Metabolism 6- Fatty Acids

Question 1

What is the end product of fatty acid metabolism and where does this take place

Answer 1

Acetyl CoA- which can enter the Krebs Cycle

This takes place in the mitochondrial matrix.

Question 2

Describe the structure of fatty acids

Answer 2

Hydrophilic carboxyl head

Hydrophobic hydrocarbon tail

Question 3

What are the features of unsaturated fatty acids

Answer 3

They contain one or more double bonds, giving the fatty acid chain a kink, this makes it more difficult for the fatty acids to pack together, hence unsaturated fatty acids are likely to be liquid at room temperature.

Question 4

How are fatty acids stored within cells

Answer 4

They are stored as triacylglycerol- three fatty acids linked to a glycerol head group via an ester bond. In mammals, specialised cells known as adipocytes take on the role of the storage of fatty acids.

Question 5

Where is fat derived from

Answer 5

De Novo synthesis in the liver
Diet
Storage depots in adipocytes.

Question 6

Why are bile salts important

Answer 6

Fatty acids are insoluble and so cannot be transported in the blood stream. Bile salts (generated by the live and stored in the gall bladder) pass through the bile duct and into the small intestine, where they emulsify fats, aiding the digestion and absorption of fats as well as fat soluble vitamins (A,D,E and K)

Question 7

What is a consequence of a lack of bile salts

Answer 7

A lack of bile salts results the majority of fat passing through the gut undigested and unabsorbed resulting in steatorrhea (fatty stool).

Question 8

Describe the process of fat digestion and absorption

Answer 8

a - Lipid digestion by lingual gastric and pancreatic lipase results in the formation of Monoacylglycerol (MAG), diacylglycerol (DAG) and free FAs that are released by lipid hydrolysis join BS (Bile Salts), CL, (Cholesterol) lysophosphatidic acid (LPA) and fat-soluble vitamins to form mixed micelles.

b - Digested dietary products are absorbed by enterocytes that line the brush border of the small intestine. TAGs are resynthesized under the control of several enzymes prior to incorporation into chylomicrons (CM) and transportation

Question 9

What is Orlistat

Answer 9

A potent inhibitor of gastric and pancreatic lipases.

It is a chemically synthesized derivative of lipstatin, a product of Streptomyces toxytricini.

Orlistat reduces fat absorption by 30% which is almost completely excreted by the faecal route.
Main side effects include abdominal pain, urgency to defecate, increased flatus and steatorrhea.

Question 10

Which organ does not use energy from fatty acid oxidation

Answer 10

The Brain

Question 11

What is the first step in the beta-oxidation of fatty-acids

Answer 11

They are converted into an acyl-CoA species
Fatty acid + ATP + Acetyl CoA — Acyl CoA + AMP + PPi
The reaction is catalysed by acyl CoA synthesase.

This reaction occurs on the outer mitochondrial membrane

Question 12

Describe the reactions in the carnitine shuttle

Answer 12

Carnitine acyltransferase 1:
Acyl CoA + Carnitine — Acyl Carnitine + CoA
Translocase: Acyl Carnitine in. Carnitine out

Carnitine transferase II:
Acyl Carnitine + CoA — Acyl CoA + Carnitin

Question 13

Describe simply the process of beta oxidation

Answer 13

The acyl CoA undergoes a sequence of oxidation, hydration, oxidation and thiolysis reactions (collectively called β-oxidation).

This results in the production of one molecule of acetyl CoA and an acyl CoA species which is 2 carbons shorter than the original.

Question 14

What are the products of each cycle

Answer 14

The β-oxidation reactions continue to consecutively remove 2-carbon units from the acyl CoA thereby producing acetyl CoA.
On the final cycle (4-carbon fatty acyl CoA intermediate), two acetyl CoA molecules are formed.
During each cycle one molecule each of FADH2 and NADH are produced.
Odd chain fatty acids give propionyl CoA and Acetyl CoA on the final cycle. Propionyl CoA undergoes a series of reactons to yield succinyl CoA which enters the TCA cycle.

