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16 β-Oxidation of Fatty Acids
R – R – CH2 – CH2 –C–OH
AMP + PPi
Acyl – CoA
(Active fatty acid) R – CH – CH – C~S – CoA
Acyl – CoA
O Respiratory chain
A b – Unsaturated R – CH = CH – C~S – CoA
Acyl – CoA
R–CH–CH2–C~S – CoA
acyl – CoA
R–C–CH2–C~S – CoA
R–C~S–CoA + CH3–C~S – CoA
The acetate molecules can be completely oxidised via citric acid cycle or may be utilised to synthesise
glucose and other complex carbohydrates as per needs of the animal. Some important steps of boxidation scheme are shown in Fig. 6.15.
It must be borne in mind that fatty acid oxidation takes place in the mitochondria, but before FA
enters the mitochondria it has to be made ready for oxidation reactions. The FA in the cytosol is
activated by a molecule of ATP in the presence of acylcoenzyme A(CoASH). The reaction occurs
either in the endoplasmic reticulum or at the outer mitrochondrial membrane, resulting in the
formation of fatty acyl CoA derivative. The fatty acyl CoA derivative is then transported inside the
mitochondria with the help of carnitine, a carrier molecule. This reaction is catalysed by an enzyme,
acyl CoA transferase. Once the fatty acyl CoA enters the mitochondrial matrix, there follows the
removal of 2 hydrogen atoms from the = and b carbons, catalysed by a dehydrogenase, resulting in
the formation of unsaturated acyl CoA. The unsaturated fatty acyl CoA derivative is subsequently
hydrated and dehydrogenated at the expense of specific enzymes to form corresponding b-keto-acyl
CoA compound. Finally, b-keto-acyl CoA undergoes thiolytic cleavage by thiolase producing an acylCoA unit and the remaining acyl-CoA chain containing 2-C less than the original fatty acyl CoA
molecule. In this way, a long chain fatty acid may be degraded completely to acetyl-CoA (2-C
fragments), which can be oxidised to CO2 and water through citric acid cycle.
In case of fatty acids with odd number of carbon atoms, oxidation takes place through b-oxidation
scheme, leaving behind propionyl CoA, a 3-carbon unit. This compound can enter the citric acid cycle
after conversion to succinyl CoA.
Energetics of >-oxidation
Let us consider the oxidation of one mole of palmitic acid (C16 H32 O2), entering the mitochondria in
the form of palmitoyl CoA. Initially one mole of ATP is required to activate the acid, and at the end of
each oxidative spiral, one FADH2 and one NADH are formed along with an acetyl CoA fragment. In
order to oxidise palmitoyl CoA, 8 acetyl CoA will be formed and the energy gained in terms of ATP
will be as follows:
8 acetyl CoA + 7 FADH2 + 7 NADH + H+
8 acetyl CoA oxidised via citric acid cycle
ATP initially used for activation
Net gain of ATP
35 ATP formed
96 ATP formed
01 ATP consumed
The overall equation is represented as:
C16 H32 O2 + ATP + 7 FAD + 7 NAD+ + 8 CoASH + 7 H2O ® AMP
+ PPi + 7 FADH2 + 7 NADH + H+ + 8 CH3COSCoA.
Since each ATP molecule has 7.6 kcal of bond energy, the net gain would be 130 ´ 7.6 = 988
kilocalories. The calorific value of palmitic acid is 2340 kcal/mole, the system receives at least 42%
of high phosphate bond energy (988/2340 ´ 100) of the total energy of combustion of the fatty acid.
Oxidation of Unsaturated Fatty Acids
Body lipids are rich in unsaturated fatty acids and these are oxidised more slowly. Some examples are:
palmitoleic acid (16:1), oleic acid (18:1), linoleic acid (18:2), linolenic acid (18:3) and arachidonic
acid (20:4). All double bonds in naturally occurring unsaturated fatty acids are in cis-configuration.
Palmitoleic and oleic acids are not essential as they can be formed in the body, but the remaining three
acids come under the essential fatty acids category and have to be supplied in the diet. Oxidation of
unsaturated fatty acids proceeds the usual b-oxidative pathway until the double bond is reached. The
double bond in cis-configuration is not vulnerable to enzymic attack unless it is isomerised to transconfiguration. Polyunsaturated fatty acids, such as linoleic, arachidonic etc. are more complex and
require additional enzyme for oxidation. They are normally found as structural components in
association with cholesterol and phospholipids (e.g. membranes and reproductive organs). In
mammals, arachidonic and some related C-20 fatty acids are known to give rise to unique compounds
like prostaglandins which have hormone-like activity.
