Oxidation of fatty acids:
Muscles of the body oxidize normally the glucose (glycogen) as their energy fuel but fatty acids may also be oxidized when glucose (glycogen) concentration falls below normal.
Fatty acids on their oxidation form acetyl CoA or “active acetate” which later enters the citric acid cycle.
There are several explanations of how fatty acids are broken-down by oxidation. Out of them the most important one is the classical theory of ?- oxidation elaborated by Knoop.
According to this theory fatty acid chains are oxidized by the removal of two carbon atoms at a time.
The carbon atom in the ?-position to the carboxyl group is assumed to be attacked with the formation of the corresponding ?-keto add, then the two terminal carbon atoms are split off as acitic acid.
A new carboxyl group (—COOH) is formed at the site of the keto (=CO) grouping so that a fatty acid remains with two carbon atoms fewer than the original.
Again the new ?-carboo atom is attacked and two more carbon atoms are split off. Thus, the fatty acid is broken-down by the removal of two carbon atoms at a time, until finally the stage of aceto-acetic acid (?-keto butyric acid) is reached.
According to the modern views of GREEN .Lynen and Kbnnbdy the oxidation of fatty acid involves the following five chemical reactions which take place in the cell mitochondria.
During this reaction the fatty acid is converted into active fatty acid in the form of acetyl CoA derivative.
At- least three activating enzymes (thiokinases) take part in this reaction. One acts on acetic and propionic acids, the second on acids having four to twelve carbon atoms and the third on acids having more than twelve carbon atoms.
The thiokinases require a supply of ATP and Co-enzyme A, while the theophorases require NAD, as in the reaction by which a-ketoglutarate is converted to succinyl CoA. They do not require ATP. Succinyl CoA can also be formed from succinate and GTP by a thiokinase.
Once the fatty acid has been activated, it can be dehydrogenated in the a, ?-position by acyl dehydrogenases.
These enzymes contain a flavoprotein, flavin adenine nucleotide, and they show specificity in relation to the chain length of the fatty acid.
The reaction is not reversible by the same enzyme but may be reversible in the presence of reductases which require NADPH.
In the next reaction the compound takes up one molecule of water under the influence of an enoyl CoA hydratase to form ?-hydroxy acyl CoA derivative.
During this reaction the ?-hydroxy acyl Co. derivative is oxidized to a keto group in the presence of ?-hydoxyl acyl dehyrogenases and NAD.
The NADH and FADH formed in the dehydrogenation step are oxidized through the hydrogen transport system and provide pa of the energy, in the form of ATP, which is obtained from fatty act oxidation.
5. Thiolytic cleavage:
The final step in the process of ?-oxidation in cleavage of the ?-keto derivative by a molecule of Co-enzyme A.
The reaction is similar to hydrolysis, involving the sulphydryl group of CoA instead of water and is called thioiysis.
The enzymes involved in this reaction are thiolases and appear to be no specific, converting ?-keto acyl CoA esters from C4 to C18.
After the oxidation of fatty acid a molecule of acetyl CoA and a molecule of activated fatty acid which is two carbon shorter than the fatty acid at the start are formed.
The activated fatty acid may again be degraded by repetition of the process, starting at the second reaction since activation is not necessary. By successive repetition of the proceess the entire fatty acid chain can be converted to a acetyl CoA.
Fate of the acetyl CoA:
The acetyl CoA produced after the oxidation of fatty acid is very much identical with acetyl CoA formed from carbohydrate by of way pyruvate and amino acid.
Most of it combines with oxaloacetate to form citrate and is oxidized via the citric acid cycle (tricarboxylic acid cycle).
The final oxidation path- way for fat and carbohydrates is, therefore, same and the end products are CO» and H»0 with the production of ATP.
The co-enzyme A released during citric acid cycle enters the cycle again with another molecule of fatty acid, while the reduced FAD and NAD are reoxi- dized by the usual hydrogn transport systems.
Some acetyl CoA is used in the formation of cholesterol of fatty acids and of aceto-acetate. Small amounts of acetyl CoA are used in various acetylation processes. Further aspects of acetyl CoA metabolism are discussed below.
