4.1: Lipid Metabolism (Lenaz)

What of the following would you expect in very long chain fatty acyl CoA dehydrogenase deficiency?

• a) The patient develops liver steatosis (fatty liver)

• b) The patient improves with fasting

• c) The patient develops acidosis due to ketone bodies accumulation

• d) The level of acyl carnitines is decreased in blood

• e) The patient improves by forcing a high fat diet

a) The patient develops liver steatosis (fatty liver)

Fatty acyl CoA dehydrogenase is a key enzyme of mitochondrial beta-oxidation. Its deficiency does not allow fatty acids to be oxidized.

a) True. In liver, the lack of fatty acid oxidation makes them available for lipid (triacylglycerol) synthesis: hence, an excessive synthesis favours the lipid deposit in hepatocytes (fatty liver).

b) False. Fasting promotes glucagon secretion and fatty acid mobilization from adipose tissue. Fatty acids reach liver and other organs but cannot be oxidized. Thus, the patient’s condition worsens.

c) False. Since fatty acids are not oxidized, ketone bodies cannot be formed.

d) False. The accumulation of acyl CoA’s within mitochondria blocks their further entry: since the carnitine system is upstream with respect to beta oxidation, acyl carnitines will accumulate and eventually be released into the blood stream

e) False. The excess of fatty acids available will worsen the situation

Chylomicron triacylglycerols are hydrolysed by

• a) Pancreatic lipase

• b) Phospholipase C

• c) Lipoprotein lipase

• d) Hormone-sensitive lipase

• e) Phospholipase A2

c) Lipoprotein lipase

a) Pancreatic lipase digests dietary triacylglycerols in the intestine with the aid of bile salts, producing fatty acids and monoacylglycerols.

b) Phospholipase C digests glycerol phospholipids breaking the ester bond between glycerol and phosphate, producing a diacylglycerol and a phosphoric ester.

c) Lipoprotein lipase digests triacylglycerols from chylomicrons and VLDL in the capillaries of adipose tissue and muscles, releasing fatty acids to the cells.

d) Hormone-sensitive lipase is activated by cAMP in adipocytes and produces free fatty acids that are released into the blood.

e) Phospholipase A2 removes the internal fatty acid in phospholipids.

Calculate the ATP yield for the complete oxidation of hexanoic acid (C6:0) and compare it with the ATP yield of complete oxidation of one molecule of glucose.

Hexanoic acid (C6:0) undergoes 2 beta-oxidations and produces 3 acetyl CoA.

Initial activation consumes 2 high-energy bonds = -2 ATP

2 beta oxidations yield (1.5 + 2.5) × 2 = 8 ATP

3 acetyl CoA in the Krebs cycle yield 10 × 3 = 30 ATP

Total = -2 + 8 + 30 = 36 ATP

Glucose by glycolysis yields 2 net substrate-level ATP + twice the aspartate/malate shuttle (5 ATP), twice pyruvate oxidation (5 ATP) + twice the Krebs cycle (20 ATP),

Total = 32 ATP

Why some glycogen storage diseases are accompanied by ketone bodies formation?

If the glycogen in liver is not broken down, glucose availability may be too low and gluconeogenesis would be activated. Such an activation removes oxaloacetate from the Krebs cycle and prevents oxidation of acetyl CoA deriving from fatty acid oxidation in the liver. Thus, acetyl CoA is addressed to ketone bodies formation.

Acetoacetate

• a) in muscle is largely reduced to β-hydroxybutyrate

• b) in liver is formed by action of HMG lyase

• c) is a good energetic substrate for liver mitochondria

• d) in the cytosol of hepatocytes is an intermediate of cholesterol biosynthesis

• e) may be a precursor for gluconeogenesis

b) in liver is formed by action of HMG lyase

a) False. Acetoacetate synthesized in the liver is largely reduced to β-hydroxybutyrate by excess mitochondrial NADH; β-hydroxybutyrate is released in the blood and taken up by peripheral tissues where it is largely re-oxidized to acetoacetate.

b) True. In liver mitochondria

c) False. Liver produces ketone bodies but does not use them

d) False. Free acetoacetate is not produced in the cytosol (no HMG lyase)

e) False. There is no way in humans by which acetyl CoA and ketone bodies can become glucogenic precursors.

Why newly synthetized fatty acids are not directed to β-oxidation?