6.17 METABOLISM OF GLYCEROL
One of the hydrolysis products of triglycerides is glycerol which is metabolised or utilised in
organs/tissues where specific enzyme glycerol kinase is abundantly present. Certain organs such as
liver, kidney, intestinal mucosa and lactating mammary glands are rich in the enzyme, while muscles
and adipose tissue contain very little activity. Glycerol is predominantly converted into carbohydrate
through glycerol phosphate, formed by a specific glycerol kinase at the expanse of ATP.
CHOH + ATP
CHOH + ADP
Glycerol phosphate is then oxidised to triosephosphate by a glycerol phosphate dehydrogenase
which ultimately forms glycogen through glycogenesis. Triosephosphate may be, however, oxidised
to pyruvic acid by way of glycolysis (Fig. 6.16). In diabetic or phlorrhizinised animals, glycerol is
converted almost quantitatively to glucose.
Fate of glycerol metabolism.
6.18 SYNTHESIS OF GLYCERIDES AND FATTY ACIDS
It has been known for a long time that fats are synthesised from metabolites such as acetate and
acetoacetate. Fats are also synthesised from protein and carbohydrate sources. All naturally occurring
fatty acids possess even number of carbon atoms; thus it is logical that fatty acids must be synthesised
from 2-carbon fragments. It has been shown that the starting material is acetyl-CoA which can be
derived from pyruvate. Alternatively, pyruvate can also be derived from free, acetate when it reacts
with coenzyme A and ATP.
Synthesis of Glycerides
Triglycerides are formed by reactions between acyl-CoA compounds and µ-glycerophosphate, which
is formed by specific glycerol kinase. There are other enzymes which catalyse the formation of
monoand diphosphatidic acids at the expense of fatty acyl CoA derivatives:
In the next stage the phosphate group is removed by a specific phosphatase followed by
replacement by a third fatty acyl residue:
+ Acyl CoA
Synthesis of Fatty Acids
Fatty acid oxidation occurs in the mitochondria, but fatty acid synthesis takes place not only in
mitochondria but also in mitochondria-free systems. The pathway for synthesis is not exactly reversal
of b-oxidation scheme, but it involves, some modifications. Under anaerobic conditions, mitochondria
catalyse the incorporation of acetyl-CoA units into long chain fatty acids (viz. stearic acid, palmitic
acid), requiring ATP, NADH and NADPH. Synthesis in extra-mitochondrial system, especially in the
liver, brain, kidney etc., acetyl-CoA units are incorporated into fatty acids, catalysed by cytosolic
enzymes and cofactors such as ATP, NADPH and Mg2+ or Mn2+ ions. This system is dependent on
CO2 supplied by bicarbonate. Chain elongation usually takes place in the microsomes.
6.19 METABOLISM OF PHOSPHOLIPIDS
Phospholipids are found in all cells and are synthesised either from phosphatidic acid or phosphatidyl
choline. Lecithin is the most important phospholipid in the body and is synthesised in the liver.
Synthesis of phospholipids from fats involves mobilisation of fats in and out of the cells. Hence the
actual sites are liver, intestine and kidney. Phospholipids are largely found in combination with
proteins and are transported in the blood in the form of protein complexes.
The most common among phospholipids are lecithins and cephalins, while where are others in
which bases are replaced by serine or inositol. Catabolism of lecithin is accomplished in the following
manner (Fig. 6.17):
Sphingomyelins are phospholipids containing a fatty acid, phosphoric acid, choline, and a
complex of amino alcohol, sphingosine, but are devoid of glycerol. Abnormal quantitities of phosphoand sphingolipids in certain tissues, especially in the nervous system, cause diseases like
sphingolipidoses, and demyleinating diseases that are inherited.
6.20 METABOLISM OF CHOLESTEROL
Cholesterol occurs in various body tissues and in the plasma either in the free form or storage form as
long chain fatty acid esters belonging to the class of sterols. It is also an important component of the
a - Glycerophosphate + Choline
Catabolism of lecithin.
membrane system of the cell and is the precursor of steroid hormones, excreted from the body in the
form of bile acids (salts). The cholesterol content of the blood ranges from 150 250 mg/100 ml.
