Introduction  lipids are a good source of energy as 1 gm supplies 9.1 calories, which is over double that supplied by carbohydrates or protein.  Dietary lipids are ingested in form of triglycerides, phosholipids, cholesterol and free fatty acids.

Lipids are esters of fatty acids with glycerol called triglycerides because they are tri- esters of glycerol

 It can be divided into 3 processes: 1)Biosynthesis of glycerol. 2)Biosynthesis of fatty acids. 3)Biosynthesis of the triacylglycerol.  It occurs in most tissues especially adipose tissue, liver, lactating mammary gland and brain.

 Glucose is oxidized via glycolysis to dihydroxy acetone phosphate reduced to glycerol-3 phosphate by the enzyme glycerol-3 phosphate dehydrogenase.

 Glucose is oxidized via glycolysis to pyruvic which undergoes oxidative decarboxylation, forming acetyl-CoA (building block of fatty acid synthesis).

 This is the only system responsible for de novo synthesis of FA from active acetate.  Free palmitate is the main product.

 Site: Many tissues, especially liver, kidney, brain, lung, lactating mammary gland and adipose tissue.  Source of acetyl-CoA Acetyl-CoA, the main building block for FA synthesis is formed from carbohydrate via oxidation of pyruvate within mitochondria.

Main requirements de novo synthesis of FA : Acetyl CoA (Active Acetate) Acetyl CoA carboxylase Fatty acid synthase ATP & NADPH

1) Step 1: Acetyl-CoA, formed from pyruvate within mitochondria, does not diffuse readily to cytoplasm (principle site for FA synthesis). 2) Step 2 : Translocation of acetyl CoA from mitochondria to the cytoplasm involves condensation with oxalacetate to form citrate which can pass out mitochondrial membrane.

II. In cytoplasm : Step 3: citrate splits again by ATP citrate lyase enzyme into acetyl- CoA and oxalacetate.

Step 4: Acetyl CoA is converted to malonyl CoA, an important intermediate in fatty acid synthesis, by acetyl CoA carboxylase that consumes ATP and requires biotin as a cofactor.

Step 5: Condensation of Acetyl CoA and malonyl CoA By Fatty acid synthase Fatty acid synthase multienzyme complex. It is a dimer. Each unit contains 7 enzymes and a protein (acyl carrier protein) Palmitate, a 16-carbon saturated fatty acid, is the final product of fatty acid synthesis.

Step 6: OAA is converted to malate. Step 7: Malate is converted to pyruvate by malic enzyme, producing 1 NADPH. a) NADPH is required for synthesis of palmitate and elongation of fatty acids. b) NADPH is produced in the cytosol by both malic enzyme and the pentose phosphate pathway, which is the primary source.

Palmitate, a 16-carbon saturated fatty acid, is the final product of fatty acid synthesis. may be assembled in a repeating four-step sequence

 To initiate FA biosynthesis, malonyl and acetyl groups are activated on to the enzyme fatty acid synthase.

 When reaches 16 carbons, the product leaves the cycle.  All the reactions in the synthetic process are catalyzed by a multi- enzyme complex, fatty acid synthase.

 Then the seven cycles of condensation and reduction produce the 16-carbon saturated palmitoyl group Acetyl-CoA + 7malonyl-CoA + 14NADPH + 14H + palmitate + 7CO 2 + 8CoA + 14NADP + + 6H 2 O  The biosynthesis of FAs requires acetyl-CoA and the input of energy in the form of ATP and reducing power of NADPH.

 Most of the fatty acids synthesized have one of two fates: incorporation into triacylglycerols for the storage of metabolic energy  or incorporation into the phospholipid components of membranes.  Triacylglycerol is formed by estrification of 3 molecules fatty acids with one molecule of glycerol-3 phosphate

 Adipose cells are specialized for synthesis and storage of TG.  LIPOLYSIS complete hydorlysis of triglyceride yeild gelycerol and 3 fatty acids.

The hydrolysis of TG is done by 3 tissue lipases: 1.Hormone sensitive lipase initiates the process of lipolysis in the adipose tissue 2.Diacylglycerol lipase. 3.Monoacylglycerol lipase. Diacylglycerol and monoacylglycerol lipases rapidly complete the hydrolysis of di and mono- acylglycerols releasing free fatty acids and glycerol

The activity of hormone sensitive lipase in the adipose cells is regulated by different hormones.  Glucagon, Epinephrine, norepinepherine hormones stimulate lipolysis activate adenylate cyclase. increased production of cAMP activates protein Kinase A subsequently activates lipases found in adipose tissue.

 After lipolysis glycerol and free fatty acids diffuse through the plasma membrane and enters the blood stream.