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The following points highlight the three main reactions involved in the synthesis of fatty acids. The reactions are: 1. Initiation Reaction 2. Chain Elongation Reactions 3. Termination Reaction.
1. Initiation Reaction:
In this reaction, acetyl CoA transfers its acetyl group to one of the —SH groups of multienzyme complex, i.e., fatty acid synthetase.
2. Chain Elongation Reactions:
Six different reactions involved are:
(i) Malonyl transfer,
(ii) Condensation.
(iii) Reduction.
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(iv) Dehydration,
(v) Reduction, and
(vi) Acyl Transfer.
Chain elongation starts with the transfer of malonyl group from malonyl-CoA to second – SH group of the multi-enzyme complex. Thereafter, there is condensation of the latter so that a 4-C unit is produced.
This unit by next three reactions, reduction, dehydration and reduction converts into a saturated 4-C unit (i.e., butyryl-CoA). In acyl transfer reaction the fatty acid residue is transferred back to the -SH group to which the acetyl group was transferred in initiation reaction.
The cycle is repeated again and again with malonyl transfer, condensation, etc., till the tatty acid residue consists of up to 16 to 18 C atoms. Each such turn elongates fatty acid chain by 2-C atoms.
Chain elongation reactions are as follows:
3. Termination Reaction:
When the fatty acid residue attains a desired length, the chain elongation stops at reaction (v) and the cycle is not repeated. The acyl group instead of being transferred to the -SH of the enzyme, is transferred to-SH group of Co-enzyme A (CoA SH) molecule. Thus, CoA derivate of the fatty acid is produced which can then be utilised in fat synthesis. The enzyme becomes free.
