Download Lec6 Fatty acid oxid..

Sources pof energy in fasting state
In adipose tissue:
In fasting state, the stored TAG will be the major source of energy.
-Stored TAG in adipose tissue is hydrolysed by hormone sensitive
lipase (HSL) into free fatty acids and glycerol.
-Glycerol will move to liver
-Adipose tissue oxidize some free fatty acids to get energy. The
remaining fatty acids will move to blood, carried by albumin and enter
peripheral tissues (except brain) to be oxidized and produce energy.
-N.B. Blood brain barrier is impermeable to fatty acids, so brain can’t
use fatty acids as source of energy.
Liver in fasting state: liver can use the following sources of energy:
1- Free fatty acids (from adipose tissue) is oxidized to produce
energy
2- Glycerol (from adipose tissue), amino acids (from degradation of
muscle protein), and lactate (from muscles), all are used as
substrates of gluconeogenesis in liver to produce glucose. This
glucose could be released into blood and up-taken by brain to be
used as source of energy.
3- Liver can degrade the stored glycogen (by glycogenolysis) to
produce glucose that is used as source of energy.
**** In fasting state, liver synthesize ketone bodies, which is then
released and up-taken by all peripheral tissues including brain to
be oxidized to produce energy. Liver can’t use ketone bodies as
source of energy.
Muscles in fasting state:
Muscles can use either fatty acids (from adipose tissues) or
ketone bodies (from liver)
Metabolism of lipids in fasting state
I-Fatty acid oxidation
II- Ketone bodies synthesis and degradation
I-Fatty acid oxidation
1- hydrolysis of TAG to release free fatty acids (lipolysis) and
mobilization of fatty acoids to peripheral tissues :
Fatty acids are the part to be oxidized so the first step is release of
fatty acids from TAG. This process is initiated by hormone- sensitive
lipase (HSL) which removes fatty acids from TAG. HSL is activated
by anti-insulin hormones (adrenaline, glucagon, GH, cortison) and
inhibited by insulin.
HSL has two forms, phosphorylated form (active form) and
dephosphirylated form (inactive).
In
fasting
hormones
state,
anti-insulin
(glucagon,
adrenaline,
thyroxine) are released, bind to their
cell membrane receptors, activate
adenylate cyclase which convert ATP
into cAMP. cAMP activate protein
kinase A which phosphorylate HSL
and activate it. However in fed state,
insulin
is
released and
activate
protein phosphatase enzyme which
dephosphorylate HSL and inactivate
it.
Activation of adipose tissue HSL
2- Activation of free fatty acids:
Free fatty acids is mobilized from adipose tissue to all tissues (except
brain) to be oxidized. Before oxidation, fatty acids should be activated
into acyl CoA.
Site of activation:
Long chain fatty acids are activated in cytoplasm of cells.
In the presence of ATP and CoA, the enzyme thiokinase (or called
Acyl CoA synthetase) catalyses the activation of long chain fatty acids
into acyl CoA.
RCOOH+2ATP + CoASH
→ RCO~SCoA
acyl CoA
3- Beta oxidation of fatty acids:
It is the major pathway of oxidation (catabolism or breakdown) of
saturated fatty acids in which two carbons are removed from activated
fatty acid, producing acetyl CoA, NADH and FADH2
Site: in the mitochondria of all tissues particularly in the liver. So
there is no fatty acid oxidation in RBCs which have no mitochondria.
Note that: fatty acids with less than 12 carbons (short and medium
chain fatty acids) are activated in mitochondria then oxidized.
Since activation of long chain fatty acids (more than 12 C) occurs in
cytoplasm (their thiokinase are cytoplasmic enzymes) and the
oxidation occur in mitochondria, so long chain acyl CoA should be
transported.
Transport of long chain fatty acyl CoA into mitochondria:
Carnitine shuttle
Long chain fatty acyl CoA cannot penetrate mitochondrial membrane (as
mitochondrial membrane is impermeable to CoA). They need a carrier to transport
them into mitochondria. This carrier is Carnitine which transports active fatty acid
by the help of 3 enzymes:
Carnitine acyltransferase I (CAT-1)
Carnitine - acylcarnitine translocase
Carnitine acyltransferase II (CAT-2)
Steps of transport: 1) acyl group is transferred from acylCoA into carnitine by CAT-1
to give acyl carnitine and free CoA which remains in cytoplasm.
2) Acyl carnitine is transported into mitochondria by the help of Carnitine acylcarnitine translocase.
3) CAT-2 catalyses the transfer of acyl group from acyl carnitine to CoA to give acyl
CoA and free carnitine which go back to cytoplasm by translocase enzyme.
