Lipid Metabolism

Question 1

A fatty acid designated as 20:0 is _____ , while one that is designated 20:3 Δ5,8,11 is _____

(a) simple; complex
(b) complex; simple
(c) saturated; unsaturated
(d) unsaturated; saturated
(e) monounsaturated; polyunsaturated

Answer 1

C

Question 2

Dietary triacylglycerols are digested as a result of:

(a) lipase action.
(b) bile salts.
(c) micelle formation.
(d) diffusion and absorption by intestinal cells.
(e) all of the above

Answer 2

E

Question 3

Adipocytes contain fat droplets which serve to provide an animal with:

(a) increased cell volume.
(b) insulation.
(c) chemical energy.
(d) (a) and (b) above
(e) (b) and (c) above

Answer 3

E

Question 4

Why are triacylglycerols able to provide more energy than carbohydrates (gram for gram)?

(a) Triacylglycerols have an extremely high group transfer potential.
(b) Carbohydrates are already in a more oxidized state than the triacylglycerols.
(c) Carbohydrates contain fewer carbon-carbon bonds.
(d) Triacylglycerols are less soluble in water than the carbohydrates.
(e) Triacylglycerols contain a larger number of carbon-carbon bonds

Answer 4

B

Question 5

The largest lipoproteins are the _____.

(a) chylomicrons
(b) VLDLs
(c) LDLs
(d) HDLs
(e) IDL’s

Answer 5

A

Question 6

Which of the following best describes the cholesterol molecule?

(a) Amphipathic
(b) Nonpolar, charged
(c) Nonpolar, uncharged
(d) Polar, charged
(e) Polar, uncharged

Answer 6

A

Question 7

Fatty acid groups are referred to as groups.

(a) acetyl
(b) acyl
(c) prenyl
(d) isoprenoid
(e) isopentenyl

Answer 7

B

Question 8

The largest usable energy store in a well-nourished human is:

(a) ATP in all tissues.
(b) blood glucose.
(c) liver glycogen.
(d) muscle glycogen.
(e) triacylglycerols in adipose tissue

Answer 8

E

Question 9

Phospholipids are

(a) structural components of membranes.
(b) cell surface active agents.
(c) rich energy sources.
(d) both (a) and (b) are correct.
(e) (a), (b) and (c) are correct

Answer 9

D

Question 10

Triacylglycerols are not found in cell membranes because they are

(a) amphipathic.
(b) not amphipathic.
(c) not abundant in cells.
(d) charged at biological pH.
(e) none of the above are correct, because TG’s are, in fact, present in cell membranes

Answer 10

B

Question 11

Cholesterol is synthesized from:

(a) acetyl CoA.
(b) choline.
(c) arachidonic acid.
(d) apolipoprotein.
(e) cis fatty acids

Answer 11

A

Question 12

Which vitamin is derived from cholesterol?

(a) A
(b) B12
(c) D
(d) E
(e) K

Answer 12

C

Question 13

Which of the following is not a fat-soluble vitamin?

(a) A
(b) C
(c) D
(d) E
(e) K

Answer 13

B

Question 14

Which of the following is a true statement, concerning HMG-CoA reductase?

(a) It is regulated by covalent modification of a serine in its active site.
(b) It catalyzes the first committed step in cholesterol biosynthesis.
(c) It causes the production of NADPH.
(d) The product of the reaction it catalyzes is acetoacetyl CoA.
(e) There is no true statement

Answer 14

B

Question 15

Cholesterol is converted to cholesteryl esters for in cells and are (more, less) hydrophobic than phospholipids.

(a) transport; more
(b) transport; less
(c) storage; more
(d) storage; less
(e) synthesis; more

Answer 15

C

Question 16

The double bonds in naturally occurring fatty acids are usually isomers.

(a) cis
(b) trans
(c) both cis and trans
(d) essential
(e) nonessential

Answer 16

A

Question 17

Which of the following statements about sterols is true?