• a) because fatty acid biosynthesis occurs in the cytosol

• b) because fatty acids inhibit acyl CoA dehydrogenase

• c) because malonyl CoA inhibits carnitine acyl transferase

• d) because fatty acids are incorporated in triglycerides

• e) because they cannot be activated to acyl CoA

c) because malonyl CoA inhibits carnitine acyl transferase

Newly synthesized fatty acids are not oxidized but mainly used for lipid synthesis.

a) False. It is true that fatty acid biosynthesis occurs in the cytosol and their oxidation occurs in mitochondria, but this fact per se would not prevent their oxidation.

b) False

c) True. Malonyl CoA is an intermediate of fatty acid biosynthesis and its level is high during the process. Since malonyl CoA inhibits CAT-1, the newly synthesized fatty acids cannot enter into mitochondria for β-oxidation.

d) False. The statement is true, but this is not the cause preventing oxidation, rather it is the effect (fatty acids cannot be oxidized and therefore they are used to make lipids)

e) False. Fatty acids are activated to acyl CoA without any problem.

Only one answer is wrong. HMG-CoA reductase

• a) in mitochondria is involved in ketone body synthesis

• b) may be inhibited by phosphorylation

• c) is involved in the synthesis of Coenzyme Q

• d) catalyses two consecutive reductions

• e) is a cytosolic enzyme

a) in mitochondria is involved in ketone body synthesis

HMG CoA reductase forms mevalonate and is the key enzyme of isoprenoid biosynthesis, including cholesterol. It is a cytoplasmic enzyme.

a) False. The enzyme is not present in mitochondria. Ketone bodies formation requires HMG CoA lyase.

b) True. cAMP-dependent phosphorylation of the enzyme is an important mechanism of its inhibition.

c) True. The isoprenoid chain of CoQ is synthesized by progressive elongation of isoprenoid units deriving from mevalonate.

d) True. The thioester carboxyl is reduced by two consecutive NADPH forming an intermediate aldehyde and then a primary alcohol

e) True

Which of these do not derive from cholesterol? More than one answer may be correct

• a) Vitamin D3

• b) Coenzyme Q

• c) Progesterone

• d) Cholic acid

• e) Bilirubin

• f) Aldosterone

b) Coenzyme Q, e) Bilirubin

a) Vitamin D3 derives from cholesterol → 7-dehydrocholesterol

b) Coenzyme Q is synthesized through the mevalonate pathways, but its synthesis diverges from the synthesis of cholesterol.

c) Progesterone derives from cholesterol via pregnenolone

d) Cholic acid and other bile salts derive from cholesterol oxidation reactions

e) Bilirubin is a product of heme catabolism

f) Aldosterone and all other corticosteroids derive from cholesterol

In 1958, Salih Wakil accidentally discovered that fatty acid biosynthesis cannot take place in vitro using decarbonated water. What was the conclusion?

Bicarbonate is required for fatty acid biosynthesis (acetyl CoA carboxylase reaction)

Under some conditions, oleate can be further desaturated to 18:2 and 18:3, but these compounds cannot be used for synthesis of arachidonic acid. Why?

Oleate is 18:1 with the double bond at ω-9. Its desaturation will add further double bonds toward the COOH, since it belongs all to the ω-9 family. Thus, 18:2 and 18:3 synthesized belong to the ω-9 family. Desaturation cannot proceed toward the methyl end. Arachidonic acid belongs to the ω-6 family (with the first double bond at position 6 from the methyl group: hence, it cannot be formed from oleic acid in any way.

Acyl carrier protein (ACP)

• a) contains a water-soluble vitamin

• b) is involved in phospholipid biosynthesis

• c) in humans is an individual protein

• d) binds fatty acids with a phosphoanhydride bond

• e) has two SH groups

a) contains a water-soluble vitamin

a) True. ACP contains 4-phosphopantetheine, a derivative of pantothenic acid that is a water-soluble vitamin of the B group

b) False. It is involved in fatty acid biosynthesis de novo.

c) False. In mammals, all polypeptide chains making up fatty acid synthase are linked in a single protein that is folded in such way to form several active sites

d) False. ACP binds fatty acids with a thioester bond.

e) False. It has one SH group, used to make the thioester bond with fatty acids

One of these statements is false. Explain all answers.

