Question 1

All of the following molecules are either substrates, cofactors or intermediates in the synthesis of triglyceride EXCEPT which one?

Accepted Answer

CDP-diglyceride is involved in the synthesis of phospholipids, not triglycerides. Triglyceride synthesis primarily involves glycerol 3-phosphate, fatty acyl-CoA, phosphatidic acid, and water as substrates, cofactors, or intermediates.

Question 2

Oxidation of acetoacetic acid to carbon dioxide and water

Accepted Answer

The oxidation of acetoacetic acid to carbon dioxide and water requires transfer to the CoA residue of succinyl-CoA. This reaction is catalyzed by the enzyme succinyl-CoA:3-oxoacid CoA transferase (SCOT), which transfers the CoA residue from succinyl-CoA to acetoacetate, forming acetoacetyl-CoA. The acetyl-CoA formed from this reaction can enter the TCA cycle and be oxidized to carbon dioxide and water, producing ATP through oxidative phosphorylation.

Question 3

The β-oxidation of a 16 carbon fatty acyl CoA to 8 acetyl CoAs would generate:

Accepted Answer

The β-oxidation of a 16 carbon fatty acyl CoA involves the cleavage of the fatty acid into 8 acetyl CoA molecules through a series of reactions. Each cycle of β-oxidation (except the last cycle) generates 1 FADH2 and 1 NADH, resulting in 7 cycles for a 16 carbon fatty acid, thus producing 7 FADH2 and 7 NADH.

Question 4

All of the following enzymes are involved in the beta-oxidation of palmitic acid EXCEPT

Accepted Answer

3 cis --> 2 trans enoyl-CoA isomerase is involved in the metabolism of unsaturated fatty acids, not in the beta-oxidation of saturated fatty acids like palmitic acid.

Question 5

The function of fatty acyl-CoA dehydrogenase of beta-oxidation is most analogous to that of

Accepted Answer

Succinate dehydrogenase is involved in the citric acid cycle and catalyzes the conversion of succinate to fumarate while transferring the electrons to FADH2. Similarly, fatty acyl-CoA dehydrogenase catalyzes the first step in beta-oxidation, converting a fatty acyl-CoA to trans-enoyl-CoA while transferring electrons to FADH2. Therefore, the function of fatty acyl-CoA dehydrogenase is most analogous to that of succinate dehydrogenase.

Question 6

Each of the following processes occurs predominantly in mitochondria EXCEPT

Accepted Answer

Mitochondria are not the site of fatty acid biosynthesis. This occurs primarily in the cytoplasm of cells.

Question 7

The primary action of leptin is the

Accepted Answer

Leptin primarily acts on the hypothalamus to regulate feeding behavior and energy homeostasis through modulation of polypeptide secretion.

Note: While leptin does play a role in regulating adipose tissue metabolism (choice A) and insulin secretion (choice B), these are secondary actions. Leptin does not promote skeletal muscle mass (choice D) or stimulate ketone body utilization by the brain (choice E).

Question 8

Citrate regulates fatty acid metabolism by

Accepted Answer

Citrate stimulates acetyl-CoA carboxylase, which is a key enzyme in fatty acid synthesis, converting acetyl-CoA to malonyl-CoA.

Question 9

Lipids are the most efficient forms of biologically stored energy because they are

Accepted Answer

Lipids are highly reduced or have many available electrons which can be used in metabolism to produce ATP. They are more efficient at storing energy than carbohydrates because they contain more carbon-hydrogen bonds per molecule, which release more energy when oxidized.

Question 10

The synthesis of fatty acids by the de novo pathway from acetyl-CoA

Accepted Answer

The de novo synthesis of fatty acids from acetyl-CoA requires NADPH, ATP, and bicarbonate. NADPH provides the reducing equivalents needed for the synthesis, ATP provides the energy, and bicarbonate is a source of carbon dioxide for the carboxylation reactions involved in the pathway. None of the other choices provided are accurate.

Question 11

Carnitine is involved in all of the following processes EXCEPT

Accepted Answer

Carnitine is not involved in the transport of citrate from mitochondria for synthesis of malonyl-CoA in the cytosol. This process is mediated by citrate synthase and acetyl-CoA carboxylase.

