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 ,trigly

cerides,  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

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 CO2

Degradation of fatty acids:

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 = 35 + 96 = 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.