• a) The reducing power for fatty acid biosynthesis is obtained by glucose oxidation

• b) All carbon atoms of glucose will be found in palmitic acid

• c) Fatty acid biosynthesis is stimulated by insulin

• d) Acetyl CoA carboxylase may be phosphorylated

• e) Acetyl CoA carboxylase is activated by citrate

b) All carbon atoms of glucose will be found in palmitic acid

a) True. Palmitate biosynthesis requires 14 NADPH. Of these, 8 may be obtained by glycolytic oxidation of 4 glucose molecules: the hydrogens from 8 NADH will be found in 8 NADPH by consecutive action of malate dehydrogenase and malic enzyme:

oxaloacetate + NADH → malate + NAD⁺

malate + NADP⁺ → pyruvate + CO₂ + NADPH

The additional 6 NADPH required may derive by glucose oxidation via the pentose phosphate pathway

b) False. Glycolytic pyruvate is decarboxylated to form acetyl CoA, thus 2 carbons per glucose will be found in CO₂

c) True. Insulin acts both by activating acetyl CoA carboxylase (by dephosphorylation) and inducing enzymes of the biosynthetic pathway

d) True. By action of glucagon

e) True. Citrate is an allosteric activator

Only one of these statements is true. Explain all answers

• a) Citrate activates fatty acid biosynthesis

• b) Phosphorylation of fatty acid synthase is required for palmitate biosynthesis

• c) Excess acetyl CoA inhibits fatty acid biosynthesis

• d) Fatty acid synthase may synthetize C16:0, C18:0, C20:0

• e) The Carbon atom of CO₂ used for acetyl CoA carboxylase will be incorporated into palmitic acid

a) Citrate activates fatty acid biosynthesis

a) True. Citrate acts as a feed-forward allosteric activator of acetyl CoA carboxylase

b) False. Fatty acid synthase is not phosphorylated

c) False. Long chain acyl CoA inhibits fatty acid biosynthesis

d) False. The only product of fatty acid biosynthesis from acetyl CoA is palmitate (16:0)

e) False. The CO₂ used to make malonyl CoA is removed during condensation of acetyl with malonyl during the action of fatty acid synthase.

Construct a scheme containing the following terms:

• acetoacetyl CoA

• acetyl CoA

• cholesterol

• Coenzyme Q10

• decaprenyl PP

• dimethylallyl PP

• farnesylated proteins

• farnesyl PP

• geranyl PP

• HMG CoA

• isopentenyl PP

• mevalonate

• mevalonate-PP

• squalene

In phosphatidyl choline biosynthesis, CDP-choline acts as

• a) Donor of a cytidyl group

• b) Donor of choline

• c) Donor of phosphoryl choline

• d) Donor of choline diphosphate

• e) As a catalytic component not incorporated into the products

c) Donor of phosphoryl choline

CDP choline reacts with diacylglycerol donating the phosphoryl choline group

Sphingomyelin and cerebroside have in common: (More than one answer may be correct)

• a) a phosphate group

• b) a fatty acyl group

• c) a carbohydrate

• d) a sphingosine

• e) a choline group

b) a fatty acyl group, d) a sphingosine

a) False. Cerebrosides do not contain phosphate

b) True. Both contain a fatty acyl group linked by amide bond with sphingosine

c) False. Sphingomyelin does not contain carbohydrate

d) True. Sphingosine is common to both

e) False. Cerebrosides do not contain choline

The nitrogen of sphingosine derives from

• a) glutamine

• b) serine

• c) choline

• d) aspartate

• e) tyrosine

b) serine

Sphingosine is built from condensation of palmitoyl CoA and serine with subsequent decarboxylation and oxidation

Adipocytes contain many enzymes of gluconeogenesis from pyruvate to triose-phosphates, but cannot synthesise glucose. Considering the function of adipocytes, what purpose can the presence of these enzymes have?

To synthesise glycerol phosphate, required for triacylglycerol biosynthesis.

Fatty acids deriving from lipolysis are released in blood

• a) in the form of monoglycerides

• b) bound to albumin

• c) bound to a specific fatty acid-binding protein

• d) in form of micelles

• e) as fatty acyl CoA derivatives

b) bound to albumin

Lipolysis results in complete hydrolysis of triacylglycerols to glycerol and fatty acids. Therefore:

a) False

b) True. The free fatty acids in blood bind to serum albumin

c) False

d) False. Binding to albumin is required to prevent micelle formation, since micelles would act as detergents

e) False. Fatty acyl CoA’s are only intracellular

Four molecules of glucose (24 C) are required for biosynthesis of palmitate (16:0). Where will you find the 8 carbon atoms not incorporated in palmitate?