Question 12

All of the compounds listed below are ketone bodies EXCEPT which one?

Accepted Answer

Alpha-ketoglutarate is not a ketone body. It is an intermediate of the Krebs cycle, which is involved in the metabolism of amino acids. The other options are all ketone bodies.

Question 13

De novo fatty acid biosynthesis occurs in the

Accepted Answer

De novo fatty acid biosynthesis occurs in the cytosol of the cell. Most of the enzymes involved in this process are located in the cytosol, including acetyl-CoA carboxylase, fatty acid synthase, and acyl carrier protein. Therefore, the correct answer is (D) cytosol.

Question 14

An intermediate produced during the oxidation of palmitic acid to carbon dioxide and water in skeletal muscle mitochondria is

Accepted Answer

The oxidation of palmitic acid to carbon dioxide and water occurs via the process of beta-oxidation in mitochondria. One of the intermediate products generated during this process is acetoacetyl-CoA, which is formed when two molecules of acetyl-CoA are combined. Therefore, option D is the correct choice. Options A, B, C, and E are not intermediate products of palmitic acid oxidation.

Question 15

Acetyl-CoA for de novo fatty acid biosynthesis is formed by the energy-dependent cleavage of

Accepted Answer

Acetyl-CoA for de novo fatty acid biosynthesis is formed by the energy-dependent cleavage of citrate by the enzyme ATP citrate lyase.

Question 16

The most immediate impact of malonyl CoA inhibition of carnitine: fatty acyl CoA transferase is to

Accepted Answer

Malonyl CoA inhibition of carnitine: fatty acyl CoA transferase (also known as carnitine palmitoyltransferase 1, CPT1) prevents the transport of fatty acids into the mitochondria for β-oxidation. This action effectively turns back fatty acids heading for the mitochondrion, while simultaneously stimulating fatty acid elongation in the endoplasmic reticulum, as these fatty acids are now available for other processes such as elongation.

Question 17

Enzymes for the synthesis of oleic acid from palmitic acid are located in the

Accepted Answer

Enzymes for the synthesis of oleic acid from palmitic acid are located in the endoplasmic reticulum. The endoplasmic reticulum is the site of lipid synthesis, modification, and transport in eukaryotic cells. The enzymes involved in fatty acid synthesis are located on the cytosolic side of the endoplasmic reticulum membrane, while the enzymes involved in fatty acid desaturation (i.e., the conversion of saturated fatty acids to unsaturated fatty acids) are located within the endoplasmic reticulum membrane.

Question 18

How may of the carbon atoms from stearic acid may contribute, through cellular metabolism, to the net synthesis of glucose?

Accepted Answer

Stearic acid is a long-chain fatty acid, and in humans, fatty acids cannot be converted into glucose because humans lack the pathway to convert acetyl-CoA (the breakdown product of fatty acids) into pyruvate or any other gluconeogenic precursors. This is due to the irreversible nature of the pyruvate dehydrogenase reaction that converts pyruvate into acetyl-CoA, making it impossible for acetyl-CoA to serve as a substrate for gluconeogenesis.

Question 19

All of the following are intermediates or enzymes in the production of triglycerides in a cell of fatty tissue from a lipoprotein of blood plasma EXCEPT which one?

Accepted Answer

Hormone-sensitive lipase is involved in the breakdown of stored triglycerides in adipocytes to release free fatty acids, not in their synthesis or uptake from lipoproteins.

Question 20

Glucose metabolism is necessary in adipose tissue to supply which of the following for the formation of triglycerides?

Accepted Answer

Alpha-glycerol phosphate is necessary for the formation of triglycerides in adipose tissue. It is produced from the breakdown of glucose through the glycolysis pathway in adipose tissue. The glycerol backbone of a triglyceride is derived from alpha-glycerol phosphate. Ribose 5-phosphate is a precursor for nucleotide synthesis, succinyl-CoA is involved in the TCA cycle, and phosphoenolpyruvate is an intermediate in gluconeogenesis. None of these are directly involved in triglyceride formation in adipose tissue.

Quiz 17: Lipid Metabolism I: Synthesis, Storage, and Utilization of Fatty Acids and Triacylglycerols