Cellular Biochemistry and Metabolism (CLS 333 ) Dr. Samah Kotb Nasr Eldeen

The Oxidative Degradation of Fatty Acids in Animal Tissues Chapter 10

INTRODUCTION

Lipids are a group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E, andK),monoglycerides,diglycerides,triglycerides, phospholipids and others.

The main biological functions of lipids include storing energy, signaling, and acting as structural components of cell membranes.

The Oxidative Degradation of Fatty Acids in Animal Tissues There are 2 major lipid molecules that are regarded as rich sources of energy in animal tissues. These are: 1. Triglycerides 2. Free fatty acids

Triglycerides have the highest energy content of the major nutrients (9 kcal/g ). Triglycerides are deposited in cells as fat droplets present in adipose tissue. 40% of the daily energy requirements in humans are met by dietary triglycerides.

The Oxidative Degradation of Fatty Acids in Animal Tissues The liver, heart & skeletal muscle obtain half of their energy requirements from the catabolism of triglycerides. Excess carbohydrates after glycogen storage are converted into triglycerides.

Chemistry of triglycerides: - fatty acid esters of glycerol

95% of the biologically available energy of triglycerides is derived from the 3 fatty acid molecules. Only 5% is provided by the glycerol backbone.

Chemistry of fatty acids: Fatty acids are long hydrocarbon acyl chains that terminate with a carboxyl group at one end and a methyl group at the other:-

Chemistry of fatty acids:

Different fatty acids differ in: 1. Length of chain. 2. Presence or absence of double bonds (saturated & unsaturated). 3. Number and positions of double bonds.

Degradation of fatty acids: 1. Degradation of fatty acids involves a process of fragmentation starting at the β carboxyl group of fatty acids. There is successive removed of 2 C units that appear as acetyl- CoA molecules. This process of fragmentation is repeated sufficient number of times until all the fatty acid is fragmented into acetyl- CoA.

2. The process of fragmentation requires ATP. The fatty acid chain is changed into a fatty acyl-CoA derivative in an enzymatically catalyzed reaction that requires ATP.

Degradation of fatty acids:  The enzyme is present on the outer mitochondrial membrane. As a result of this reaction:- A. The fatty acid becomes activated (ready for fragmentation). B. Is able to cross the double mitochondrial membrane into the matrix where the process of fragmentation occurs.

Degradation of fatty acids: 3. The process of fragmentation involves oxidative removal of successive 2 C units by a catabolic pathway made of 4 reactions known as β-oxidation.

β-oxidation will repeat it self sufficient member of times (known as PASSES) until all the fatty acid chain becomes fragmented into acetyl-CoA molecules.

4. Finally the acetyl-CoA molecules will enter the TCA cycle for complete degradation into CO 2

Degradation of fatty acids:

 Reactions of β-oxidation:- 1.The First Dehydrogenation Step. 2.The Hydration Step. 3.The Second Dehydrogenation Step. 4.The Cleavage Step.

Degradation of fatty acids:

Bioenergetics of fatty acid oxidation As seen from pathway every pass (turn) of β-oxidation will yield 5 ATP molecules.  Thus complete degradation of 1 molecule of palmitic acid (C:16) will produce:- Stage 1: (16/2) -1 = 7 Passes of β-oxidation. = 7 × 5 = 35 ATP molecules. Stage 2: (16/2) = 8 acetyl-CoA molecules. = 8 × 12 = 96 ATP molecules.  Thus Net ATP gain = = 131 ATP molecules.

Bioenergetics of fatty acid oxidation  The longer the fatty acid chain the higher the number of ATP molecules synthesized.

 If the fatty acid is unsaturated auxillary enzyme are required to remove the double bond. Once this is done the normal β- oxidation enzymes will come into play. Number of ATP molecules synthesized still depends on the number of C atoms making up the fatty acid.