Beta-oxidation

Stages of beta-oxidation

We will see the case of the saturated fatty-acids with even number of Carbone, degraded in the Mitochondrie.

Activation of the fatty-acid

This first stage takes place in the Cytoplasme, on the level of the external membrane mitochondriale where the enzyme catalyzing is located this reaction (acylCoA synthétase). The compound formed is a acylcoenzyme has in C _N. This reaction also produces AMP and Pyrophosphate. This last, under the action of a pyrophosphatase (Hydrolase) is hydrolized in two groupings phosphates (we represent only the assessment of these two reactions).

Penetration of the acylCoA in mitochondrion

This stage relates to the acylCoA with long chain (12 to 18 carbons), the acylCoA penetrate rather easily. It is about a molecular system of shuttle. The fatty-acid penetrates in the form of acylcarnitine.

Dehydrogenation of the acylCoA

The Dehydrogenation, in the presence of FAD, catalyzed by a Oxydo-réductase (acylCoA déshydrogénase), takes place between carbons β (from where the name of the site of reaction) and α (carbons 2 and 3 in the official nomenclature). There is formation d'α-β-déshydroacylCoA in C _N.

Reduced FAD will be réoxydé by the respiratory Chaîne with formation of energy in the form of ATP.

Hydration of the double connection

This reaction of addition is catalyzed by a crotonase (group of the Lyase S) and led to the β-hydroxyacylCoA in C _N.

Because of proximity of the grouping Ketone, the double connection is polarized (carbon β is δ+, carbon α is δ-): grouping OH of water binds to carbon β and forms a β-hydroxyacylCoA in C _N. The crotonase is a énoyl-CoA hydratase specific déhydroacyl-CoA to short chain, another énoyl-CoA hydratase is specific déhydroacyl-CoA to long chain. This reaction is reversible and stereospecific.

Dehydrogenation of the β-hydroxyacylCoA

This reaction, catalyzed by a Oxydoréductase (β-hydroxyacylCoA déshydrogénase) in the presence of NAD leads to the βcétoacylCoA in C _N.

Reduced NAD will be réoxydé by the respiratory Chaîne with formation of energy in the form of ATP.

Cut of the carbonaceous chain

This reaction, catalyzed by a Transférase (β-cétothiolase), in the presence of HSCoA led to the formation of:

Acétylcoenzyme has which undergoes the Cycle of Krebs;
Of acylCoA with C _{N -2} again which can undergo β-oxidation (except the two initial reactions).

The complete degradation of the fatty-acid occurs until the carbonaceous chain is completely cut out in molcéules of acétylcoenzymes a: it is the propeller of Lynen .

Assessment of beta oxidation

For a Fatty-acid in C _N:

${N \ over 2} - 1$ turns of whorls of propeller of Lynen;
${N \ over 2} - 1$ NADH, H⁺;
${N \ over 2} - 1$ FADH₂;
${N \ over 2}$ AcétylCoA;
1 AMP;
2 P_i;
- 1 ATP (activation of the fatty-acid).

For the hexanoic acid ( $H_ {3} C (CH_ {2}) _ {4} - COOH$ ):

2 NADH, H⁺;
2 FADH₂;
3 AcétylCoA;
1 AMP;
2 P_i;
- 1 ATP (activation of the fatty-acid).

That is to say 45 ATP. The catabolism of the Glucose (in C_{6 also) generates 38 ATP.}

With number equal of carbons, a Fatty-acid is energy than a Ose.

The Beta-oxidation generates many acétylCoA. The Cycle of Krebs can be overflowed in the presence of many acétylCoA which is used for the synthesis of ketonic compounds in particular of the Acétone. See Cétoacidose diabetic.

External bonds

The chemical logic behind fatty acid metabolism

See too

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