(a) All sterols share a fused-ring structure with four rings.
(b) Sterols are found in the membranes of all living cells.
(c) Sterols are soluble in water, but less so in organic solvents such as chloroform.
(d) Stigmasterol is the principal sterol in fungi.
(e) The principal sterol of animal cells is ergosterol

Answer 17

A

Question 18

Which of the following is a true statement, concerning HMG-CoA reductase?

(a) It is regulated by covalent modification of a serine in its active site.
(b) It catalyses the first committed step in cholesterol biosynthesis.
(c) It causes the production of NADPH.
(d) The product of the reaction it catalyses is acetoacetyl CoA.
(e) Both (b) and (d) are correct

Answer 18

B

Question 19

Drugs called statins lower cholesterol levels because they __________.

(a) degrade HMG-CoA reductase
(b) bind serum cholesterol
(c) inhibit HMG-CoA reductase
(d) bind bile salts
(e) activate HMG-CoA reductase

Answer 19

C

Question 20

Cholesterol is a precursor of _____

(a) aldosterone
(b) cortisol
(c) testosterone
(d) estrogen
(e) All of the above are correct

Answer 20

E

Question 21

A patient is found to have a high concentration of cholesterol in the blood and deposits of cholesterol under the skin. The patient is diagnosed with familial hypercholesterolemia. What is the likely cause of this condition?

(a) A deficiency in insulin production by the pancreas.
(b) Insufficient chylomicron concentration in the blood.
(c) Overproduction of lysosomal lipases.
(d) Lack of LDL receptors on the surfaces of nonhepatic and liver cells.
(e) Deficiency of HMG-CoA reductase

Answer 21

D

Question 22

Circle the fatty acid in each pair that has the higher melting temperature.

(a) 18:1Δ9 18:2Δ9,12
(b) 18:0 18:1Δ9
(c) 18:0 16:0

Answer 22

(a) 18:1Δ9
(b) 18:0
(c) 18:0

Question 23

Describe the dependence of the melting point of a fatty acid upon
(a) chain length;
(b) unsaturation;
(c) explain these dependencies in molecular terms

Answer 23

All other things being equal,
(a) the longer the acyl chain, the higher the melting temperature; and
(b) the more unsaturation, the lower the melting temperature.
(c) The melting temperature is a measure of the thermal energy needed to break the intermolecular interactions that stabilise the “solid” form of a lipid, which depends upon how well the individual lipid molecules fit into the nearly crystalline array of lipids. When a shorter acyl chain lies between two longer chains in a nearly crystalline array of lipid molecules, there is a cavity at the end of the short acyl group that allows freer motion to the neighbouring acyl chains. A cis double bond introduces a “kink” into the acyl chain, so that it does not pack as easily with its straighter neighbours.

Question 24

Fatty acids have a number of specific physiological roles. State four physiological roles for fatty acids?

Answer 24

Important physiological roles for fatty acids include:
(1) building blocks of phospholipids and glycolipids (membrane constituents),
(2) modification of proteins via covalent lipid attachment,
(3) fuel molecules for metabolism,
(4) building blocks and derivatives for hormones and chemical messengers (such as leukotrienes, prostaglandins and thrombaxanes)

Question 25

In cells, fatty acids are stored as triacylglycerols for energy reserves.
(a) What is the molecule to which fatty acids are esterified to form triacylglycerols?
(b) Define the logic behind cells storing fatty acids in esterified form.
(c) Describe three functions of triacylglycerols in mammals

Answer 25

a) Three fatty acids are esterified to glycerol.
(b) Triacylglycerols are uncharged and insoluble in water. They form lipid droplets within adipocytes, which do not contribute to the osmolarity of the cytosol in those cells, and do not require any water of hydration.
(c) Triacylglycerols provide mammals with (1) stored fuel, (2) insulation, and (3) a source of metabolic water. In some animals, such as camels and desert rats, the oxidation of stored lipids provides water; in hibernating animals, oxidation of stored lipids generates heat to maintain body temperature

Question 26

What are plasma lipoproteins? What is their general role in mammalian metabolism?