• a) in short-chain fatty acids

• b) in lactate

• c) in CO₂

• d) in acetate

• e) in glycerol-3-P

c) in CO₂

Glycolytic pyruvate is decarboxylated to form acetyl CoA, thus 2 carbons per glucose will be found in CO₂

Beta-oxidation of pentadecanoic acid (15:0) yields

• a) 5 molecules of propionyl CoA

• b) 7 acetyl CoA + CO2

• c) 6 acetyl CoA and 1 propionyl CoA

• d) Pentadecanoic acid cannot be oxidized

c) 6 acetyl CoA and 1 propionyl CoA

Beta oxidation proceeds regularly releasing acetyl CoA until the last thiolase reaction on 5C acyl CoA that is cleaved to acetyl CoA and propionyl CoA

Which of these fats has the highest level of monounsaturated fatty acids?

• a) Butter

• b) Lard

• c) Corn oil

• d) Olive oil

d) Olive oil

a) Butter is rich in saturated fatty acids, largely short-chain.

b) Lard has both saturated and monounsaturated fatty acids, but the latter are less abundant than in olive oil.

e) Corn oil is rich in polyunsaturated fatty acids (mainly linoleic acid, 18:2 ω-6)

d) Olive oil has the highest content of monounsaturated fatty acids (oleic acid).

Which of these lipids has the highest content of fatty acids?

• a) Lecithin

• b) Sphingomyelin

• c) Cardiolipin

• d) Triacylglycerols

c) Cardiolipin

a) Lecithin is phosphatidyl choline (2 chains)

b) Sphingomyelin has only one chain

c) Cardiolipin is diphosphatidyl glycerol; therefore it has 4 fatty acyl chains.

d) Triacylglycerols have 3 chains.

Which of these is/are not involved in digestion of lipids? More than one answer may be applicable:

a. Bile salts

b. Bilirubin

c. Pancreatic lipase

d. Lipoprotein lipase

b) bilirubin, d) lipoprotein lipase

a) Involved. Bile salts help lipid digestion by increasing the surface of lipids to be hydrolysed by forming mixed micelles.

b) Not involved. Bilirubin is excreted in the bile as glucuronide, but it is not involved in digestive processes

c) Involved. Pancreatic lipase is the key enzyme for digestion of triglycerides

d) Not involved. Lipoprotein lipase hydrolyses triacylglycerols in chylomicrons and VLDL in blood capillaries.

Cyclic AMP

• a) activates hormone-dependent lipase

• b) activates acetyl CoA carboxylase

• c) activates HMG reductase

• d) increases acetyl CoA available for fatty acid biosynthesis

a) activates hormone-dependent lipase

a) True. By phosphorylation of perilipins of adipocytes

b) and c) False. It induces phosphorylation of the enzyme and its inhibition

d) False. Acetyl CoA available for fatty acid biosynthesis in liver derives from glucose oxidation, that is inhibited by cAMP through decreasing the levels of fructose-2,6-bisP

Electron Transfer Flavoprotein (ETF)

• a) is an enzyme of the respiratory chain

• b) contains FMN

• c) is reduced by acyl CoA dehydrogenases

• d) is contained in the intermembrane space of mitochondria

c) is reduced by acyl CoA dehydrogenases

a) False. It acts as a coenzyme (substrate-like); in the reduced form it reduces ETF dehydrogenase, that is an enzyme of the respiratory chain

b) False. It is a flavoprotein in which FAD (not FMN) is alternatively reduced and reoxidised

c) True

d) False. It is contained in the matrix

Calculate the ATP balance of complete oxidation of propionyl CoA compared to acetyl CoA. (Keep in mind that malate is converted to pyruvate by malic enzyme)

propionyl CoA → methylmalonyl CoA (-1 ATP) → succinyl CoA → succinate (+1 ATP) → fumarate (+1.5 ATP) → malate → pyruvate → acetyl CoA (+2.5 ATP) → Krebs cycle (+10 ATP)

Total = 14 ATP compared with 10 ATP from acetyl CoA

Write the reactions for palmitate biosynthesis from alanine.

1. How many alanine molecules are required?

2. Write the ATP balance.

3. What is the major source of NADPH?

Alanine → pyruvate → acetyl CoA.

