BC10-2(part4)

OXIDATION of FATTY ACIDS

A. -Oxidation of Fatty Acids

- major (mitochondrial) pathway for saturated fatty acid catabolism

- 2-carbon fragments are successively removed from the carboxyl end of the fatty acyl CoA  acetyl

CoA

- generates

- NADH

- FADH2

- yields acetyl CoA (substrate for the TCA cycle)

- principal pathway for fatty acid catabolism

- occurs in the mitochondrial matrix

- involves oxidation of -carbon to-keto acid

1. Fatty Acid Transport into Mitochondria

a. Long Chain Fatty Acid Transport into the Mitochondria

- 12-20 carbons

- free fatty acid take-up into the cell  activated to acyl-CoA

i. 2 Activating Systems

ia. Endoplasmic Reticulum Fatty Acyl CoA Synthase (Thiokinase)

- activates long-chain fatty acids

ib. Inner Mitochondrial Acyl CoA Synthases

- activates fatty acids of

- medium-chain length (6-12 carbons)

- short-chain length (2-4 carbons)

- acetate

- propionate

- these fatty acids freely enter the

mitochondria from the cytosol

b. Long Chain Fatty Acid Translocation (Carnitine Shuttle)

- -oxidation occurs in the mitochondria  long-chain fatty acids must be

transported across mitochondrial inner membrane (generally impermeable to

bulky polar molecules such as CoA)  specialized carrier (carnitine shuttle) in

the membrane transports the acyl group from cytosol  mitochondrial matrix

Carnitine

- exists in the inner mitochondrial membrane

1. Sources

- dietary - red meat

- dairy products

2. Synthesis

- in the body from

- lysine

- methionine

- by - liver

- kidney

3. Skeletal Muscles

- contain about 97% of all carnitine in the body

4. Functions

- receives the transferred fatty acyl group  long-chain fatty

acids in the mitochondrial matrix

- allows export from the mitochondria of branched-chain acyl

groups

- trapping and excretion via the kidney of acyl groups that

cannot be metabolized by the body

i. Acyl group is transferred from cytosolic CoA to carnitine by carnitine palmitoyl

transferase I (CPT-I)/carnitine acyltransferase I (CAT-I) in the outer

surface of the inner mitochondrial membrane  acylcarnitine

ii. Acylcarnitine transported across the membrane to mitochondrial matrix  acyl group

transferred to mitochondrial CoA by carnitine palmitoyl transferase II (CPT-

II)/carnitine acyltransferase II (CAT-II) in the inner surface of the inner

mitochondrial membrane

c. Carnitine Shuttle Inhibitor

i. Malonyl CoA

- inhibits carnitine acyltransferase I  inhibiting entry of acyl groups into the

mitochondrial matrix

- when fatty acid synthesis is occurring in the cytosol (as indicated by the

presence of malonyl CoA), newly made fatty acids cannot be

transferred into the mitochondria for degradation

d. Genetic Defects in the Carnitine Shuttle

- congenital absence of carnitine acyltransferase in skeletal muscles

- defective carnitine synthesis  low carnitine concentration

 inability to use long-chain fatty acids as a metabolic fuel  accumulation of

- toxic amounts of free fatty acids

- branched-chain acyl groups in cells

i. CPT I Deficiency

- affects the liver

- inability to use long chain fatty acids for fuel  impaired glucose synthesis (an

endergonic process) during fasting  - severe hypoglycemia

- coma

- death

ii. CPT II Deficiency

iia. Primarily in

- cardiac muscles

- skeletal muscles

iib. Symptoms of Carnitine Deficiency

- cardiomyopathy

- muscle weakness

- myoglobinemia following exercise

iii. Treatment

- avoidance of prolonged fasts

- diet - high carbohydrate

- low in long chain fatty acids

- supplementation with medium chain fatty acids

e. Carnitine Deficiency

i. 2 Categories

ia. Systemic

- carnitine levels are reduced in all tissues

ib. Myopathic

- carnitine levels are reduced only in muscle tissue including the heart

ii. Congenital Deficiencies

- defective tubular carnitine reabsorption

- deficient cellular carnitine uptake

iii. Secondary Deficiencies

- liver disease  decreased carnitine synthesis

- malnutrition

- strict vegetarian diet

- increased carnitine requirements

- pregnancy

- severe infections

- burns

- trauma

- hemodialysis  carnitine removal

iv. Treatment

- carnitine supplementation

v. Case (Systemic Carnitine Deficiency)