Answer 26

Plasma lipoproteins are protein-lipid aggregates that circulate in the blood, carrying phospholipids, triacylglycerols, cholesterol, and cholesteryl esters from their points of synthesis or absorption to
the tissues in which they will be used. The lipid/protein ratio, and therefore the density, of plasma lipoproteins varies, giving rise to particles separable by ultracentrifugation: HDL, LDL, VLDL, and
chylomicrons, for example

Question 27

Describe the process by which the Low-Density Lipoproteins in the bloodstream enter cells

Answer 27

The process is receptor-mediated endocytosis. LDL particles containing cholesterol esters interact via apoB-100 with specific LDL receptors in the cell surface, initiating endocytosis that brings the LDL into the cell within endosomes. Endosomes fuse with lysosomes, and lysosomal enzymes degrade the apoproteins; cholesterol esters are released into the cytosol, and LDL receptors recirculate, reappearing on the cell surface

Question 28

Determine the order of the following events involving lipoprotein-mediated transport of dietary triacylglycerols and cholesterol.

Triacylglycerols are removed from circulating VLDL by lipoprotein lipase.

Chylomicrons are transported through the lymphatic system and enter the bloodstream.

Chylomicrons are formed in the intestinal mucosa.

Cells take up cholesterol via receptor-mediated LDL endocytosis.

Chylomicrons are degraded by lipoprotein lipase to chylomicron remnants.

LDL components are rapidly degraded by lysosomal enzymes.

VLDL are synthesized in liver

Answer 28

1. Chylomicrons are formed in the intestinal mucosa.
2. Chylomicrons are transported through the lymphatic system and enter the bloodstream.
3. Chylomicrons are degraded by lipoprotein lipase to chylomicron remnants.
4. VLDL are synthesised in liver.
5. Triacylglycerols are removed from circulating VLDL by lipoprotein lipase.
6. Cells take up cholesterol via receptor-mediated LDL endocytosis.
7. LDL components are rapidly degraded by lysosomal enzymes

Question 29

Why are cholesteryl esters located in the interior of a lipoprotein while cholesterol is located in the outer lipoprotein shell?

Answer 29

Cholesteryl esters are highly hydrophobic and therefore occupy the nonaqueous interior of the lipoprotein. Cholesterol, with an hydroxyl (-OH) group, is weakly polar and therefore can interact with water molecules at the surface of the lipoprotein in the outer lipoprotein shell

Question 30

Free fatty acids in the bloodstream are:

(a) bound to hemoglobin.
(b) carried by the protein serum albumin.
(c) freely soluble in the aqueous phase of the blood.
(d) nonexistent; the blood does not contain free fatty acids.
(e) present at levels that are independent of adrenaline (epinephrine).

Answer 30

B

Question 31

Transport of fatty acids from the cytoplasm to the mitochondrial matrix requires:

(a) ATP, carnitine, and coenzyme A.
(b) ATP, carnitine, and pyruvate dehydrogenase.
(c) ATP, coenzyme A, and hexokinase.
(d) ATP, coenzyme A, and pyruvate dehydrogenase.
(e) carnitine, coenzyme A, and hexokinase

Answer 31

A

Question 32

Which of these is able to cross the inner mitochondrial membrane?

(a) Acetyl–CoA
(b) Fatty acyl–carnitine
(c) Fatty acyl–CoA
(d) Malonyl–CoA
(e) None of the above can cross the inner mitochondrial membrane

Answer 32

B

Question 33

Which of the following is not required in the synthesis of fatty acids?

(a) Acetyl-CoA
(b) Biotin
(c) HCO 3 (CO 2)
(d) Malonyl-CoA
(e) NADH

Answer 33

E

Question 34

How many reaction cycles of β-oxidation are required to completely process a saturated C20 fatty acid?

(a) 8
(b) 9
(c) 10
(d) 11
(e) 20

Answer 34

B

Question 35

Enzymes that catalyse the synthesis of long chain fatty acids in vertebrate cells:

(a) act as seven separate proteins.
(b) are encoded in mitochondrial genes.
(c) are localized in the mitochondrial matrix.
(d) are part of a single polypeptide chain containing several distinct enzyme activities.
(e) have none of the characteristics above.

