Week 11-Lipi

LIPID METABOLISMWeek 11

FATE OF GLYCEROL

� From action of lipoprotein lipase

and hormone-sensitive lipase

�Glycerol is converted to the

Glycolysis/ gluconeogenesis Glycolysis/ gluconeogenesis

intermediate-- dihydroxyacetone

phosphate

Glycerol is converted to dihydroxyacetone phosphate,

by reactions catalyzed by:

1 Glycerol Kinase

2 Glycerol Phosphate Dehydrogenase.

Martius & Knoop (1902) fed dogs even- and odd-carbon fatty acids labelled with a benzene ring in place of the terminal methyl group.

ββββ-Oxidation of fatty acids

Fatty acid oxidation occurs by removal of 2-C

units at a time with oxidation at the β-carbon

of the fatty acid

Fatty Acid β-Oxidation

�All cells except for RBCs and brain can use

fatty acids for energy.

� β-Oxidation occurs in Mitochondria

�Three Steps� A.activation

B.transport into mitochondria� B.transport into mitochondria

� C.oxidation

A.Fatty acid

activation

Acyl-CoA

Synthetase of

ER & outer

mitochondrial

membranes,

catalyzes fatty catalyzes fatty

acid activation

FATTY ACID ACTIVATION

� fatty acid + ATP � acyladenylate + PPiPPi � 2 Pi

� acyladenylate + HS-CoA � acyl-CoA + AMP

Overall: Overall: fatty acid + ATP + HS-CoA � acyl-CoA + AMP + 2 Pi

Exergonic hydrolysis of PPi (P~P), catalyzed by Pyrophosphatase, makes the coupled reaction spontaneous.

Fatty acids are linked to CoA in the cytosol. Enzymes of the b-Oxidation Pathway are in the mitochondrial matrix.

Transfer of the fatty acid across the inner membrane involves carnitine

� The transfers long-chain fatty acyl CoA from the

cytosol into the mitochondria C

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1.Acyl-CoA Dehydrogenase

catalyzes oxidation of the

fatty acyl-CoA, to form a

C2 to C3 double bond.

• FAD is the e−

acceptor for Acyl-CoA Dehydrogenase.

• FADH is reoxidized • FADH2 is reoxidized by transfer of 2 e− to an electron transfer flavoprotein (ETF), which passes 2 e− to coenzyme Q of the respiratory chain.

2.Enoyl-CoA Hydratase

catalyzes hydration of the

trans double bond, yielding L-

hydroxyacyl-Coenzyme A.

3.Hydroxyacyl-CoA

Dehydrogenase

catalyzes oxidation of

the hydroxyl in the βposition (C3) to a ketone.position (C3) to a ketone.

NAD+ is the electron

acceptor.

Process similar to

succinate oxidation in

the Citric Acid Cycle

(dehydrogenation,

hydration,

dehydrogenation)

4. β4. β4. β4. β-Ketothiolasecatalyzes thiolytic

cleavage yielding fatty

acyl-CoA (2 C shorter)

and releasing Acetyl-CoA.

� The β oxidation

pathway is cyclic. The

product, 2 C shorter, is

the input to another

round of the pathway

� Products: an acetyl-

CoA and a fatty acid

two carbons shorter, two carbons shorter,

FADH2, NADH

� If the fatty acid

contains an even

number of C atoms,

Final cleavage product

is acetyl CoA.

� If the number is

odd,final cleavage

product is propionyl

CoA

one round of the β-oxidation pathway:

fatty acyl-CoA + FAD + NAD+ + HS-CoA

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fatty acyl-CoA (2 C less) + FADH + fatty acyl-CoA (2 C less) + FADH2 +

NADH + H+ + acetyl-CoA

Acetyl-CoA can enter Krebs cycle, yielding

additional NADH, FADH2, and GTP

Complete β- oxidation of 1 mol palmitic acid yields 106 mol ATP

How many cycles occurred in the oxidation of

palmitic acid? (#C/2) – 1 (16/2) –1 = 7

14 ATP per cycle 7 * 14 = 98

the last 2 carbons + 10

Subtract 2 for the initial activation –2Subtract 2 for the initial activation –2

Net ATP formed 106

Fatty acid oxidation is a major source of cell ATP

It also produces large amounts of metabolic

water( 130 H2O per palmitoyl-CoA).

“The ship of the desert” sails on its own metabolic

water.

β-Oxidation of Unsaturated and Odd Chain Fatty Acids

�Variations on β-Oxidation - extra enzymes required

�Unsaturated FA (C18:1, C18:2, C18:3 and others)others)� require two extra enzymes to get double bonds in the right place for enzymes of β-oxidation to work（cis-trans isomerase, reductase)

� skip one or more oxidation steps -Slightly less Energy derived as these molecules are slightly more oxidized to begin with

ββββ−−−− Oxidation of Odd-Chain Fatty Acids

• Odd-chain fatty acids occur in plants and

microorganisms

• Final cleavage product is propionyl CoA

rather than acetyl CoA

• Three enzymes convert propionyl CoA to

succinyl CoA (citric acid cycle intermediate)

Conversion of propionyl CoA to

succinyl CoA

Succinyl CoA --> oxalacetate--> glucose

(gluconeogenesis)

During CHO starvation, oxaloacetate in liver is depleted due to

gluconeogenesis. This impedes acetyl-CoA entry to Krebs cycle.

