2. FACULTY OF PHARMACY &
PHARMACEUTICAL
SCIENCES.
DEPARTMENT OF
PHARMACOLOGY.
TEACHER; MA’AM
DR.AFSHAN SIDDIQ.
PREPARED BY;
SYED UL ARFEEN.
ASSIGNMENTOF
BIOCHEMISTRY.
TOPICS;
1.STEROIDS AND
STEROLS
2.METABOLISM
OF
CHOLESTEROL.

3. STEROIDS& STEROLs

4. STEROIDS:
 A steroid is biologically active organic compound with four rings arranged
in a specific molecular configuration.
 The large number of compounds in the nature are called steroids. The
steroid is typically composed of seventeen carbon atoms, bonded in four
fused rings: three six-member cyclohexane rings(rings A,B & C in the first
illustration) and one five-member cyclopentane ring(the D ring).
The ring is termed as ‘Cyclopentanophenanthrene ring’. One methyl
group is usually attached at carbon no 10 & second methyl group at
carbon no 13.

5.  Hundreds of steroids are found in plants, animals and fungi. All steroids
are manufactured in cells from the sterols lanosterol (opisthokonts) or
cycloartenol (plants). Lanosterol and cycloartenol are derived from the
cyclization of the triterpene squalene.
 Steroids vary by the functional groups attached to this four-ring core and
by the oxidation state of the rings.
Biological significance;
 Steroids and their metabolites often function as signaling molecules
and are components of cell membranes. Steroids such as cholesterol
decrease membrane fluidity. Similar to lipids, steroids are highly
concentrated energy stores. However, in mammals, they are normally
metabolized and excreted.
 Steroids play critical roles in a number of disorders, including
malignancies like prostate cancer, where steroid production
promotes cancer cell aggressiveness.

6. Catabolism and excretion;
 Steroids are primarily oxidized by cytochrome P450 oxidase
enzymes, such as CYP3A4. These reactions introduce oxygen
into the steroid ring, allowing the cholesterol to be broken up
by other enzymes into bile acids. These acids can then be
eliminated by secretion from the liver in bile. The expression of
the oxidase gene can be up regulated by the steroid sensor
PXR when there is a high blood concentration of steroids.
Steroid hormones, lacking the side chain of cholesterol and bile
acids, are typically hydroxylated at various ring positions or
oxidized at the 17 position, conjugated with sulfate or
glucuronic acid and excreted in the urine.

7. Sterols;
 Sterol is a chemical compound with formula C17H28O, whose
molecule is derived from that of gonane by replacement of a
hydrogen atom in position 3 by a hydroxyl group. It is therefore an
alcohol of gonane.
 Sterols, also known as steroid alcohols are a subgroup of steroids &
an important class of organic molecules. They occur naturally in
plants, animals, and fungi, and can be also produced by some
bacteria (however likely with different functions).The most familiar
type of animal sterol is cholesterol, which is vital to cell membrane
structure, and functions as a precursor to fat-soluble vitamins and
steroid hormones.
 While technically alcohols, sterols are classified by biochemists as
lipids (fats in the broader sense of the term).
 Due to the presence of Hydroxyl (-OH) group, sterol can form
esters. It is either in free form or bonded form in the body which
is termed as “Esters of the fatty acids”.

8. Types;
 Sterols of plants are called phytosterols
and sterols of animals are called
zoosterols. The most important zoosterol
is cholesterol; notable phytosterols include
campesterol, sitosterol, and stigmasterol.
Ergosterol is a sterol present in the cell
membrane of fungi.
Role in biochemistry;
 Sterols and related compounds play essential roles in the
physiology of eukaryotic organisms. For example, cholesterol forms
part of the cellular membrane in animals, where it affects the cell
membrane's fluidity and serves as secondary messenger in
developmental signaling. In humans and other animals,
corticosteroids such as cortisol act as signaling compounds in
cellular communication and general metabolism.
Sterols are common components of human skin oils.

9. Phytosterols as a nutritional
supplement;
 Phytosterols, more commonly known as plant sterols, have been
shown in clinical trials to block cholesterol absorption sites in the
human intestine, thus helping to reduce cholesterol absorption in
humans. They are currently approved by the U.S. Food and Drug
Administration for use as a food supplement; however, there is
some concern that they may block absorption not only of
cholesterol, but of other important nutrients as well. At present, the
American Heart Association has recommended that supplemental
plant sterols be taken only by those diagnosed with elevated
cholesterol, and has particularly recommended that they not be
taken by pregnant women or nursing mothers. Preliminary
research has shown that phytosterols may have anticancer effects.

10. Examples of Sterol:-
 Cholestrol
 Bile acids & Bile salts(which are present in the body & they will be
synthesize in gall bladder).
 Ergosterol (which is present in the cell membrane of fungi or yeast,
where it serves a role similar to cholesterol on animal cells).
Q. Why we use the term “Sterol” for subgroup?
Ans. Because it is containing an Hydroxyl moiety.

11.  There are various hormones especially sex hormones i.e.
Progestogens:
Progesterone, which regulates cyclical changes in the endometrium of
the uterus and maintains a pregnancy.
Androgens:
Testosterone, which contributes to the development and maintenance
of male secondary sex characteristics
Estrogens:
Estradiol, which contributes to the development and maintenance of
female secondary sex characteristics.
are sterol in nature.
Difference b/w cholesterol & ergosterol

12.  7-dehydrocholestrol is present in many tissues especially under the layer
of skin. Whenever skin exposes to ultraviolet radiation (UV rays), it is
converted to vitamin D₃(Cholecalciferol).
 7-dehydrocholestrol:-
 It is zoosterol (a sterol produced by animals rather than plants).
 It is basically the preliminary form of vitamin D.
 There are some other vitamins for example; vitamin E & vitamin K which
are also exist in the form of such type of ring structures.
NOTE: 7-Dehydrocholestrol is an intermediate from cholesterol
biosynthesis that is a precursor of Vitamin D₃ (Cholecalciferol).

