Part 5 - Fatty acids

Part 5  -  Fatty acids

==================================================

v Fatty acids are acids which can form fat with glycerol

Fatty acids are characterized by one methyl group (CH3) at the one end and, one carboxyl group (COOH) at the other end. In between them only carbons (C) and hydrogens (H). Three fatty acid molecules and one glycerol molecule together can form one fat molecule. This happens when the 3x COOH groups and the 3x OH groups join together and 3x H₂O split off. This is called a ‘condensation reaction’.

- A fatty acid with only single bonds (-CH₂-CH₂-CH₂-CH₂-) is called a saturated fatty acid.

- A fatty acid with one or more double bonds (-CH₂-CH = CH-CH₂-) is called a unsaturated fatty acid.

l To clarify, pyruvic acid (CH₃-CO-COOH), lactic acid (CH₃-CHOH-COOH), oxaloacetic (2 –COOH groups) and citric acid (3 –COOH groups) are not fatty acids. They have a different structure and they can not form fat with glycerol.

v Fatty acid is an important component of the cell membrane.

The cell membrane is constructed of phosfolipids. A phosfolipid consists of a head and a tail of 2 fatty acids (lipid = fat ). We have seen this already in Part 4.

v  Sources fatty acids

For cell multiplication, a lot of fatty acids will be needed. For this purpose, cells may hydrolyze (=split with water) fat into glycerol and fatty acids. Usually cells synthesize fatty acids from Acetyl-CoA. Cells can obtain Acetyl-CoA in many ways, e.g.

(1) from glucose (see glucose metabolism).

(2) from citric acid in the must (grape juice), which will be converted in oxaloacetic acid and acetic acid. The acetic acid will then be attached to coenzyme A to become Acetyl-CoA.

(3) from amino acids (see image below) and proteins which are basically made up of amino acids.  Must poor of amino acids or proteins (e.g. by excessive ‘débourbage’) may have problems to start the fermentation.

v  Fatty acid synthesis from Acetyl-CoA in cytoplasm.

l In order to synthesize fatty acids, Acetyl-CoA will first replace  the coenzyme CoA with the ACP (acyl carrier protein). Acetyl-CoA has become Acetyl-ACP. This Acetyl-ACP will be carboxylated to become Malony-ACP and the synthesis can begin.

l The synthesis occurs in 4 steps. (1) The Malonyl-ACP will be decarboxylated, then linked up with a 2^nd Acetyl-ACP by releasing H-S-ACP. They become an Acetoacetyl-ACP. This will then be (2) reduced, (3) dehydrated and (4) reduced, resulting in a  4-carbon compound, Butyryl-ACP. When this is linked up with a 3^rd Acetyl-ACP, these 4 steps will be repeated, with the difference that there is no decarboxylation in the first step. Each addition of a new Acetyl-ACP will make the ACP-compound  2 carbons longer. When the ACP is hydrolyzed (=split with water) it becomes a fatty acid.

l In general, NADP and NADPH are used in anabolism (building reactions, such as fatty acid synthesis), while NAD and NADH are used in catabolism (breaking reactions, such as glycolysis).

v Fatty acid chains

Fatty acid chains differ by length. The smallest fatty acid, with 3 carbon atoms, is called propionic acid. Animal fatty acids usually contain a long and an even number of C-atoms (almost always 14 -24).

Fatty acid chains are often categorized as short to very long.

- Short-chain fatty acids (SCFA) are fatty acids with less than 6 carbons. 

- Medium-chain fatty acids (MCFA) are fatty acids with 6 to 12  carbons.

- Long-chain fatty acids (LCFA) are fatty acids with 13 to 21 carbons.

- Very long chain fatty acids (VLCFA) are fatty acids with more than 22 carbons.

v  Diagrammatic representation of a long fatty acid

v  Fatty acids C-atoms numbering

The C-atoms of the fatty acids are numbered starting at the C-atom of the-COOH group. The C-atoms 2 and 3 are often indicated by α and β. The C-atom of the methyl group at the end of the chain is called the omega (Ѡ).

                                

v  Saturated or unsaturated fatty acids? Long or medium chain fatty acids?

Saturated fatty acids (SFA), as it shows in fatty acids synthesis above, do not need oxygen. They have only single bonds 

(-CH2-CH2-)

For unsaturated fatty acids (UFA), however, oxygen is needed to dehydrogenate at a defined position in the SFA (-CH₂-CH₂- + O à -CH=CH- + H₂O). Due to the double bond, UFA has a lower melting point and is more flexibility than SFA.

During alcoholic fermentation, the ethanol concentration increases progressively which reduces drastically the fluidity of the cell membrane. The yeasts need to adapt their cell membrane fluidity by increasing the proportion of UFA in the phospholipids. In red wines, these changes can be done without problems because oxygen is introduced during the racking process. In white wines which are usually made without aeration, the lack of oxygen makes yeasts unable to synthesize UFA. Consequently, yeasts need to use another strategy to fluidize their cell membranes by  synthesizing medium chain fatty acids (MCFA). The effect of a short chain is similar to that of the double bond of a long chain.

v  Fatty acids and wine aroma

Fatty acid can bind with ethanol to form an ester. Fatty acid esters, depending on concentration, can contribute to the aromas of wine, especially in young white wines (more about esters in Part7 – Esters).

 -------------------------------------------------------------------------------------------------------------------------------------------------------

P.S.

- What are amino acids?

- What are higher alcohols? How are they formed?  And why are they formed?

These will be set out in Part 6, coming next month.