Fatty acid metabolism is a complex multi-step process that occurs in different cellular locations. Triglycerides are first hydrolyzed by lipases to release fatty acids. The fatty acids are then bound to coenzyme A and transported into the mitochondrial matrix using carnitine. Inside the mitochondria, the fatty acids undergo beta-oxidation where they are broken down in a cycle of reactions that cleave two carbon units at a time, producing acetyl-CoA. If fatty acid breakdown outpaces the citric acid cycle, ketone bodies like acetoacetate can be produced from acetyl-CoA. Acetoacetate is an important energy source for tissues like the heart and kidney.

1. FATTY ACID METABOLISM
Because the breakdown of fats is a complicated process, this part is divided in a number of
different parts. Below the different parts are indicated.
Triglycerides are hydrolysed by cyclical AMP-regulated lipases
Fatty acids are bound to coenzyme A before they are oxidised
Carnitine transports long-chain activated fatty acids the mitochondrial matrix in
Fatty acids are broken by splitting-off of always two carbon atoms
oxidation of unsaturated fatty acids yet an isomerase and a reductase are necessary
If the fat breakdown dominates acetyl CoA keton bodies are formed
Acetylacetate is an important fuel in some tissues

2. Triglycerides are hydrolysed by cyclical AMP-regulated lipases
The first event in the use of fat as energy source is the hydrolysis (= break down by water) of
triglycerides by the enzymes that are called lipases. This process is also called lipolyse.
Lipases convert triglycerides into glycerol and fatty acids. The hydrolyse by lipases of
triglycerol in glycerol and fatty acids.
The activity of lipase in fat cells is regulated by hormones like epinephrine and glucagon.
These hormones activate the enzyme adenylate cyclase. This enzyme converts ATP in cyclical
AMP. This cyclical AMP activates the enzyme protein kinase A (PKA). The
enzyme PKAphosphorylyse the lipase enzyme and gets activated because of this
phosphorylation. Like in thebreak down of glycogen cyclical AMP is here "the second
messenger". The hormone insulin inhibits the hydrolysis of triglycerids.
Glycerol, that by the break down of triglyceride arise, is phosphorylated by glycerolkinase and is
then oxidised by glycerol phosphate dehydrogenase to dihydroxyacetone phosphate. This is an
intermediary of the glycolysis and will be broken down further in this glycolysis
.

3. Fatty acids are bound to coenzyme A before they are oxidised
A fatty acid reacts with ATP and coenzyme A to acyl CoA, AMP and pyrophosphate.
A fatty acid reacts with ATP and coenzyme A to form acyl CoA, AMP and
pyrophosphate. This reaction is catalysed by acyl CoA synthetase.
The enzyme acyl CoA synthetase has been bound at the outer membrane of the
mitochondria. The balance of the total reaction lies in the direction of acyl CoA
because of the fast hydrolysis of pyrophosphate
Carnitine transports long-chain activated fatty acids the mitochondrial
matrix in
Fatty acids are activated at the outer membrane of the mitochondria, but are oxidised inside the
mitochondria. Because long-chains fatty acids are not easily going through the outer membrane
of the mitochondria a special transport mechanism is necessary to transport these fatty acids into
the mitochondria.
Activated long-chain fatty acids are combined with carnitine. The acyl group is transferred by
the sulphur atom of coenzyme A on the hydroxyl group of carnitine under formation of
acylcarnitine. This reaction is catalysed by carnitine acyltransferase I, that is bound at the outer
membrane of the mitochondria. Activated long-chain fatty acids are combined with carnitine.

4. Acylcarnitine is then moved through the outer membrane by a translocase enzyme (membrane
protein). The acyl group is transferred back to coenzyme A at the matrix side (in the
mitochondria) by the membrane. This reaction is catalysed by carnitine acyltransferase
II. Ultimately carnitine is transported back into the cytoplasm by the enzyme translocase in
exchange for a coming in of acylcarnitine

