Synthesis of Fats

 Synthesis of Fats

Fats can be synthesized from both carbohydrates and proteins as is given below-

1. from carbohydrates

It has been found that Palmitic and Stearic acids are rapidly synthesized but the synthesis of unsaturated acids is very slow. The body is able to introduce one or two double bonds but not more. So that the highly unsaturated fatty acids like linoleic acid or linolenic acid or arachidonic acid which are known as essential fatty acids cannot be synthesized.

Synthesis of saturated fatty acids

Fatty acids are synthesized from acetate which is activated by reaction with CoASH and ATP in presence of acetyl CoA synthetase to form active acetyl CoA. This is carboxylated to form malonyl CoA in presence of CO₂+Mn⁺⁺ and acetyl CoA carboxylase (dependent upon enzyme-bound biotin). Then acetyl and malonyl groups are transferred to the acyl carrier protein (ACPSH) with the help of enzyme fatty acyl transacylase and the CoASH becomes free.

Acetyl ACP and malonyl ACP thus formed condense by a sulphydryl enzyme to form acetoacetyl ACP with the liberation of CO, and one molecule of ACPSH. Acetoacetyl ACP is reduced by dehydrogenase. Glycerol is formed from carbohydrate absorbed as such from the intestine in presence of NADPH+H⁺ (formed during glucose oxidation in hexose monophosphate shunt, HMS) to 3-hydroxybutyryl ACP which is further unsaturated at α, β-position by the enzyme enoyl hydrase.

Then α-β-unsaturated butyryl ACP is reduced by dehydrogenase in presence of NADPH+H⁺ tobutyryl ACP. The 2C acetate is thus elongated to 4C chain, i.e., butyryl ACP which may further be elongated by 2C in each above cycle of reactions adding of 16C fatty acyl ACP, i.e., palmityl ACP, Palmitic acid and ACPSH.

Acetic acid derived from glucose, fatty acids and amino acids go to form fatty acids in the body. The mitochondrial and microsomal systems are concerned with the synthesis of fatty acids. In the former system elongation of fatty acid carbon chain and unsaturated fatty acid formation takes place which is sensitive to arid in and not biotin dependent one and in the latter system carboxylation of acetyl CoA, conversion of malonyl CoA to fatty acid and phospholipid formation takes place.

Several types of the vitamin B complex are involved in the process. Thiamine is necessary for the conversion of sugar into fats. Riboflavin, pyridoxine and nicotinic acid play a similar but subsidiary role.

Synthesis of unsaturated fatty acids

Schoenheimer and his associates have reported that unsaturated fatty acids are synthesized from saturated fatty acids. A microsomal system is involved for such synthesis. The liver contains such system for the synthesis of oleic and palmitic acids from stearyl CoA and palmityl CoA respectively. NAD or NADP is required for carrying out the process.

2. from proteins

Since nearly 60% of proteins can be converted into carbohydrate, it is reasonable to expect that some of this sugar may be available for conversion into fats. Certain members of vitamin B family are also important in this respect,