Question 15

Describe the first reaction of beta oxidation

Answer 15

Fatty acyl dehydrogenase adds a double bond between the second and third carbon atoms. FADH— FADH2

Question 16

Describe the second reaction of beta oxidation

Answer 16

Hydration of this compound to give 3-hydroxyacyl CoA.

This reaction is catalysed by 3-Hydroxyacyl CoA hydrolase.

Question 17

Describe the third reaction of beta oxidation

Answer 17

3-hydroxyacyl CoA undergoes an oxidation reaction, where a ketone is produced, catalysed by L- Hydroxyacyl CoA dehydrogenase

Question 18

Describe the final reaction of beta oxidation

Answer 18

Beta ketothiolase hydrolyses thioester bond of HS- CoA which joins to give Acyl CoA, removing Acetyl CoA.

Question 19

When will the Acetyl CoA generated by beta oxidation enter the Krebs Cycle

Answer 19

Acetyl CoA generated by β-oxidation enters the TCA cycle only if β-oxidation and carbohydrate metabolism are balanced, since oxaloacetate is needed for entry

Question 20

When will fat breakdown predominate

Answer 20

during fasting or in diabetes, gluconeogenesis will take oxaloacetate to generate glucose. Hence the need for ketonebodies. Non-enzymatic decrboxylation of Acetoacetate gives acetone. Occurs in the liver. This increases the acidity of the blood.

Question 21

Describe the reactions involved in the production of ketone bodies.

Answer 21

Reaction 1:
Condensation of two acetyl CoA molecules to give acetoacetyl CoA. Catalysed by 3-ketothiolase. CoA group released
Reaction 2:
Acetoacetyl CoA + Acetyl CoA + H2O — 3-hydroxy-3-methyl glutaryl CoA. Catalysed by hydroxymethylglutaryl CoA synthase
Reaction 3:
Removal of acetyl CoA from 3-hydroxy-3-methyl glutaryl CoA to give acetoacetone. Catalysed by hydroxymethyl CoA cleavage enzyme.
Reaction 4: Spontaneous breakdown of acetoacetone into acetone. Acetoacetone can also be oxidised to 3-hydroxybutyrate by D-3 hydroxybutyrate dehydrogenase. NAD—NADH. This reaction is reversible.

Question 22

What enzymes are involved in lipogenesis

Answer 22

Acetyl CoA Carboxylase

Fatty acid synthase (FAS) - polypeptide with seven different enzymatic activities.

Question 23

When is lipogenesis required

Answer 23

Most fatty acids obtained from a Western diet. Some synthesis needed during embryogenesis and also within mammary gland during lactation.

Question 24

Describe reaction 1 of lipogenesis

Answer 24

Acetyl CoA + HCO3- + ATP — Malonyl CoA (3C) + ADP +Pi. Catalysed by Acetyl Co-A carboxylase

Question 25

Describe reaction 2 of lipogenesis

Answer 25

The transfer of Malonyl from Malonyl CoA-to acyl carrier protein (ACP) to form Malonyl-ACP (catalyzed by Malonyl-CoA-ACP transferase).

Question 26

Describe reaction 3 of lipogenesis

Answer 26

The transfer of acetyl from a CoA species to acyl carrier protein (ACP) to form Acetyl-ACP (catalyzed by Acetyl-CoA-ACP transferase).

Question 27

Describe reaction 4 of lipogenesis

Answer 27

Condensation of acetyl ACP with malonyl-ACP to form a 4C fatty acid species (catalysed by β-ketoacyl ACP synthase). CO2 and ACP are released

Question 28

Describe reaction 5 of lipogenesis

Answer 28

Reduction of Acetoacetyl-ACP to D-3-hydroxylacyl-ACP (catalysed by β-ketoacyl ACP reductase).
NADPH — NADP+

Question 29

Describe reaction 6 of lipogenesis

Answer 29

Dehydration to crotonyl ACP (a.k.a. trans-∆2-Enoyl-ACP which is catalysed by 3 hydroxyacyl-ACP dehydrase).