5. Fatty acids are broken by splitting-off of always two carbon atoms
Fatty acids are broken down by repetitions of separations of parts of two carbon atoms. The reactions
that repeat are oxidation, hydration, oxidation (dehydrogenation) and thiolyse.
The three reactions from acyl CoA to 3-ketoacyl CoA are comparable to the reactions of Succinate to
Oxalacetate in the citric acid cycle.
The break down of fatty acids with a chain of an odd number of carbon atoms leads to the formation
of propionyl CoA in the last thiolyse reaction step. In the last reaction step of the fatty acid break
down 3-ketopentanoyl CoA (5 carbon atoms) is split up in propionyl CoA (3 carbon atoms) and
acetyl CoA (2 carbon atoms). Propionyl CoA is converted in methylmalonyl-CoA by the enzyme
propionyl-CoA carboxylase. This enzyme needs biotin as an assistant-factor (and bicarbonate and
ATP) to catalyse the reaction. Methylmalonyl-CoA is converted in succinyl-CoA by the enzyme

6. methylmalonyl-CoA mutase. This enzyme needs coenzyme B12 (a product ofvitamin B12) to
catalyse this reaction. Succinyl CoA can be further broken down in the citric acid cycle.
oxidation of unsaturated fatty acids yet an isomerase and a reductase
are necessary
The first reaction in the cycle of the break down ( -Oxidation) of a fatty acid under formation of
an enoyl CoA with a trans double bond between carbon number 2 is the oxidation of an acyl
CoA and 3.By the break down of an unsaturated fatty acid, the presence of a double bond
between C-3 and C-4 prevents the formation of a trans double bond between C-2 and C-3. A
trans double bond is necessary for the formation of L-3-hydroxyacyl CoA, because the enzyme
dehydrogenase is specific for this. An isomerase changes a double bond between C-3 and C-4
into a trans double bond between C-2 and C-3. By the break down of a plural unsaturated fatty
acid, a cis- 4 double bond forms another problem. Through dehydrogenation of this part, a 2,4-
dienoyl intermediate product is raised, that is no substratum for the following enzyme in the -
Oxidation. This problem is solved by the enzyme 2,4-dienoyl CoA reductase that with NADPH
3
as coenzyme reduces the intermediate product to a cis- -Enoyl CoA. The earlier called
3
isomerase converts cis- -Enoyl CoA in the trans form, see the figure below.

7. If the fat breakdown dominates acetyl CoA keton bodies are formed
All by the fatty acid break down formed active acetyl CoA can only be sufficient fast broken
down in the citric acid cycle when sufficient oxalacetate is present. By fasting or by diabetes
oxalacetate is used for the gluconeogenesis. Then there is insufficient oxalacetate available to
react with acetyl CoA.
Under these circumstances, from two molecules acetyl CoA one molecule acetoacetyl CoA is
formed and from that the keton bodies are formed: acetylacetate (diacete), D-3-hydroxybutyrate
and acetone.

8. Acetylacetate is an important fuel in some tissues
The keton bodies appear to be important energy sources, it is the primary fuels for the heart
muscle and the kidney salt marsh. By fasting or diabetes the brains change from the use of
glucose to the use of acetylacetate as fuel.
Acetylacetate is activated by the transfer of the CoA of succinyl CoA to acetylacetate.
Acetoacetyl CoA is then thiolysed to two molecules acetyl CoA that go into the citric acid
cycle.

9. The use of acetoacetate as a fuel. Acetoacetate is converted in 2 molecules acetyl CoA what the citric acid cycle
can enter.
The liver can supply acetylacetate (not thiolysed) to other organs because the liver itself has
not the enzyme CoA transferase. Other tissues do have this enzyme.
Acetylacetate has a regulating role. High concentrations in the blood are a signal for an
excess of acetyl-units and lead to a delayed lipolyse (fat breakdown) in fat tissue (negative
feedback). Humans and animals cannot convert fatty acids into glucose. Humans and animals
can not convert fatty acids into glucose because they cannot use the acetyl CoA to make
pyruvate or oxalacetate. The both carbon atoms are taken up in the citric acid cycle, but is
formed by two decarboxylations per balance no extra oxalacetate (no gluconeogenesis).
Plants can do that with help of the glyoxylate cycle.

10. Characteristic differences between the break down and synthesis of fatty
acids.
Break down of fatty Structure of fatty
acids acids
In which part of the cell mitochondria Cytoplasm
Bond of intermediate acyl transport protein
coenzyme A
products on ACP
enzymes in one protein
Enzyme system separate enzymes
chain
separation of C2 (acetyl addition of C2
Change of the chain length
CoA) donor: malonyl ACP
Oxidizers: FAD and
Redox Reducers: NADPH
NAD +