Fatty acid synthesis is the creation of fatty acids from acetyl-CoA and NADPH through the action of enzymes called fatty acid synthases. This process takes place in the cytoplasm of the cell. Most of the acetyl-CoA which is converted into fatty acids is derived from carbohydrates via the glycolytic pathway. The glycolytic pathway also provides the glycerol with which three fatty acids can combine (by means of ester bonds) to form triglycerides (also known as "triacylglycerols" – to distinguish them from fatty "acids" – or simply as "fat"), the final product of the lipogenic process. When only two fatty acids combine with glycerol and the third alcohol group is phosphorylated with a group such as phosphatidylcholine, a phospholipid is formed. Phospholipids form the bulk of the lipid bilayers that make up cell membranes and surrounds the organelles within the cells (such as the cell nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, etc.).
Straight-chain fatty acids
Straight-chain fatty acid synthesis occurs via the six recurring reactions shown below, until the 16-carbon is produced. The fatty acids are synthesized in microorganisms and list the enzymes found in Escherichia coli. Fatty acid synthesis is performed by fatty acid synthase II (FASII), which in general contain multiple enzymes that act as one complex. FASII is present in prokaryotes, plants, fungi, and parasites, as well as in mitochondria.
Acetyl-CoA is coming from Citric Acid Cycle.
Step 1 Acetyl-CoA:ACP transacylase; (acyl-carrier-protein) S-acetyltransferase; EC 2.3.1.38
+ [acyl-carrier-protein] ⇌ CoA + acetyl-[acyl-carrier-protein]
Step 2 Malonyl CoA:ACP transacylase; (acyl-carrier-protein) S-malonyltransferase; EC 2.3.1.39
+ acyl carrier protein ⇌ CoA + malonyl-[acyl-carrier-protein]
Step 3 3-ketoacyl-ACP synthase; Beta-ketoacyl-ACP synthase I; Acyl carrier protein synthase; EC 2.3.1.41
Acyl-acyl-carrier-protein + malonyl-acyl-carrier-protein ⇌ 3-oxoacyl-acyl-carrier-protein + CO2 + acyl-carrier-protein
Step 4 3-ketoacyl-ACP reductase; 3-oxoacyl-[acyl-carrier-protein] reductase; EC 1.1.1.100
3-oxoacyl-[acyl-carrier-protein](ACP) + NADPH + H+ ⇌ (3R)-3-hydroxyacyl-[acyl-carrier-protein](ACP) + NADP+
Step 5 3-Hydroxyacyl ACP dehydrase; Beta-hydroxyacyl-acyl carrier protein dehydratase
Step 6 Enoyl-ACP reductase; EC 1.3.1.9
Conversion of carbohydrates into fatty acids
In humans, fatty acids are formed from carbohydrates predominantly in the liver and adipose tissue, as well as in the mammary glands during lactation. The pyruvate produced by glycolysis is an important intermediary in the conversion of carbohydrates into fatty acids (see Lipids: structure and classification) and cholesterol.