Under normal metabolic conditions when acetyl coenzyme A, produced after the oxidation of fatty acids, pyruvate and from other sources, is not required for the synthesis of cholesterol or fatty acid or acetyl derivatives combines with oxaloacetic acid and is oxidized to CO* and HiO through tricarboxylic acid cycle.
While some of the oxaloacetic acid is of course regeneratd in the operation of tricarboxylic acid cycle, this amount may be supplemented by additional supplies formed in the Jiver by the carboxylation of pyruvic acid derived from carbohydrate metabolism.
However, in circumstances when the metabolism of carbohydrates is impaired or operating at loss level such as in diabetes mellitus, starvation or prolonged subsistence on low carbohydrate diet, the fate of the acetyl CoA is altered by two reasons,
(i) The oxaloacetic acid available to combine with acetyl CoA is in limited supply and
(ii) A much greater proportion of the body’s energy needs is being supplied by the oxidation of fatty acids, leading to the production of acetyl CoA in greater than normal amounts.
Because of this combination of circumstances (large amounts of acetyl CoA and small amounts of oxaloacetic acid) acetyl CoA metabolism proceeds to a greater extent than normally via a different route.
This consists of the condensation of two molecule of acetyl CoA to form aceto-acetyl CoA which, in turn, is hydrolyzed by deacylase in the liver to yield acetoacetic acid which may be reduced to ?-hydroxybutyric acid in the presence of ?-hydroxybutyrlc dehydrogenase and reduced NAD or decarboxylated to form acetone.
These three compounds are collectively known as “ketonebodies” or “acetone bodies” and the process is called “ketogenesis”.
They are disposed of by oxidation in the process of “ketolysis” which takes place mainly in the extra-hepatic/tissues especially muscles.
In this process the acetoacetic acid is activated by succinyl Co-enzyme A produced by operation of the citric acid cycle to form acetoacetyi CoA which can act as immediate precursor of acetyl CoA which in turn, is oxidized in usual way to CO2 and H20.
The ketone bodies are normal end products of fatty acid oxidation in the liver, but the amount formed is relatively small.
The acetoacetic acid produced in the liver is not further utilized by the organ, except during fisting.
Other tissues, however, readily metabolize acetoacetic acid to CO2 and H20 and there appears to be no impairment in this respect in diabetes (Soskin).
If adequate amount of carbohydrate is available, the liver apparently prefers carbohydrate oxidation as a source of energy, and ketone body production is small. Carbohydrate is, therefore, an “antiketogenic” substance.
In the diabetic where there is impaired glucose metabolism, the operation of the citric acid cycle is impaired by a decrease in oxaloacetic acid.
The degradation of fatty acids, however, continues uninterruptedly, and the concentration of acetyl CoA which requires oxaloacetic acid to enter the citric acid cycle, increases.
As a results acetyl CoA is shunted into the formation of acetoacetyi CoA and subsequently into ketone bodies.
The term “ketosis” is applied to the condition in which ketone bodies accumulate in the blood (ketonaemia) and appear in urine (ketonnria).
This condition appears most commonly in starvation and clinical or experimental diabetes mellitus when the carbohydrate metabolism is very low.
The ketosis of diabetes mellitus is accompanied by profound metabolic disturbances which lead to gradually depending coma and finally to death.
It is now clear that the accumulation of ketone bodies may be due to an abnormally high rate of formation of acetyl CoA together with a diminished capacity of its disposal.
The capacity for disperse I is limited by the availability of oxaloacetic acid and is, therefore dependent upon carbohydrate metabolism.
It was felt at one time that ketosis was harmful and that it could be controlled by the proper ratio in the diet of ketogenic material (fats) to antiketogenic material (carbohydrates).
This view is no longer held. The major effect of ketosis on the animal body appears to be in relation to acid-base balance, excretion of large amounts of acetoacetic acid and ?-hydroxybutyric acid in the urine as their alkali salts deplete the body of available base and may lead to the development of an acidosis.
The oxidation of fat is not dependent on the simultaneous oxidation of carbohydrate because, when fatty acids and ketones are beint extensively oxidized in muscle tissue, the resulting accumulation d acetyl CoA and citrate depress glycolysis and thus diminish the oxidation of glucose.