2 FA Oxidation and Ketone

8/8/2019 2 FA Oxidation and Ketone

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FA oxidation and ketone bodies


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Roles of Lipids

principal form of stored energy

major constituents of cell membranes

vitamins

messengers intra and extracellular


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Glycerol-P Glycerol

Triacylglycerol

Fatty acyl CoA Fatty acid

Malonyl CoA

Acetyl CoA

Glucose

Pyruvate

TCA cycle

Starved state

gluconeogenesis

Ketone bodies


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Oxidation of fatty acids

Central energy-yieldingpathway in animals.

Generates acetyl-CoA

Generates electrons

which pass through the

respiratory chain driving

ATP synthesis.

CH3-C-CoA=O


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Sources of fatty acid fuels

1. de novo synthesis

Fatty acids may be synthesized and areconverted to triacylglycerols

Made in liver and exported to muscle or fat cells

In muscle, used as fuel; In fat cells, stored asdroplets


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Sources of fatty acid fuels

2. diet

Diet on average 40% or more of the daily

energy requirement of humans is supplied inthe diet in the form of triacylglycerols

Stored in cells as lipid droplets


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Extremely suitable as a energy storagemolecule:

1. Contain highly reduced hydrocarbonchains with an energy more than twicethat of the same weight of carbohydrate orprotein

2. Extremely insoluble in water: less heavy

than hydrated molecules, such ascarbohydrates.

3. Chemically inert and so can be stored inlarge quantity in cells.

Triacylglycerols


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Absorption of dietary fat


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Dietary fats are absorbed in the small

intestine

Ingested triacylglycerolsare converted from

insoluble fat particles tofinely dispersed micelles

Bile salts are amphipathiccholesterol derivatives

made in the liver andstored and released by thegall bladder perform thisfunction


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Micelle formation allows lipid

molecules to be accessible towater-soluble lipases (secreted by

pancreas)

Ingested

triacylglycerols

Mono- and di-

acylglycerols,free fatty acids,

glycerol

Lipases


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The products of lipase action diffuse into theepithelial cells lining the intestinal surface

Then they are reconverted to triacylglycerols and packaged with dietary cholesterol and specific lipoprotein aggregates calledchylomicrons.


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Molecular structure of a chylomicron

The surface is a layer ofphospholipids, withhead groups facing theaqueous phase.

Triacylglycerols are inthe interior make upmore than 80% of themass.

Apolipoproteins lipidbinding proteins


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Chylomicrons move through thelymphatic system, enter the bloodstream and are carried to muscleand adipose tissue.

In the capillaries of these tissues,

the extracellular enzymelipoprotein lipase hydrolyzetriacylglycerols to fatty acids andglycerol, which are taken up bycells in the target tissues.


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In muscle, the fatty

acids are oxidized

for energy. In adipose tissue,

they are esterified for

storage.


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Fate of dietary fat

Dietary fat may be utilized immediately or

stored in adipocytes

In fed state, fatty acid synthesis occurs and

the products are also stored in adipocytes

In starved state, these stored forms of fat are

mobilized


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The Mobilization of

triacylglycerols

Triacylglycerols stored in the adipose tissues aremobilized and transported to tissues where fatty

acids can be oxidized for energy production.

Triacylglycerols

Fatty acids

+Glycerol


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What signals the mobilization of

stored triacylglycerols?

Hormones signal the need for metabolic

energy.

Hormones epinephrine and glucagon are

secreted in response to low blood glucoselevels


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These hormones activate the enzyme adenylylcyclase in the adipocyte plasma membrane.

Results in increased amount of the secondary

messenger cAMP.

Capillary

Glucagon and epinephrine act by a

signal transduction mechanism


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cAMP activateshormone-sensitive

triacylglycerol lipase


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The fatty acids released

passed into the bloodwhere they bind to blood

protein serum albumin.

the insoluble fatty acids

are carried to tissues,

dissociated from albumin

and transported into cellsto serve as fuels.