Sources of Cholesterol in the Body
While major part of the body cholesterol (about 1 g/day) is synthesized in the body, a small portion is
(about 0.3 g/day) provided by various foods intake such as egg yolk, meat, liver, and brain and its
uptake is through low density lipoproteins (LDLs). Synthesis occurs from acetyl-CoA (two-carbon
units) produced by beta oxidation of fatty acids in the liver, intestine and almost all cells. About 700
mg/d is synthesized by a well regulated mechanism. Free cholesterol is removed from the body by
plasma high-density lipoproteins (HDLs).
Synthesis of Cholesterol
Although cholesterol is synthesized by many tissues such as adrenal cortex, skin, intestine testes etc.
liver is considered to be the main site. Two molecules of acetyl-CoA condense to form acetoacetylCoA, which again reacts with another molecule of acetyl-CoA to form 3-hydroxy-3-methylglutarylCoA (HMG-CoA), leading to the synthesis of mevalonate (Fig. 6.18). It must be noted that all carbon
atoms of cholesterol originate from acetyl-CoA enzyme. Mevalonate is the crucial compound and
through a series of reactions, gives rise to cholesterol.
Transport and Excretion of Cholesterol
Cholesterol, in association with other lipids, is absorbed in the intestine and thereafter incorporated
into chylomicrons and very low-density lipoproteins (VLDLs). A greater part of cholesterol (about
80%) in the lymph undergoes esterification with long-chain fatty acids and transported as lipoproteins
in the plasma. Highest proportion of cholesterol is found in the LDL. Free cholesterol in the plasma is
2NADPH + H+
CoA HMG CoA reductase
To synthesis of cholesterol
Steps in the synthesis of mevalonic acid, the precursor of cholesterol synthesis (steps not shown).
equilibrated with the cholesterol esters and about half of the free cholesterol is eliminated by
excretion in the faeces, while remainder is excreted as neutral steroid.
The liver contains a pool of unesterified cholesterol from where it is transported to the intestine
and then back to liver through chylomicrons. Cholesterol is excreted from the body as cholesterol or
bile acids. Coprostanol is the main sterol secreted in the faeces, a product of cholesterol, arising from
the intestinal bacteria activity. Much of the cholesterol secreted in the bile is either reabsorbed or gets
deposited in the arteries in the form of esters or may participate in steroid synthesis.
During conditions of starvation and diabetes, oxidation of free fatty acids is enhanced in the
mitochondria. In vertebrates, the acetyl-CoA originating from the b-oxidation can leave the
mitochondrion in the form of citrate. However, a major part of this acetyl-CoA is used for the
synthesis of ketone bodies in the liver (ketosis). There are three types of ketone bodies: acetone,
acetoacetic acid, and 3-hydroxybutyric acid, which are generated through a process called
ketogenesis, taking place in the mitochondria.
Formation of Ketone Bodies
Long-chain fatty acid are transported to the inner mitochondrial membrane through carnitine
derivatives. b-oxidation of fatty acids takes place in the mitochondrion, releasing acetyl-CoA, with
the consequent production of large amount of ATP. The initial steps are similar to those of cholesterol
synthesis (Fig. 6.19). The 3-hydroxy-3-methylglutaryl-CoA formed is split into acetyl-CoA and free
The acetoacetic acid is either spontaneously decarboxylated to acetone or reduced to 3hydroxybutyric acid:
CO2 + CH3–CO–CH3
CH3–CO–CH2–COOH + NADH + H
CH3–CHOH–CH2–COOH + NAD
Long-chain fatty acids (even number) produce acetyl-CoA units through beta oxidation, but odd
chain fatty acids produce acetyl-CoA and propionyl-CoA, which is glucogenic. Peroxisomes oxidize
long-chain fatty acids.
Ketone Bodies Serve as Energy Source
Acetoacetic acid and 3-hydroxybutyric acid are produced in large amount and are utilized as energy
source by various tissues such as muscles, kidney and the brain (extrahepatic tissues). Acetoacetic
acid diffuses freely across the cell membranes and cannot be reactivated unless they reach the cytosol
where they can participate in cholesterol synthesis as happens with acetoacetate.
Acetoacetate is reactivated in extrahepatic tissues:
Acetoacetate + Succinyl-CoA ® Succinic acid + Acetoacetyl-CoA. Acetoacetyl-CoA splits by
thiolysis into two molecules of acetyl-CoA which enters the Krebs cycle. Ketogenesis is an extremely
important physiological process and is exclusively hepatic.