Acetyl-CoA in liver mitochondria is converted then to ketone

bodies.

KETONE BODIES FORMATION

METABOLISM OF KETONE BODIES

� Acetone, Acetoacetate, β- hydroxybutyrate are

called ketone bodies.

� Produced in liver,Diffuse out of the liver into

the blood .Acetone is exhaled by the lungs,

Acetoacetate and beta hydroxybutyrate are Acetoacetate and beta hydroxybutyrate are

taken up by extrahepatic tissues and

catabolized for energy.

�Fuel molecules

�derived from excess acetyl CoA

�Water solubleWater soluble

�readily and quickly transported to

other tissues for energy

�Major source of energy for brain in starvation

(skeletal muscle and kidney, also)

KETONE BODIES OXIDIZED IN

MITOCHONDRIA OF EXTRAHEPATIC TISSUES

FATTY ACID BIOSYNTHESIS

� Not exactly the reverse of degradation

by a different set of enzymes , in a different part of the cell

� Primarily in the cytoplasm of the following tissues: liver, kidney, adipose, central nervous system and lactating mammary gland

� Rule: Fatty acid biosynthesis is a stepwise assembly of acetyl-CoA units (mostly as malonyl-CoA) ending with palmitate (C16 saturated)

� 3 Phase: Activation, Elongation, Termination

�Liver is the major organ for fatty acid synthesis

FATTY ACID BIOSYNTHESIS

� Cytosol

� Requires NADPH

� Acyl carrier protein

� Mitochondria

� NADH, FADH2

� CoA

Beta OxidationSynthesis

� D-isomer

� CO2 activation

� Keto → saturated

� L-isomer

� No CO2

� Saturated → keto

CH3C~SCoA

O

ACTIVATION

-OOC-CH C~SCoA

HCO3-

NN

O

SCH2CH2CH2CH2CO

HH

LYS

NHCH2CH2CH2CH2 ENZYME

Biotin

CO

Biocytin

Cofactor

ATP

ADP + Pi

-OOC-CH2C~SCoA

O

NN

O

SCH2CH2CH2CH2CO

HC

O

O

Carboxybiocytin

active carbon

Acetyl-CoA carboxylase

CO2

Acyl carrier protein

10 kDa

Cysteamine

Phosphopantetheine

Acyl Carrier ProteinAcyl Carrier Protein

HS-CH2-CH2-N-C-CH2-CH2-N-C-C-C-CH2-O-P-O-CH2-Ser-

O O O

OH H

H

HO CH3

H

ACP

HS-CH2-CH2-N-C-CH2-CH2-N-C-C-C-CH2-O-P-O-P-O-CH2

O O O

OH H

H

HO CH3

H

O

OO Adenine

O-P-O

O

OH

OH

H

Coenzyme A

Overall Reaction

CH3C~SCoA

O

CH3C- CH2C~S- ACP

HS-CoACO2

Acyl Carrier

Protein

Malonyl-CoA + ACP

-OOC-CH2C~S-

O

ACP + HS-CoA

Initiation

CH3C-

O

CH2C~S-

O

ACP

NOTE:

Malonyl-CoA carbons become new COOH end

Nascent chain remains tethered to ACP

CO2, HS-CoA are released at each condensation

CH3C-

O

CH2C~S-

O

ACP

NADPH

CH3C- CH2C~S-

O

ACP

HO

H

-H O

ββββ-Carbon Elongation

D isomer

Reduction

Dehydration

ββββ-Ketoacyl-ACP reductase

CH3CH2CH2C~S-

O

ACP

CH3C- = C- C~S-

O

ACP

H

H

-H2O

NADPH Reduction

ββββ -Hydroxyacyl-ACP dehydrase

Enoyl-ACP reductase

-KS

-S-ACP

TERMINATION Ketoacyl ACP

Synthase

Transfer to KSTransfer to Malonyl-CoA

-CH2CH2CH2C~S- ACP

CO2

Free to bind

Malonyl-CoASplit out CO2

O

When C16 stage is reached, instead of transferring to KS,

the transfer is to H2O and the fatty acid is released

Acetyl-CoA is delivered to cytosol from the

mitochondria as CITRATE

METABOLISM OF CHOLESTEROLS

SOURCES OF CHOLESTEROL

Diet De novo synthesisCholesterol synthesizedin extrahepatic tissues

Liver cholesterolLiver cholesterolpool

Free cholesterolIn bile

Conversion to bile salts/acids

Secretion of HDLand VLDL

CHOLESTEROL SYNTHESIS

�80 % in liver, ~10% intestine, ~5% skin

Occurs in cytosol

�Requires 18Acetyl-CoA、16NADPH、�Requires 18Acetyl-CoA、16NADPH、

36ATP

�Similar to ketogenic pathway

Highly regulated

Cholesterol

Synthesis

(C2)

(C5)

(C30)

(C27)