MCAT Biochemistry Review

Chapter 11: Lipid and Amino Acid Metabolism

Practice Questions

1.    Which of the following enzymes is NOT used for lipid digestion in the digestive tract?

1.    Pancreatic lipase

2.    Hormone-sensitive lipase

3.    Cholesterol esterase

4.    Colipase

2.    What is the fate of long-chain fatty acids that are contained within micelles?

1.    Transport into chylomicrons released into the lymphatic system

2.    Transport into chylomicrons released into the circulatory system

3.    Direct diffusion across the intestine into the circulatory system

4.    Direct diffusion across the intestine into the lymphatic system

3.    During fatty acid mobilization, which of the following occur(s)?

1.    HSL is activated.

2.    Free fatty acids are released.

3.    Gluconeogenesis proceeds in adipocytes.

1.    I only

2.    III only

3.    I and II only

4.    II and III only

4.    How do chylomicrons and VLDLs differ?

1.    Chylomicrons contain apoproteins, VLDLs do not.

2.    Chylomicrons are synthesized in the intestine, VLDLs are synthesized in the liver.

3.    Chylomicrons transport triacylglycerol, VLDLs transport cholesterol.

4.    VLDLs are another term for chylomicron remnants; they differ in age.

5.    Which of the following could result from an absence of apolipoproteins?

1.    An inability to secrete lipid transport lipoproteins.

2.    An inability to endocytose lipoproteins.

3.    A decreased ability to remove excess cholesterol from blood vessels.

1.    I only

2.    III only

3.    I and II only

4.    I, II, and III

6.    Statin drugs inhibit HMG-CoA reductase. As such, they are likely prescribed for:

1.    hypercholesterolemia (high cholesterol).

2.    hypertriglyceridemia (high triacylglycerol).

3.    hypocholesterolemia (low cholesterol).

4.    visceral adiposity (obesity).

7.    What is the function of LCAT?

1.    LCAT catalyzes the production of cholesteryl esters.

2.    LCAT catalyzes the production of cholesterol.

3.    LCAT catalyzes the breakdown of cholesteryl esters.

4.    LCAT catalyzes the breakdown of cholesterol.

8.    Which fatty acid can be synthesized by humans?

1.    12:0

2.    16:0

3.    16:1

4.    18:3

9.    Which of the following best characterizes the process of fatty acid synthesis?

1.    Two reductions followed by a dehydration and bond formation.

2.    Reduction followed by activation, bond formation, dehydration, and reduction.

3.    Activation followed by bond formation, reduction, dehydration, and reduction.

4.    Activation followed by bond formation, oxidation, dehydration, and reduction.

10.Where does β-oxidation of fatty acids occur within the cell?

1.    Cytosol

2.    Mitochondria

3.    Smooth endoplasmic reticulum

4.    Plasma membrane

11.The majority of triacylglycerol stored in adipocytes originates from:

1.    synthesis in the adipocyte.

2.    dietary intake.

3.    ketone bodies.

4.    synthesis in the liver.

12.2,4-dienoyl-CoA reductase is used in the oxidation of:

1.    saturated fatty acids.

2.    monounsaturated fatty acids.

3.    polyunsaturated fatty acids.

4.    cholesterol.

13.Which of the following is true regarding ketolysis?

1.    Ketolysis occurs only in the brain.

2.    Ketolysis occurs in the liver.

3.    Ketolysis generates acetyl-CoA.

4.    Ketolysis increases glucose metabolism.

14.Which of the following amino acids will provide the most energy when degraded?

1.    Glycine

2.    Alanine

3.    Valine

4.    Isoleucine

15.Which of the following is LEAST likely to result from protein degradation and processing by the liver?

1.    Fatty acids

2.    Glucose

3.    Acetoacetate

4.    3-Hydroxybutyrate

PRACTICE QUESTIONS

Answers and Explanations

1.    BHormone-sensitive lipase responds to low insulin levels as well as cortisol and epinephrine to mobilize fatty acids from adipocytes. It is not involved in digestion, but rather mobilization of fatty acids.

2.    AShort-chain fatty acids are soluble in the intestinal lumen, and thus do not interact with micelles as longer fatty acid chains do. The long-chain fatty acids are taken up by the intestinal cells and packaged into triacylglycerols for transport as chylomicrons. Chylomicrons exit the intestine through lacteals that feed into the lymphatic system, which joins with the bloodstream in the base of the neck through the thoracic duct.