Answer 35

D

Question 36

If the 16-carbon saturated fatty acid palmitate is oxidised completely to carbon dioxide and water (via the β-oxidation pathway and the citric acid cycle), and all of the energy-conserving products are used to drive ATP synthesis in the mitochondrion, the net yield of ATP per
molecule of palmitate is:

(a) 16.
(b) 32.
(c) 106.
(d) 108.
(e) 128

Answer 36

C

Question 37

Which of the following is (are) true of the oxidation of 1 mol of palmitate (a 16-carbon saturated fatty acid; 16:0) by the β-oxidation pathway, beginning with the free fatty acid in the cytoplasm?

1. Activation of the free fatty acid requires the equivalent of two ATPs.
2. Inorganic pyrophosphate (PPi) is produced.
3. Carnitine functions as an electron acceptor.
4. 8 mol of FADH2 are formed.
5. 8 mol of acetyl-CoA are formed.
6. There is no direct involvement of NAD+

(a) 1 and 5 only
(b) 1, 2, and 5
(c) 1, 2, and 6
(d) 1, 3, and 5
(e) 5 only

Answer 37

B

Question 38

Which of the following statements concerning the β oxidation of fatty acids is true?

(a) About 1,200 ATP molecules are ultimately produced per 20-carbon fatty acid oxidised.
(b) One FADH2 and two NADH are produced for each acetyl-CoA.
(c) The free fatty acid must be carboxylated in the β position by a biotin-dependent reaction before the process of β oxidation commences.
(d) The free fatty acid must be converted to a thioester before the process of β oxidation commences.
(e) Two NADH are produced for each acetyl-CoA

Answer 38

D

Question 39

Which of the following is not true of the fatty acid synthase and the fatty acid β-oxidation systems?

(a) A derivative of the vitamin pantothenic acid is involved.
(b) Acyl-CoA derivatives are intermediates.
(c) Double bonds are oxidized or reduced by pyridine nucleotide coenzymes.
(d) The processes occur in different cellular compartments.
(e) The processes occur in the mitochondrial matrix

Answer 39

E

Question 40

Which of the following is not true of the reaction producing malonyl-CoA during fatty acid synthesis?

(a) It is stimulated by citrate.
(b) It requires acyl carrier protein (ACP).
(c) It requires CO 2 (or bicarbonate).
(d) One mole of ATP is converted to ADP + Pi for each malonyl-CoA synthesized.
(e) The cofactor is biotin

Answer 40

B

Question 41

The rate-limiting step in fatty acid synthesis is:

(a) condensation of acetyl-CoA and malonyl-CoA.
(b) formation of acetyl-CoA from acetate.
(c) formation of malonyl-CoA from malonate and coenzyme A.
(d) the reaction catalysed by acetyl-CoA carboxylase.
(e) the reduction of the acetoacetyl group to a β-hydroxybutyryl group

Answer 41

D

Question 42

What is the fate of most glycerol that is released during the hydrolysis of triacylglyceride from adipocytes?

(a) It is transported to the kidneys and excreted in the urine.
(b) It is used in the synthesis of sphingolipids.
(c) It is transported to the liver and converted to glucose via gluconeogenesis.
(d) It is absorbed by the chylomicrons.
(e) It is metabolised by glycerolysis in the Vesuvian ducts of the spleen

Answer 42

C

Question 43

_____, a condition in which acetoacetate production exceeds its metabolism and results in a sweet breath odour due to nonenyzmatic _____ of acetoacetate to _____

(a) ketosis; decarboxylation; hydroxylbutyrate
(b) ketosis; carboxylation; hydroxylbutyrate
(c) ketosis; carboxylation; acetone
(d) ketogenesis; decarboxylation; hydroxylbutyrate
(e) ketogenesis; decarboxylation; acetone

Answer 43

C

Question 44

Which of the following is the starting metabolite in ketone body biosynthesis?

(a) acetyl CoA
(b) malonyl CoA
(c) propionyl CoA
(d) acetate
(e) acyl CoA

Answer 44

A

Question 45

Which of the following is TRUE regarding the enzyme acetyl-CoA carboxylase?