Inhibitor of carnitine shuttle: malonyl CoA which inhibits carnitine acyltransferase I
Sources of carnitine: diet (meat products), synthesized from amino acids lysine and
methionine in liver and kidney.
Mitochondria
Steps of β- oxidation:
β-oxidation consists four sequential steps. These steps are
repeated until all the carbons of an even-chain fatty acyl-CoA
are converted to acetyl-CoA.
Reactions:
1. Oxidation: FAD accepts 2 hydrogens from a fatty acyl-CoA in
the first step. A double bond is produced between the α- and βcarbons, and an enoyl-CoA is formed.
-Enzyme: acyl-CoA dehydrogenase
2. Hydration: H2O is added across the double bond, and a βhydroxyacyl-CoA is formed.
-Enzyme: enoyl-CoA hydratase
3. Oxidation: β-Hydroxyacyl-CoA is oxidized by NAD+ to a βketoacyl CoA.
‒Enzyme: L-3-hydroxyacyl-CoA dehydrogenase
4. Thiolysis: The bond between the (α- and β-carbons of the βketoacyl CoA is cleaved by a thiolase that requires coenzyme A.
Acetyl CoA is produced from the two carbons at the carboxyl end of
the original fatty acyl CoA, and the remaining carbons form a fatty
acyl CoA that is two carbons shorter than the original.
-Enzyme: β-ketothiolase
5. The shortened fatty acyl CoA repeats these four steps.
Repetitions continue until all the carbons of the original
fatty acyl CoA are converted to acetyl CoA.
a. The 16-carbon palmitoyl CoA undergoes seven repetitions.
b. In the last cycle, a four-carbon fatty acyl CoA (butyryl CoA) is
cleaved to two acetyl CoAs.
Energy yield from β- oxidation:
Each β- oxidation cycle yields :
-acyl CoA with 2 carbon atoms less
-one NADH+H+ (give 3 ATP via respiratory chain)
-one FADH2
(which give 2 ATP via respiratory chain)
- one acetyl CoA (oxidized through Kreb's cycle to yield 12 ATP).
Remember that the last cycle produce two acetyl CoA.
Question: Oxidation of one molecule of palmitic acid yields 129 ATP (
How?).
Palmitic acid need 7 cycles to be completely converted into acetyl CoA.
7x 3ATP (from NADH+H+) = 21
7x 2ATP (from FADH2 ) = 14
8 x12 (from a cetyl CoA) = 96
Total 131, since 2ATP are used for activation, so the net ATP yield is
131-2 = 129 ATP
Energy yield from one molecule of palmitic acid
(16C): palmitic undergo 7 cycles of oxidation
With Production of 8 acetyl CoA
8 acetyl CoA x12 ATP = 96 ATP
7 NAH x3 = 21
7FADH2 x 2= 14
Total =131
2ATP utilized for FA activation
, so net is 129 ATP
C16
→ C14 + Acetyl CoA
Regulation of β- oxidation:
Oxidation of fatty acids is controlled by the rate of release of free fatty
acids from adipose tissues (lipolysis), which is inhibited by food
intake and insulin, and stimulated by starvation, adrenaline, glucagon,
thyroxin, glucocorticoids and growth hormone (the factors that
regulate the action of HSL).
Comparison between Fatty acid oxidation, Fatty acid synthesis
Greatest flux of pathway (diet
regulation)
fatty acid β-oxidation
Increase in fasting state
Decrease in fed state
fatty acid synthesis
Increase in fed state
Decrease in fasting state
Hormonal state that favor pathway
Anti-insulin hormones
Insulin
Major tissue site (organ)
All tissues especially liver
Liver (mainly), adipose tissue
Site inside the cells
Mitochondria
cytoplasm
Precursor
Acyl CoA
Acetyl CoA
Regulatory enzyme
CAT-1
Acetyl CoA carboxylase (ACC)
Two-carbon donor/product
Acetyl CoA (two carbons
product)
Allosteric Activator
Malonyl CoA (two carbons
donor)
Citrate, ATP
Allosteric Inhibitor
Palmitate
Carrier of acyl/acetyl groups between
mitochondria and cytosol
H-carrier (Coenzymes)
End product
Steps of the pathway
Carnitine (caryy long chain
acyl CoA from cytoplasm to
mitochondria
FAD, NAD
Acetyl CoA
Oxidation,
Hydration
Oxidation
Thiolysis (cleavage)
Citrate (caryy acetyl CoA from
mitochondria into cytoplasm)
NADPH
Palmitate
Condensation
Reduction
Dehydration
Reduction