3.    CDuring fatty acid mobilization, there is a breakdown of triacylglycerols in adipocytes by hormone-sensitive lipase (HSL). This breakdown results in the release of three fatty acids and a glycerol molecule. The glycerol may be used by the liver for gluconeogenesis, but adipocytes do not have the ability to carry out gluconeogenesis.

4.    BChylomicrons and VLDLs are very similar. Both contain apolipoproteins and primarily transport triacylglycerol, eliminating choices (A) and (C). The only major difference between them is the tissue of origin. Chylomicrons transport dietary triacylglycerol and originate in the small intestine, while VLDLs transport newly synthesized triacylglycerol and originate in the liver.

5.    DWhile the transport and lipid binding functions of most lipoproteins are independent of the apolipoprotein component, the interaction of these lipoproteins with the environment is controlled almost exclusively by apolipoproteins. Lipoproteins cannot exit or enter cells without apolipoproteins, and are unable to transfer lipids without specialized apolipoproteins or cholesterol-specific enzymes.

6.    AStatins are drugs that are prescribed to treat high cholesterol and act as competitive inhibitors of HMG-CoA reductase. HMG-CoA reductase is the rate-limiting enzyme of de novo cholesterol synthesis; inhibition of this enzyme lowers production of cholesterol, thus lowering overall levels of cholesterol.

7.    ALCAT adds a fatty acid to cholesterol, producing cholesteryl esters, which dissolve in the core of HDL, allowing HDL to transport cholesterol from the periphery to the liver.

8.    BHumans can only synthesize one fatty acid, palmitic acid. Palmitic acid is fully saturated and therefore does not contain any double bonds. Palmitic acid has 16 carbons, and is synthesized from eight molecules of acetyl-CoA. In shorthand notation, palmitic acid is written as 16:0 (16 carbons, no double bonds).

9.    CThe steps in fatty acid synthesis are activation (attachment to acyl carrier protein), bond formation (between malonyl-CoA and the growing fatty acid chain), reduction (of a carboxyl group), dehydration, and reduction (of a double bond).

10.Bβ-Oxidation occurs within the mitochondria, along with the electron transport chain. In contrast, fatty acid synthesis occurs in the cytosol, choice (A). Fatty acyl carriers like the carnitine shuttle allow entry into the mitochondrion for breakdown.

11.DThe liver is the major metabolic organ in the body and is responsible for much of the synthesis and interconversion of fuel sources. Most of the triacylglycerols that are synthesized in the liver are transported as VLDL to adipose tissue for storage. Both the adipocytes, choice (A) and dietary intake, choice (B), constitute a minor source of triacylglycerol.

12.CIn order for the enzymes of fatty acid oxidation to operate, there can be, at most, one double bond in the area of enzyme activity, and it must be oriented between carbons 2 and 3. In order to accomplish this in monounsaturated fatty acids, an isomerase is employed. When there are multiple double bonds that fall within the enzymatic binding site, both an isomerase and 2,4-dienoyl-CoA reductase are required for the oxidative enzymes to act on the fatty acid.

13.CKetolysis is the breakdown of ketone bodies to acetyl-CoA for energy. This process occurs in the brain and muscle tissues, but cannot occur in the liver, which lacks an enzyme necessary for ketone body breakdown. Ketolysis is not associated with an increase in glucose metabolism because it most often occurs under conditions of starvation.

14.DThe energy contribution of an amino acid depends on its ability to be turned into glucose through gluconeogenesis (glucogenic amino acids), ketone bodies (ketogenic amino acids), or both. All of the amino acids listed in the answer choices are glucogenic; isoleucine is also ketogenic. The energy acquired from an amino acid will also depend on the number of carbons it can donate to these energy-creating processes, which depends on the size of its side chain. Isoleucine has the largest side chain of the answer choices, and will thus contribute the most energy per molecule.

15.AThe degradation of protein and processing by the liver implies a prolonged starvation state; protein will not be used for energy unless absolutely necessary. Thus, gluconeogenesis is the most likely process. When gluconeogenesis is not possible, easily metabolized molecules, such as ketone bodies, are synthesized. Fatty acid production occurs when energy is being stored; proteins would not be broken down to store energy in fatty acids.