I. It catalyses the first committed step in fatty acid oxidation.
II. It requires S-adenosylmethionine.
III. It produces malonyl CoA
IV. It uses acetyl CoA

(a) I, II, III, IV
(b) I only
(c) I, III, IV
(d) III, IV
(e) III only

Answer 45

D

Question 46

List the four basic reaction steps in the oxidation of a saturated fatty acid (the β-oxidation pathway). Briefly, describe the main chemical changes and reaction partners

Answer 46

Oxidation – Formation of double bond between α and β carbons of the activated fatty acid. FAD reduced to FADH2 , which passes electrons to electron-transfer chain (physical link to Oxidative Phosphorylation).

Hydration – Addition of H2O resulting in hydroxyl group on β carbon

Oxidation – Hydroxy group oxidized to ketone on β carbon. NAD+ reduced to NADH + H+ ).

Thiolysis – bond between α and β carbons cleaved yielding the release of acetyl-CoA and a new acyl-CoA shortened by two carbons (the latter will be further broken down by repeating the same 4 reactions of the β-oxidation pathway)

Question 47

What is the fate of glycerol released during lipolysis?

Answer 47

The glycerol is absorbed by the liver, phosphorylated, and then oxidised to DHAP, which is isomerised to glyceraldehyde 3-phosphate. It can then become either glucose or pyruvate

Question 48

The synthesis of fatty acids and their breakdown by β oxidation occur by separate pathways. Compare the two paths by filling in the blanks below. (Some blanks may require more than one answer.)

                                                         Synthesis                β oxidation Activating group Electron carrier coenzyme(s) Basic units added or removed Cellular location of process

Answer 48

Synthesis β oxidation

Activating group acyl carrier protein CoA—SH
Electron carrier coenzyme(s) NADPH NAD+
Basic units added or removed malonyl- and acetyl- acetyl-
Cellular location of process cytosol mitochondrial matrix

Question 49

Describe the mechanism for moving acetyl-CoA produced in the mitochondrial matrix into the cytosol for fatty acid synthesis

Answer 49

Acetyl-CoA in the mitochondrial matrix condenses with oxaloacetate to form citrate in a reaction catalysed by citrate synthase. Citrate moves out of the matrix via the citrate transporter. Citrate in the cytosol is cleaved by citrate lyase, yielding acetyl CoA and oxaloacetate. To complete the cycle, oxaloacetate in the cytosol is reduced to malate which moves into the mitochondrial matrix on the malate-α−ketoglutarate transporter, and is converted to oxaloacetate in the matrix

Question 50

What pathological conditions can arise from a large increase in blood levels of ketone bodies in diabetics?

Answer 50

In insulin-resistant diabetes mellitus (T1D), the lack of insulin means the liver cannot absorb glucose, no oxaloacetate is made, and the fatty acid mobilisation is slowed. The large amounts of ketone bodies actually cause acidosis. This pH change causes impairment of tissue function. If the acidosis is not rectified then ketoacidosis can become life-threatening

Question 51

Calculate the amount of ATP generated from the total oxidation of an activated fatty acid with an acyl chain of 18 carbons?

Answer 51

The pathway would generate 8 NADH and 8 FADH2 and 9 acetyl CoA molecules, which would be further metabolised in the TCA cycle. Thus, the net gain in ATP would be 20 ATP (NADH) plus 12 ATP (FADH 2) plus 90 ATP (acetyl CoA) = 122.
Remember: The question states ‘activated fatty acid’, so the 2 ATP’s required for activation need not be subtracted

Question 52

Match each term on the left with its function in the short-term regulation of fatty acid metabolism.

Malonyl-CoA A. Activates acetyl CoA carboxylase
Palmitate B. Activates hormone-sensitive lipase
Citrate C. Inhibits acetyl CoA carboxylase
cAMP-dependent D. Inhibits carnitine palmitoyl transferase
phosphorylation

Answer 52

A Citrate
B cAMP-dependent phosphorylation
C Malonyl-CoA
D Palmitate