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The products of fat mobilization

Triacylglycerols are broken down toglycerol and fatty acid

95% of the biologically available energy oftriacylglycerols resides in their long-chain

fatty acids 5% is contributed by the glycerol moiety


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Entry of glycerol into the glycolytic

pathway

The glycerol released isphosphorylated by glycerolkinase to glycerol 3-phosphate

oxidized to dihydroxyacetonephosphate

then converted toglyceraldehyde 3-phosphate,which is oxidized viaglycolysis


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Fatty acids are activated and

transported into mitochondria

FA oxidation enzymes

are located in

mitochondria Free FA cannot pass

directly through the

mitochondrialmembranes


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1. Activation of fatty acid by CoA

Acyl CoA synthase

Fatty acid + CoA + ATP fatty acyl-CoA +AMP + PPi


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2. Esterification to carnitine

Then transferred to carnitine

Fatty acyl-CoA + carnitine Fatty acyl- carnitine + CoASH


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The fatty acyl group is transferred to

carnitine by carnitine transferase I on the

outer face of the inner membrane The fatty acyl-carnitine ester then enters the

matrix through the acyl-carnitine/carnitine

transporter


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3. Esterification to CoA

The fatty acyl group is enzymatically transferred fromcarnitine to coenzyme A by carnitine acyltransferase II

Regenerates fatty acyl-CoA and free carnitine


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Mitochondrial oxidation of fatty

acids takes place in three stages

These stages result in massive ATP

production


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Stage 1. -oxidation: Oxidative removal

of successive two-

carbon units to formacetyl-CoA startingfrom carboxyl end ofthe fatty acyl chain

Also generatesNADH and FADH2


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Stage 2

Acetyl groups of

acetyl-CoA are

oxidized to CO2 in

the TCA cycle

NADH is also

generated from theTCA cycle


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Stage 3

The NADH and

FADH2produced

mitochondrialrespiratory chain


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The -oxidation of saturated

fatty acids has four basic steps

The -oxidation sequence is a

mechanism of breaking stable bondbetween methylene (-CH

2-) groups.

The first three steps create a bond that is

more easily broken.


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Step 1

Dehydrogenation of fatty acyl-CoA produces a doublebond between C-2 and C-3

This double bond is in the trans configuration

Yields FADH2


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Step 2

Hydration - water is added to the double bondto form -hydroxyacyl-CoA.


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Step 3

-hydroxyacyl-CoA is dehydrogenated(oxidized) to form ketoacyl-CoA.(yields NADH + H+)


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Step 4

ketoacyl-CoA reacts with freecoenzyme A to split off the carboxyl-

terminal two-carbon fragment of the

original FA as acetyl CoA.


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The products for one pass through -

oxidation

Palmitoyl-CoA + CoA + FAD + NAD+ + H20

myristoyl-CoA + acetyl-CoA + FADH2

+ NADH + H +

Myristate is a C14 fatty acid


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The myristoyl-CoA can go through another

set of four -oxidation reactions to yield asecond molecule of acetyl-CoA and lauryl-

CoA (C-12).

The four steps are repeated to

yield acetyl-CoA and ATP


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The complete -oxidation of

palmitate

Palmitoyl-CoA + 7CoA + 7FAD + NAD+ + 7H20

8acetyl-CoA + 7FAD2 + 7NADH + 7H+


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The fate of FADH2 and NADH

Each molecule of FADH2---2 molecules of

ATP .

Each molecule of NADHdelivers a pair of

electrons---3 molecules of ATP.


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The fate of acetyl-CoA

Each molecule of acetyl-CoA can be

oxidized to CO2and H

2O by the TCA cycle

to yield 12 molecules of ATP.


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The overall ATP yield

Palmitoyl-CoA + 23O2 + 108Pi + 131ADP

CoA + 131ATP + 16CO2 + 23H2O

Palmitoyl-CoA + 7CoA + 7FAD + NAD+ + 7H20

8acetyl-CoA + 7FAD2 + 7NADH + 7H+

becomes


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Oxidation of special fatty acids

Mono and polyunsaturated fatty acids

Odd chain fatty acids


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Oxidation of unsaturated fatty acids

requires additional reactions

Most fatty acids in triacylglycerols and phospholipids

are unsaturated. Being in the cis position these double bonds cannot

be acted upon by the -oxidation enzymes

By the action of two auxiliary enzymes such

substrates may be broken down

Oxidation of a monounsaturated


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Oxidation of a monounsaturated

fatty acid

Example: oleic acid

Requires enoyl-CoAisomerase to reposition thedouble bond.

Converting the cis isomer

to a trans isomer, a normalintermediate in -oxidation


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Oxidation of a polyunsaturated

fatty acid

Example, linoleic

acid

The first step is the

same as that

described above


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Complete oxidation requiresa second auxiliary enzyme, a

NADPH dependent reductasethat removes an unsaturated

bond

The isomerase is required toconvert the double bondsfrom a cis to a transconfiguration


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Complete oxidation of odd-

number fatty acids

Odd numbered lipids are present in

plants and marine organisms

Oxidized as even chain but end up

withproprionyl-CoA


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Proprionate metabolism

Proprionyl-CoA is

carboxylated toform D-methyl-

malonyl-CoA


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D-methyl-malonyl-CoA is epimerized to

its L-stereoisomer

L-methyl-malonyl-

CoA is converted to

succinyl-CoA, which

can enter TCA cycle.