Ketogenesis is regulated chiefly by two hormones: insulin and glucagon. When blood glucose
levels are decreased, insulin is depressed, thereby raising glucagon levels, resulting in arresting
glycolysis, increase in gluconeogenesis and inhibiting fatty acid synthesis. As a consequence,
hydrolysis of triglycerides is affected by hormone-dependent lipase in adipose tissues. The fatty acids
thus liberated are transported to the liver.
The hormonal imbalance inhibits acetyl-CoA carboxylase, depressing malonyl-CoA
concentration, which is a repressor of acetylcarnitine synthetase, hence it gets activated. This favours
penetration of fatty acids into the mitochondria where they are catabolized by beta oxidation,
increasing the NADH/NAD ratio. This will lead to reduction of oxaloacetate to malate which leaves
mitochondria to participate in gluconeogenesis. The reduction of oxaloacetate prevents transformation
into citrate of acetyl-CoA arising from b-oxidation. Thus the only fate of acetyl-CoA is to synthesize
ketone bodies to be transported to other tissues capable of utilizing them.
Oxidation of ketone bodies takes place in the extrahepatic tissues because of the absence of key
enzymes in the liver to oxidize them, i.e. 3-oxoacid-CoA transferase. This enzyme is present in the
kidney, red muscle, brain and other peripheral tissues. Normally ketone bodies are continuously
oxidized in muscles, therefore only traces of these may be found in the blood and urine. In fasting
mammals these accumulate in blood and the levels may rise to 20-25 mg/100 ml after a weeks
fasting. 3-hydroxybutyrate apearing in the blood reaches the muscles (skeletal as well as the cardiac),
where a mitochondrial enzyme, 3-hydroxybutyrate dehydrogenase oxidizes the compound to
acetoacetic acid. The latter compound in the muscle mitochondria is activated by 3-oxoacid-CoA
transferase, catalyzing the transfer of CoA from succinyl-CoA to acetoacetic acid. Excessive
production of ketone bodies by the liver leads to ketonemia.
(2 Acetyl CoA)
H2O + Acetyl CoA
NANDH + H
Formation of ketone bodies from acetyl CoA.
In the previous chapter, we have given an account of the classification and nature of nutrients required
by animals. The food materials available to the animals are, as such, rarely in a form suitable for
cellular consumption and to achieved when the foods are subjected to mechanical processes such as
mastication, swallowing and movements of gastrointestinal tract, and chemical processes such as the
enzymatic reactions in the digestive tract.
During the process of digestion, the complex nutrients are split into their simpler substances, i.e.
the proteins into amino acids; the polysaccharides into monosaccharides; and the fats into their
constituent fatty acids and glycerols. In addition to hydrolysis of proteins, carbohydrates and fats, the
process of digestion renders food in a soluble form for abosrption and helps in separating the required
from those not required.
7.1 MODES OF NUTRITION
Based on their method of food procurement the living organisms (plants, animals and
microorganisms) are broadly classified into two major groups, viz. autotrophs and heterotrophs.
The autotrophs synthesize all essential organic compounds from inorganic constituents. They include
phototrophs and chemotrophs. The phototrophs are chlorophyll bearing plants and make the essential
organic compounds by photosynthesis. For this process they require sunlight, and inorganic
substances like carbon dioxide and nitrogenous compounds. The chemotrophs are bacteria making
their food by chemo-synthesis. Since the autotrophs synthesize the required organic substances within
their body, digestion is not required.
Digestion and Absorption
The heterotrophs require organic substances as food and their synthesizing capability is limited. For
this reason they depend on organic substances like carbohydrates, fats and proteins to carry out their
life processes. They obtain their food from plants and animals. Such a food is in a form unsuitable for
direct absorption. The food may be in the form of tiny particles (food of protozoans and other lower
organisms), large chunks or whole animals (as in higher animals), or in the liquid form (leeches,
certain insects, etc.). Therefore these food particles are to be reduced to sizes suitable for entrance
through the cellular membranes to take part in metabolic activities. This process as we have
mentioned earlier is called digestion and only in heterotrophs it is a necessary process with the
exception of certain parasites (cestodes, etc.) and commensals. The cestodes directly absorb the
digested food through its body surface, from its surrounding medium in the intestine. The male
echiurid derives its nourishment from the female echiurid. Such organisms are devoid of a digestive
tract of their own.