i. Presentation

A 3 year old boy was found to have an enlarged heart while being examined for a cough. An ECG was abnormal. The boy’s development appeared normal, although he was an irritable child. He was released and monitored. Over the next 2 years, cardiomegaly increased, and he developed symptoms of CHF. He also began to exhibit signs of weakness of skeletal muscles. A muscle biopsy showed lipid deposition and low carnitine levels. Plasma levels of carnitine were also found to be below normal.

ii. Diagnosis and Treatment

- below normal carnitine levels confirmed a diagnosis of carnitine

deficiency

- treatment was begun with daily dose of 174 mg/kg L-carnitine

- within 1 month

- activity increased

- irritability decreased

- after 12 months

- cardiac problems improved

- normal strength and exercise tolerance

iii. Discussion

- oral carnitine load  plasma carnitine levels increased to low-normal

range, urinary levels increased to 30x normal  suggests a

defect in renal and/or gastrointestinal transport of carnitine

f. Short- and Medium-Chain Fatty Acid Entry into the Mitochondria

- fatty acids < 12 carbons can cross the inner mitochondrial membrane without the aide

of carnitine or the CPT system  activated to their CoA derivative by matrix

enzymes  oxidation

i. Medium Chain Fatty Acids

- plentiful in human milk

- oxidation not dependent on CPT-I  not inhibited by malonyl CoA

2. Reactions of -Oxidation

- sequence of 4 reactions  shortening of the fatty acid chain by 2 carbons

a. Steps - oxidation that produces FADH2

- hydration

- oxidation that produces NADH

- thiolytic cleavage  acetyl CoA release

i. Acyl CoA  Enoyl CoA

- dehydrogenation reaction catalyzed by acyl CoA dehydrogenase

- prosthetic group FAD reduced to FADH2 during the reaction

ii. Enoyl CoA  3-Hydroxyacyl CoA

- hydration reaction catalyzed by enoyl CoA hydratase

iii. 3-Hydroxyacyl CoA  3-Ketoacyl CoA

- dehydrogenation reaction catalyzed by 3-hydroxyacyl CoA dehydrogenase

- NAD+

is reduced to NADH during the reaction

iv. 3-Ketoacyl CoA + CoA  Acetyl CoA + Tetradecanoyl CoA

- thiolytic cleavage reaction catalyzed by thiolase (-ketothiolase)

b. Acetyl CoA

- positive allosteric effector of pyruvate carboxylase  link between fatty acid

oxidation and gluconeogenesis

3. Energy Yield from Fatty Acid Oxidation

i. Stoichiometry of -Oxidation

a. -Oxidation of Palmitate (16 carbons)

Palmitoyl CoA + 7CoA + 7FAD + 7NAD+

+ 7H2O 

8Acetyl CoA + 7FADH2 + 7NADH + 7H+

- 7 moles FADH2  10.5 moles ATP by electron transport and oxidative

phosphorylation

- 7 moles NADH  17.5 moles ATP by electron transport and oxidative

phosphorylation

 Palmitoyl CoA + 7CoA + 7O2 + 28Pi + 28ADP 

8Acetyl CoA + 28ATP + 42H2O

- costs 2 moles ATP to activate free palmitate  26 moles ATP per mole of

palmitate for -oxidation

ii. Total Oxidation

- acetyl CoA derived from -oxidation of fatty acids  oxidized to O2 and H2O by the