Question 30

Describe reaction 7 of lipogenesis

Answer 30

Enoyl-ACP reductase catalyzes further reduction to butyryl-ACP.
NADPH — NADP+

Question 31

How are longer fatty acids created

Answer 31

The reactions of steps 4-7 are repeated (Condensation, Reduction, dehydration, reduction) In the final step butyryl- ACP is hydrolysed to give ACP and butarate.

Question 32

Differences between lipogenesis and beta-oxidation

Answer 32

Cytoplasm v Mitochondrial matrix
NADPH as reducing power vs NAD+ and FADH2
Length increased by 2C V Length decreased by 2C
ACP is carrier V CoA is carrier

Question 33

Where does the elongation of fatty acids longer than 16C take place

Answer 33

mitochondria and endoplasmic reticulum

Question 34

What does the desaturation of fatty acids require

Answer 34

the action of fatty acyl-CoA desaturases

Question 35

Describe the role of fatty acid synthesis in cancer

Answer 35

In adults, de novo FA biosynthesis is restricted mainly to the liver, adipose tissue and lactating breast.

Evidence suggests that reactivation of FA synthesis also occurs in certain cancer cells.
Inhibition of FASN by cerulenin (an antifungal antibiotic) shown to reduce tumour growth of ovarian cancer cells.

Question 36

describe the characteristics of fatty acyl CoA dehydrogenases

Answer 36

Need different conformational shapes to catalyse oxidation of fatty acids of different lengths.
Short-chain acyl-Co enzyme A dehydrogenase (<6C)
Medium-chain acyl-Co enzyme A dehydrogenase (C6-C12)
Long-chain 3-hydroxyacyl-Co enzyme A dehydrogenase (C13-C21)
Very long-chain acyl-Co enzyme A dehydrogenase (>C22)

Question 37

Describe Medium-chain acyl-coenzyme A dehydrogenase deficiency (MCADD)

Answer 37

Autosomal recessive. Predominantly occurring in Caucasians.

• Occurs 1 in 10,000 live births in the UK per year.

• If undiagnosed, can be fatal. Thought to account for 1 in100 deaths from Sudden Infant Death Syndrome (SIDS).
If diagnosed, patients should never go without food for longer than 10–12 hours (a typical overnight fast). Adhere to a high carbohydrate diet.

Patients with an illness resulting in appetite loss or severe vomiting may need i.v. glucose to make sure that the body is not dependent on fatty acids for energy.

Question 38

Describe primary carnitine deficiency

Answer 38

Autosomal recessive disorder.

• Occurs 1 in 100,000 live births in the USA per year (1 in 40,000 live births in Japan; 1 in 500 in the Faroe Islands).
• Symptoms appear during infancy or early childhood and include encephalopathies, (cardiomyopathies, muscle weakness; and hypoglycaemia).

Mutations in a gene known as SLC22A5 which encodes a carnitine transporter result in reduced ability of cells to take up carnitine, needed for the β-oxidation of fatty acids.

Question 39

Describe the structural organisation of fatty acid synthase domains

Answer 39

KS, ß-ketoacyl synthase; MAT, malonyl/acetyl ACP transferase (aka transacylase)
DH1&DH2, dehydrase
SD, structural domain
ER, enoyl reductase
KR, ß-ketoacyl reductase 
ACP, acyl carrier protein
TE, thioesterase.
Fatty acids are formed sequentially by decarboxylative condensation reactions involving the molecules acetyl-CoA and malonyl-CoA. 

Following each round of elongation, the fatty acid undergoes reduction and dehydration by the sequential action of a ketoreductase (KR), dehydratase (DH), and enol reductase (ER) activity.

The growing fatty acyl group is linked to an acyl carrier protein (ACP).