1. Since palmitate biosynthesis requires 8 acetyl CoA, the same holds for alanine → 8 alanine required.

2. ATP balance:

• 8 pyruvate → 8 acetyl CoA (8 NADH → +20 ATP)

• Exit of 8 acetyl CoA (citrate lyase, pyruvate carboxylase → -16 ATP)

• 7 times acetyl CoA carboxylase, i.e. acetyl CoA → malonyl CoA (-7 ATP)

• Total = 20 - 16 - 7 = -3 ATP

3. For glucose oxidation the major source of NADPH is glycolytic NADH; in the case of alanine this is not possible, therefore NADPH must derive from the pentose phosphate pathway.

Calculate the ATP balance for synthesis of tripalmityl glycerol starting from glucose.

Part A:

4 Glucose → 8 pyruvate (+8 ATP)

8 pyruvate → 8 acetyl CoA (+20 ATP)

8 acetyl CoA _mitochondria → 8 acetyl CoA _cytosol (-16 ATP)

7 acetyl CoA → 7 malonyl CoA (-7 ATP)

Total = 8 + 20 - 16 - 7 = +5 ATP

This must be multiplied times 3 (3 molecules of palmitate) = +15 ATP

The activation of 3 palmitate molecules to palmityl CoA consumes 3×2 = 6 high energy bonds.

Total: 15 - 6 = +9 ATP

Part B: (glucose to glycerol-P)

½ glucose → dioxyacetone P (-0.5×2 = -1 ATP)

dioxyacetone P + NADH → glycerol-3-P + NAD⁺

glycerol-3-P + 2 palmitoyl CoA → phosphatidic acid

phosphatidic acid + H₂O → diacylglycerol + P_i

diacylglycerol + palmitoyl CoA → tripalmitoyl glycerol + CoASH

Total = A + B = 9 - 1 = 8 ATP

A rat is fed a diet containing 14C-labelled linoleic acid. List the derivatives that would contain radioactivity

• Linoleic acid can be totally oxidized, so radioactivity would be found in CO₂

• If linoleic acid (18:2 ω-6) is converted to arachidonic acid (20:4 ω-6) the latter will contain radioactivity together with eicosanoid derivatives (prostaglandins, thromboxans, leukotrienes).

• Linoleic acid can be incorporated into lipids

A rat is fed glucose 14C-labelled in C₃. Under conditions favouring fatty acid biosynthesis, will most radioactivity be found in fatty acids?

No. Glucose C₃ will yield the carboxyl group of phosphoglyceric acid and hence of pyruvic acid, and will be lost as CO₂ during pyruvate oxidation

Cholesterol esters

• a) are mostly contained in the mitochondrial membrane

• b) are more hydrophobic than free cholesterol

• c) are carried mostly by VLDL

• d) are esterified with phosphoric acid

• e) are formed after activation of cholesterol with ATP

b) are more hydrophobic than free cholesterol

a) False. Being completely hydrophobic (the OH is esterified with a fatty acid), cholesterol esters are not amphipathic and are not part of membranes. The plasma membrane contains free cholesterol.

b) True.

c) False. They are carried mostly by LDL

d) False. They are esterified with a fatty acid

e) False. It is the fatty acid to be activated by acyl CoA

Bile salts help lipid digestion

• a) by forming mixed micelles

• b) by forming covalent bonds with fatty acids

• c) by optimizing pH during digestion

• d) working as coenzymes of lipases

• e) providing the energy for hydrolysis

a) by forming mixed micelles

Bile acids secreted by liver in the duodenum help lipid digestion.

a) True. Being amphipathic molecules, they act forming mixed micelles with triacylglycerols, thus increasing the surface of attack by lipases

b) False

c) False

d) False. Lipases, as most hydrolases, use no coenzyme.

e) False. Hydrolysis reactions require no energy

Chylomicrons

• a) are carried to the liver by portal circulation

• b) contain free fatty acids deriving from digestion

• c) undergo triglyceride hydrolysis by lipoprotein lipase

• d) release glycerol into adipocytes to be used for triacylglycerol re-synthesis