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Fatty acid oxidation is tightly

regulated

Fatty acid oxidation is regulated so it

occur only when the need for energy

requires it


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Two pathways fatty acyl-CoA in liver

The pathway taken

depends on the rate

of transfer of long-

chain fatty acyl-CoAinto the

mitochondria

Cytosol Mitochondria

Triacylglycerols

and

phospholipids

Fatty

acid

oxidation

Fatty acid

Carnitine

transporter

Fatty acid

synthesis


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Malonyl-CoA initiates fatty acid synthesis

Malonyl-CoA is the first

intermediate in the cytosolicbiosynthesis of long-chain

fatty acids from acetyl-CoA

Excess glucose that cannotbe oxidized or stored as

glycogen is converted in the

cytosol into FA for storage

as triacylglycerols

Glycerol-P

Triacylglycerol

Fatty acyl CoA

Malonyl CoA

Acetyl CoA

Glucose

Pyruvate

TCA cycle

M l l C A i hibit iti t f I


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Malonyl-CoA inhibits carnitine transferase I

Cytosol Mitochondria

Triacylglycerols

and

phospholipids

Fatty

acid

oxidation

Fatty acid

Carnitine

transporter

Fatty acidsynthesis

Malonyl-CoA

F d


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Fed state

Glycerol-P Glycerol

Triacylglycerol


Malonyl CoA

Acetyl CoA

Glucose

Pyruvate

TCA cycleInsulin, citrate

Carnitine

transporter

S d


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Starved state

Glucagon/

epinephrine

Malonyl CoA

Glycerol-P Glycerol

Triacylglycerol


Acetyl CoA

Glucose

Pyruvate

TCA cycle

g

lucon

eogene

si

s

Ketone bodies

Carnitine

transporter


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Ketone bodies

The acetyl-CoA formed in the

liver during -oxidation can

have two fates:

1. Enter the TCA cycle

2. Converted to ketone bodies

acetone, acetoacetone and

-hydroxybutyrate forexport to other tissues

Ketone bodies formed in the liver


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Ketone bodies formed in the liver

1. Condensation of twomolecules of acetyl-CoA,

2. The resulting

acetoacetyl-CoAcondenses with acetyl-CoA to form -hydroxy- -methylglutaryl-CoA(HMG-CoA)


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3. Cleavage of HMG-CoA yields acetyl-CoAand acetoacetate.

4. Reduction of acetoacetate yields D- -hydroxybutyrate (do not confuse with L-

-hydroxybutyrate of the b-oxidationpathway).

5. Acetoacetate is easily decarboxylated (maybe spontaneously or enzymatically) toacetone and CO2.


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Ketone bodies are exported to

other organs Acetone, produced in smaller quantities

than the other ketone bodies, is exhaled

Acetoacetate and -hydroxybutyrate aretransported in the blood to tissues other than

the liver

K b di f l


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Ketone bodies as fuels

-hydroxybutyrate maybe converted to acetyl-

CoA.

The acetyl-CoA is

Oxidized in the TCA

cycle to provide much of

the energy required bytissues


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Ketone bodies are used under

starvation conditions

The brain, which preferentially uses glucose as fuel, can

adapt to the use ofacetoacetate or -hydroxybutyrateunder starvation conditions, when glucose is

unavailable

I i l i hi d i i


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Intertissue relationships during starvation

S f li id t b li


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Summary of lipid metabolism

Sources of triacylglycerols diet and stored inadipocytes

Route taken by dietary triacylglycerols to muscle or

fat cells

Mobilization of triacylglycerols is initiated by

hormones epinephrine and glucagon

The products of mobilization are free fatty acid and

glycerol, both are used for energy production

S mmar of lipid metabolism


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Summary of lipid metabolism

Carnitine transporter mediates entry of fatty acidsinto mitochondria

-oxidation of fatty acids has four basic steps essentially fatty acid synthesis in reverse

-oxidation generates acetyl-CoA, NADH andFADH

2ATP

Ketone bodies serve as fuel molecules under

starvation conditions

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2 FA Oxidation and Ketone