fatty acid synthesis - why cant mammals synthesize essential fatty acids?

[rammbbs7]

I need a small clarification regarding fatty acid synthesis in humans. To be more clear with my question, I will consume some space to explain few things.

consider the following image:

 

 

That above diagram shows nomenclature of fatty acids:

consider the fourth one alpha linolenic acid

its w3, meaning the first double bond from the last carbon is in 3rd position. 18:3 means its 18 carbon and has 3 double bonds, delta 9, 12, 15 refers to the position of double bonds from the COOH end.

 

legend for figure 2:

1, elongase;

2, delta⁶ desaturase;

3, delta⁵ desaturase;

4, delta⁴ desaturase

 

The above figure shows the interconversion of fatty acids within the three families w9, w6, w3. The interconversion of fatty acids involves microsomal elongase which adds 2 carbons to the COOH end and various desaturases which can introduce double bonds at various positions depending on their number. It has been said that mammals can't introduce a double bond beyond 9th position as they do not have delta 12 or delta 15 desaturases.

 

 

 

In the above figure, conversion of linoleoyl co-A to arachidonic acid is shown using various desaturases and elongase.

 

Now because mammals can't introduce a double bond at 12, 15 or later as they lack a specific desaturase, linolenic and linoleic acids have become essential. Now the question is using simple elongase and existing desaturases, we can derive the essential fatty acids in theory without the need for specific desaturase. I am writing a hypothetical scenario. Please tell me why I am wrong.

 

1.start with 12 chain saturated fatty acid.

 

2. now one step of delta 9 desaturase will change this to w3 12:1 delta 9

 

3. now another step of delta 6 desaturase will change this to w3 12:2 delta 6, 9

 

4. now one step of elongase will change this to: w3 14:2 delta 8, 11

5. Now one step of delta 5 desaturase will change this to w3 14:3 delta 5, 8, 11

6. now one step of elongase will change this to w3 16:3 delta 7, 10, 13

7. now another step of elongase will change this to w3 18:3 delta 9, 12, 15

the last one is nothing but alpha linolenic acid which is one of the essential fatty acids. I want to know whats wrong in the above set of reactions. Even without specific desaturase like delta 12 or delta 15, mammals should be synthesizing the EFA, but why is it not happening?

 

Thank you for reading.

 

edit: this could probably be the reason. I might be wrong because delta 9 desaturase is actually Stearoyl-CoA desaturase-1 and might be relatively specific for the 18 carbon stearic acid. So it might not act on fatty acid chains less than 18.

Edited February 25, 2012 by rammbbs7

[John Cuthber]

I'm not certain, but I think the numbers are counted from the "methyl" end of the chain and the elongase adds to the carboxylate end so you are mistaken about step 4 but I may be mistaken.

I'm not familiar with the notations used in this field (because it's not my area).

Could you draw diagrams of the products in each of the first 4 or 5 steps please?