse across the membranes of the mitochondria, nor is there a transport mechanism to move the molecule to the cytosol. This ensures that all acetyl-CoA generated by PDH or fat oxidation will be used for energy production. However, as cellular energy demand falls, carbon atoms can be diverted into storage molecules such as glycogen and fatty acids. To move acetyl-CoA out of the mitochondria to the cytosol, it must first be converted to citrate, which can be transported by the TCA transport system.

Question 12

The rate of transcription of the mRNA of which of the following enzymes is most likely increased in an individual on a fat-free, high-carbohydrate diet?
A) fatty acid synthase
B) fructose 1,6-bisphosphatase
C) hormone-sensitive lipase
D) phosphoenolpyruvate carboxykinase
E) pyruvate carboxylase

Accepted Answer

A fat-free, high-carbohydrate diet increases the need for de novo lipogenesis, which involves the synthesis of fatty acids. Fatty acid synthase is a key enzyme in this process, and its transcription is upregulated to facilitate the conversion of excess carbohydrates into fatty acids.  Dietary composition can exert longterm effects on biochemical pathways generally exerted at the level of gene expression. A diet low in fat will trigger a need for increased de novo synthesis of fatty acids, and thus be reflected in an increased level of expression of fatty acid synthase.

Quiz 19: Lipids: Fatty Acid Synthesis

Gas chromatographic analysis of fatty acids present in cells isolated from a particular strain of mice shows little to no palmitoleic and oleic acid but expected levels of palmitic acid. A defect in which of the following proteins is most likely causing the absence of the 2 fatty acids?

You are studying the activity of acetyl-CoA carboxylase in an isolated buffered system containing the enzyme and its substrate. Addition of which of the following substances to this in vitro assay system would be expected to result in the largest decrease in malonyl-CoA production?

Glucose is known to regulate the activity of ChREBP which in turn regulates the expression of numerous genes involved in glucose and lipid homeostasis. Which of the following proteins, whose activity rises as glucose levels rise, is involved in the signaling of glucose levels to ChREBP?

Which of the following enzymes is most responsive to changes in the level of circulating insulin such that as insulin levels rise fatty acid synthesis is increased?

Acetyl-CoA is transported out of the mitochondria in order to serve as a substrate for fatty acid or cholesterol synthesis. Which of the following enzymes used in this transport process provides a cofactor required for these reductive biosynthesis reactions?

Which of the following exerts a positive allosteric effect on de novo fatty acid synthesis at the level of acetyl-CoA carboxylase?

The rate of transcription of the mRNA of which of the following enzymes is most likely increased in an individual on a fat-free, high-carbohydrate diet?