The organisms are thus broadly divided into two groups. However, there are living beings
between the typical chemotrophs and the typical phototrophs, and also between the autotrophs and the
heterotrophs. Such living beings are dependent on both the processes and hence are grouped under
mesotrophs. The Euglena, for example, is a mesotroph and can synthesize essential organic
compounds but it still requires certain growth factors or vitamins for which it is dependent on organic
7.2 INTAKE OF FOOD MATERIALS
In order to capture different kinds of food and prepare it for absorption by the cells of the body, the
heterotrophs have suitable anatomical structures and devices.
Among animals the feeding mechanisms exhibit some relation to the type of food they take. It is
reasonable to presume that food of the most primitive organisms is organic matter in solution.
Similarly organic matter in dissolved form is directly absorbed by protozoan parasites, tapeworms and
a few other animals. Studies on the absorption of dissolved organic substances in bacteria as well as
in the cells of higher animals revealed the existence of a number of active sites, each of which
receives the specific compounds to lead them into the cell. Dissolved substances are absorbed by yet
another process. In this process minute droplets of dissolved food are engulfed by the cell through
Protozoans prey upon parts of other organisms smaller than or equal to themselves. To ingest this
type of food the protozoans have a variety of mechanisms. Amoeba, for example, has a mechanism
which may possibly be an extension of pinocytosis and upon coming in contact with its food particle
the amoeba surrounds the particle and engulfs it. Another distinct mechanism of feeding can be
observed among ciliates. The ciliates produce water currents by ciliary movements and these currents
carry the impaled organisms or the particulate food to the mouth or gullet. From here the food is taken
into the body. In sponges the method of collecting food particles by water currents and their
subsequent engulfing by cells for digestion exists in a specialized form. In this method the food
particles are carried along the water currents into the paragastric cavity through body pores.
There are several coelenterates which, like sponges, are sessile animals. However, they procure
food by trapping method rather than the current method noted in sponges. The important structure
involved in the trappin mechanism is the cnidoblast (nematocyst). The tentacles in Hydra and some
other coelenterates are armed with cnidoblasts and these would burst when small animals brush or
bump over the cnidocil which is a small hair-like process serving as receptor of contact stimuli. As a
result of this burst a poisonous thread from the nematocysts is thrust into the victim to impale it. The
impaled organism is then carried to the mouth by one or more of the tentacles.
Filter feeding is characteristic of sessile animals and sedentary feeders such as bivalve molluscs,
Amphioxus, Ammocoete larva, etc. These animals have varied types of filtering and trapping devices.
Cilia and setae produce water currents which carry the food particles to the the feeding surfaces.
These particles get entangled in the mucus cord which is carried into the digestive, tract by the aid of
ciliary movement. Certain sea-cucumbers have sticky tentacles which trap small organisms. These
tentacles are then thrust into the pharynx in order to wipe off food from them.
Although filter feeding is a characteristic feature of sessile and sedentary group of animals, it is
also present in a few other animal groups. Small active copepods, sock-eye salmon, the huge basking
shark and the whalebone whale are all filter feeders. The filter feeders do not select their food, hence
they are known as nonselective feeders. However, they respond to certain chemicals and stimuli by
operating, or by preventing the filter mechanism, depending on favourable or hazaradous conditions.
In other nonselective feeders such as many annelids, some echinoderms and hemichordates, food
enters the body by water currents. From this whatever is needed by the body is absorbed and the rest
discarded. The animals of this group are omnivorous provided those which come in their way should
be sufficiently small to be collected in their filters or traps. Thus collected food is subjected in their
filters or traps. The collected food is subjected to grinding when such organs are present. If absent,
the food material is passed directly for chemical digestion.
Selective feeders employ various methods for the capture and effective utilization of bulky foods
or for withdrawing the juices from animals and plants. Each mechanism is supported by suitable
anatomical modifications and physiological alterations. While the fishes, amphibians, reptiles and
many birds swallow their food without chewing, the mammals masticate food and supply it to the
digestive system in the form of small size particles. For efficient mastication, changes have been
brought about in the jaw bones and muscles. The air passages are separated and guarded by soft
epiglottis and pharyngeal folds in order to facilitate breathing during mastication. These changes are
directly related to the changes in the feeding habits.
The dependence of selective feeders on specific types of food is so great that in its absence they
suffer from functional irregularities. The female mosquito, for instance, needs a meal of blood for the
development of eggs. The rabbit flea, Spilopsyllus cuniculi, requires the blood of breeding, or
pregnant rabbit, containing reproductive hormones, to carry out its own reproductive cycle.
Selective feeders have neurosensory and neuromuscular specializations to enable them to locate
and capture specific foods. The neurosensory abilities may be in the development of visual, chemical
and other stimuli.