• e) are the only lipoproteins that do not contain phospholipids

c) undergo triglyceride hydrolysis by lipoprotein lipase

a) False. Chylomicrons are carried by the lymphatic system

b) False. They contain triacylglycerols re-synthesized in the enterocyte

c) True. This happens in the capillaries of adipose and muscular tissue

d) False. The hydrolysis of triacylglycerols by lipoprotein lipase is complete and free glycerol is released. Since adipocytes cannot utilize glycerol, it is esterified in liver to glycerol-3-P

e) False, they contain a phospholipid monolayer as the other lipoproteins

Only one answer is wrong. Diacylglycerol

• a) is a product of phospholipase C

• b) is formed by action of phosphatidate phosphatase

• c) is a second messenger

• d) reacts with CTP for biosynthesis of phosphatidyl inositol

• e) activates protein kinase C

d) reacts with CTP for biosynthesis of phosphatidyl inositol

a) True. Phospholipase C breaks the bond between glycerol and phosphate

b) True. It is also formed during lipid biosynthesis by hydrolysis of phosphatidic acid

c) and e) True. It acts as a second messenger by activating protein kinase C

d) False. It is phosphatidic acid to react with CTP to form CDP-diacylglycerol and PP_i

Untreated ____ have high plasma _____. In fact enhanced _____ oxidation in liver yields ____ that cannot enter the _____ because ____ is diverted to ____.

The first product, ____ is reduced to ____ due to the high levels of ____ produced by ____ and ____ oxidation.

Complete using the following terms:

• acetoacetate

• acetyl CoA

• amino acid

• diabetics

• fatty acid

• fatty acid

• gluconeogenesis

• β-hydroxybutyrate

• ketone bodies

• Krebs cycle

• NADH

• oxaloacetate

Untreated diabetics have high plasma ketone bodies. In fact enhanced fatty acid oxidation in liver yields acetyl CoA. that cannot enter the Krebs cycle because oxaloacetate is diverted to gluconeogenesis.

The first product, acetoacetate is reduced to β-hydroxybutyrate due to the high levels of NADH produced by amino acid and fatty acid oxidation.

What do you expect in case of lipoprotein lipase deficiency?

• a) Decreased blood lipid levels

• b) Increased blood lipid levels

• c) Increased uptake of free fatty acids by adipocytes

• d) Increased plasma LDL

• e) Improvement by a high fat diet

b) Increased blood lipid levels

Deficiency of lipoprotein lipase prevents hydrolysis of triacylglycerols of chylomicrons and VLDL; as a consequence, triacylglycerol levels in blood increase.

a) False

b) True

c) False. The lack of lipase activity prevents free fatty acid formation

d) False. LDL form after VLDL lose their triacylglycerols

e) False. A high fat diet would worsen the lipid pattern

One answer only is wrong. Concerning VLDL

• a) They are very rich in triacylglycerols

• b) They are synthesized in the liver

• c) Their lipids are hydrolysed by lipoprotein lipase

• d) Their deficit is the cause of familial hypercholesterolemia

• e) They are larger than HDL

d) Their deficit is the cause of familial hypercholesterolemia

a) True

b) True

c) True

d) False. Familial hypercholesterolemia is due to mutations of the LDL receptor

e) True. They are the largest lipoproteins after chylomicrons

1. How many rounds of β-oxidation are required to completely convert stearic acid (18:0) into acetate units in form of acetyl CoA?

2. How many molecules of acetyl CoA, NADH and FADH₂ would be produced by this complete conversion of stearic acid into acetate units?

8 rounds of β-oxidation will produce the following for the respiratory chain:

• 9 molecules of acetyl CoA

• 8 FADH₂

• 8 NADH

Stearic acid has a molecular mass of about 280. 2 molecules of glucose have a molecular mass of about 360.

1. How many molecules of acetyl CoA and NADH would be produced by conversion of two molecules of glucose into acetate units in the form of acetyl CoA through glycolysis and pyruvate dehydrogenase?

2. Why do two molecules of glucose, in spite of having a greater molecular mass than stearic acid, yield fewer energy-rich products than stearic acid?

1. 2 molecules of glucose will yield:

• 4 acetyl CoA

• 8 NADH (4 from aspartate/malate shuttle, 4 from pyruvate dehydrogenase)

2. Glucose is more oxidized than stearic acid, therefore its oxidation yields less energy.

An individual developed a condition characterized by progressive muscular weakness and aching muscle cramps. The symptoms are aggravated by fasting, exercise and a diet high fat and very low in carbohydrates. The homogenate of a skeletal muscle specimen from the patient oxidized added oleate more slowly than did control specimens from healthy individuals.  When carnitine was added in the patient’s muscle homogenate, the rate of oleate oxidation equalled that in the control homogenates. The patient was diagnosed as having a carnitine deficiency.