TCA cycle

8Acetyl CoA + 16O2 + 80Pi + 80ADP 

8CoA + 80ATP + 16CO2 + 104H2O

- combined yield of ATP

Palmitoyl CoA + 23O2 + 108Pi + 108ADP 

8CoA + 108ATP + 16CO2 + 146H2O

- when starting with the free fatty acid palmitate, the ATP required for the production of

palmitoyl CoA must be taken into consideration  2 high-energy bonds are

broken down owing to the thiokinase reaction  total energy yield is 106

ATP

4. Respiratory Quotient

- moles of CO2 produced divided by the moles of O2 consumed during complete oxidation of a

metabolic fuel to CO2 and H2 O

a. Palmitate

RQ = 16 moles CO2 produced = 0.7

23 moles O2 consumed

b. Glucose

C6H12O6 + 6O2  6CO2 + 6H2O

RQ = 6 moles CO2 produced = 1.0

6 moles O2 consumed

c. Protein = 0.83

* Daily Energy Expenditure (DEE)

- can be determined from the oxygen consumption and the respiratory quotient (RQ)

B. Oxidation of fatty Acids with an Odd Number of Carbons

- oxidation proceeds by the same reaction steps as that of fatty acids with an even number of carbons, until

the final 3 carbons are reached (propionyl CoA)

- propionyl CoA is also produced during the metabolism of certain amino acids

1. Propionyl CoA Metabolism (Propionic Acid Pathway)

a. Synthesis of Methylmalonyl CoA

i. Propionyl CoA Carboxylase

- absolute requirement for the coenzyme biotin

- propionyl CoA  carboxylation  D-methylmalonyl CoA

b. Formation of L-Methylmalonyl CoA

i. Methylmalonyl CoA Racemase

- D-methylmalonyl CoA  L-methylmalonyl CoA

c. Synthesis of Succinyl CoA

- succinyl CoA can enter the TCA cycle

i. Methylmalonyl CoA Mutase

- requires a coenzyme form of vitamin B12 (deoxyadenosylcobalamin)

- rearranges the substrate’s carbon skeleton

ia. Vitamin B12 Deficiency

- propionate and methylmalonate are excreted in the urine

ib. Methylmalonic Acidemia and Aciduria

- 2 types - 1 in which the mutase is missing

- 1 in which the patient is unable to convert vitamin B12 into

its coenzyme form

 either defects results in

- metabolic acidosis

- developmental retardation

C. Oxidation of Unsaturated Fatty Acids

- 50% of fatty acids in the human body are unsaturated

- provides less energy because they are less highly reduced  fewer reducing equivalents produced

- degraded by the -oxidation pathway assisted by other 2 enzymes

- double bonds in naturally occurring fatty acids are in the cis configuration

- -oxidation pathway deal only with trans configuration as at the enoyl CoA hydratase step

1. Oxidation of Monounsaturated Fatty Acids

a. 3

-cis-2

-Trans Enoyl CoA Isomerase

- shifts the double bond to the preferred 2

-trans configuration [when a cis-3, 4 (ȕ,Ȗ)

double bond is encountered after several rounds of ȕ oxidation, the enzyme

converts it to a trans-2, 3 double bond]

- ex: -oxidation of

- palmitoleate (16 : 1 : 9

)

- oleic acid (18 : 1 : 9

)

2. Oxidation of Polyunsaturated Fatty Acids

- polyunsaturated fatty acids require another enzyme for complete oxidation

a. 2, 4-Dienoyl-CoA Reductase

- removes a double bond at the expense of an NADPH

i. In Mammals

- the resulting product has a trans-3, 4 double bond, which must be converted

to a trans-2, 3-double bond by 2, 3-enoyl CoA reductase before ȕ

oxidation can continue

3. -Oxidation in the Peroxisome

- very long-chain fatty acids (> 20 carbons)  preliminary -oxidation in the peroxisomes 

chain shortening  mitochondrion  further oxidation

a. Differs from Oxidation in Mitochondria

i. No carnitine is required

ii. Acyl-CoA Oxidase Reaction

- FAD-containing

- initial dehydrogenation

- FADH2 produced  direct transfer of electrons to molecular O2 H2O2 (and

a trans-2,3-enoyl-CoA)  catalase  H2O

b. Genetic Defects

 very long-chain fatty acid accumulation in

- blood

- tissues

i. Zellweger (Cerebrohepatorenal) Syndrome

- defect in peroxisomal biogenesis in all tissues

ii. X-Linked Adrenoleukodystrophy

- defect in peroxisomal activation of very long-chain fatty acids