1. Why did added carnitine increase the rate of oleate oxidation in the patient’s muscle homogenate?

2. Why were the patient’s symptoms aggravated by fasting, exercise and a high fat diet?

Carnitine deficiency presumably derives from a genetic mutation affecting its biosynthesis.

1. Added carnitine restores the carnitine/acylcarnitine shuttle and therefore restores β-oxidation.

2. Fatty acid oxidation is stimulated by fasting, muscular exercise and high fat diet, thus carnitine deficiency would be more severe under these conditions.

ETF dehydrogenase is located

• a) in the outer membrane

• b) on the outer side of the inner membrane

• c) on the inner side of the inner membrane

• d) in the matrix

c) on the inner side of the inner membrane

ETF is soluble in the matrix, like the enzymes of fatty acid oxidation, but ETF dehydrogenase is a membrane enzyme with its active site on the matrix

Only one of these statements concerning methylmalonyl CoA is true.

• a) It is involved in cholesterol biosynthesis

• b) It is formed from propionyl CoA in a B₁₂-dependent reaction

• c) It is converted into succinyl CoA

• d) It is an intermediate of fatty acid biosynthesis

• e) It is ketogenic

c) It is converted into succinyl CoA

a) False

b) False. It is formed by propionyl CoA carboxylase, a biotin enzyme

c) True. This is the B₁₂-dependent reaction

d) False. This is malonyl CoA

e) False. It is glucogenic (via succinyl CoA)

Acetoacetate in muscle mitochondria is activated by

• a) ATP and CoA

• b) Succinyl CoA

• c) GTP and α-ketoglutarate

• d) Long-chain acyl CoA

b) Succinyl CoA

Insulin induces decrease of fatty acid oxidation because

• a) increased malonyl CoA inhibits CAT-1

• b) long-chain acylCoA dehydrogenase is dephosphorylated

• c) ketone bodies production is decreased

• d) ketone bodies utilization is increased

a) increased malonyl CoA inhibits CAT-1

Since insulin activates acetyl CoA carboxylase and fatty acid biosynthesis, increased malonyl CoA inhibits CAT-1 and hence fatty acid oxidation

a) True

b) False

c) Irrelevant

d) Irrelevant

Type-2 diabetics are often obese because

• a) Lack of insulin stimulates transcription of fatty acid biosynthesis enzymes

• b) Obesity and overfeeding favour diabetes arousal

• c) Lack of insulin inhibits lipolysis in adipose tissue

• d) Lack of insulin enhances the transport of citrate in the cytosol

b) Obesity and overfeeding favour diabetes arousal

a) False. Insulin stimulates lipogenesis.

b) True. Obesity is a cause of diabetes

c) False. Insulin inhibits lipolysis

d) False. Insulin favours citrate export

A patient develops glycosuria but his glycaemia is normal (90 mg/dl). What would you deduce from this situation?

• a) The patient is diabetic but his glycaemia at fasting has normalized

• b) The patient has a defect in renal reabsorption of glucose

• c) Lipogenesis from glucose is enhanced in kidney cells

• d) There is increased gluconeogenesis in renal cortex

b) The patient has a defect in renal reabsorption of glucose

a) False. It is very unlikely that a diabetic patients with glycosuria may have normal glycaemia at fasting

b) True. This is the most likely explanation → renal glycosuria

c) False. This would decrease glucose content

d) False. Increased gluconeogenesis would increase blood glucose

Phospholipase A2

• a) hydrolyses a fatty acid from the 2-position of lecithin

• b) hydrolyses phosphorylcholine from lecithin

• c) hydrolyses choline from lecithin

• d) hydrolyses a fatty acid from lysolecithin

a) hydrolyses a fatty acid from the 2-position of lecithin

a) True.

b) False. This is phospholipase C

c) False. This is phospholipase D

d) False.

In LDL

• a) cholesterol esters are located on the external face

• b) phospholipids form a bilayer

• c) Apo B100 is recognised by LDL receptors

• d) the majority of cholesterol is not esterified

c) Apo B100 is recognised by LDL receptors

a) False. Cholesterol esters are hydrophobic and are contained in the interior of the particle

b) False. In lipoproteins, phospholipids form a monolayer with the hydrophilic end at the exterior

c) True

d) False. The majority of cholesterol is esterified with fatty acid