13.  Ergosterol:-
Ergoster oil is the main i.e.derived from fungal cell membrane or yeast &
the name of this compound is given due to it is first isolated from Ergot
fungus. In human nutrition, Ergosterol is provitamin form of vitamin D₂;
exposure to ultraviolet light (UV)light causes a chemical reaction that
produces Ergocalciferol (also known as vitamin D₂ & calciferol).

14. CHOLESTEROL

15. INTRODUCTION:
 Cholesterol is the modified form of steroid which is containing Hydroxyl (-
OH) group.
 It is extremely important biological molecule that has the role in the
membrane structure as well as precursor for the biosynthesis of steroid
hormones, bile acid & vitamin D.
 Both dietary cholesterol & cholesterol synthesize DE novo (which is
synthesizing in the body itself freshly) are transported through the
circulation in Lipoprotein particles.
Lipoprotein is one of the form of compound or complex lipid which are
conjugated with the protein particles.
 Cholesterol is the main ingredient which is circulated within the blood in
terms of Lipoprotein.

16. Types:
Types of Lipoproteins are:
1. LDL (Low Density Lipoprotein)
2. HDL (High Density Lipoprotein)
3. VLDL (Very Low Density Lipoprotein)
4. Chylomicrons (Ultra Low Density Lipoprotein)
NOTE: up to 70-80% of DE novo cholesterol synthesis occur in liver &
About 10% of DE novo cholesterol synthesis occur in small intestine.
 Once, Cholesterol is synthesized, it ultimately go to synthesize
hormones and it is also going to synthesize bile acid and bile salt which
are basically facilitating the absorption of lipid components which are
taken by the oral route of administration.

17.  The synthesis & utilization is tightly regulated in human body in order to
prevent over accumulation & abnormal deposition of Cholesterol within
the body. This is particularly clinically important, if the abnormal
deposition of cholesterol or cholesterol rich lipoprotein occur in the
coronary artery it eventually leads Atherosclerotic Cardiovascular
problem & it will ultimately result in coronary artery disease.

18. Structural difference of
Cholesterol & Ergosterol:
 If you check the structure of cholesterol it contains a Hydroxyl(-OH) group
& a side of 8 number Carbon which are attached in terms of methyl 2.
 If you check or differentiate between Ergosterol & Cholesterol so,
Ergosterol containing 2 double(=) bond in ring structure while if you
compare Cholesterol side chain as well as Ergosterol side chains, you
will find 1 double (=) bond in the side chain of Ergosterol.

19. Functions of Cholesterol:
 Cholesterol increases membrane packing, which both alters
membrane fluidity and maintains membrane integrity so that
animal cells do not need to build cell walls (like plants and most
bacteria). The membrane remains stable and durable without being
rigid, allowing animal cells to change shape and animals to move.
 Within the cell membrane, cholesterol also functions in
intracellular transport, cell signaling and nerve conduction.
 Cholesterol regulates the biological process of substrate
presentation and the enzymes that use substrate presentation as a
mechanism of their activation. (PLD2) is a well-defined example of
an enzyme activated by substrate presentation.
 Within cells, cholesterol is also a precursor molecule for several
biochemical pathways. For example, it is the precursor molecule
for the synthesis of vitamin D in the calcium metabolism and all
steroid hormones, including the adrenal gland hormones cortisol
and aldosterone, as well as the sex hormones progesterone,
estrogens, and testosterone, and their derivatives

20. BIOSYNTHSIS OF CHOLESTEROL:
 All animal cells manufacture cholesterol, for both membrane structure and
other uses, with relative production rates varying by cell type and organ
function.
 It is slightly less than half of the cholesterol in the body which is derived
from biosynthesis DENOVO.
 Biosynthesis of Cholesterol is approximately 70-80% in the liver while
10% in the Small intestine.
 Cholesterol synthesis mainly occurs in cytoplasm & in endoplasmic
reticulum.
 All the compounds as well as fatty acids are synthesized from Acetyl-CoA.
So, the 2 acetate carbon group i.e. Acetyl-CoA is the main precursor for
the synthesis of Cholesterol too.
 The synthesis of Cholesterol just similar like fatty acid & β-oxidation of
fatty acid. NADPH2 as a Co-factor, several different enzymes & Co-
enzyme such as Co-enzyme Q (which is basically oxidative
phosphorylating Enzyme) are required for the synthesis of
Cholesterol.

21.  Palmitic Acid which is well known and commonly available fatty acid in
the body which is undergoing into β-oxidation and once it undergoes β-
oxidation, it will synthesize and give an ultimately product i.e. Acetyl-CoA
and this Acetyl-CoA is the building block for the synthesis of Cholesterol.
 Once, Cholesterol is synthesized, it ultimately go to synthesize
hormones & it is also going to synthesize bile acid & bile salts which are
basically facilitating the absorption of lipid components which are taken
by the oral route of administration.
SYNTHESIS OF CHOLESTEROL NEEDS:
ATP,ADPH2, Magnesium, Manganese ion

22. STEPS OF CHOLESTROL SYNTHESIS:
The process of Cholestrol synthesis has following major steps:
 Synthesis of HMG-CoA:
• In this first step, Acetyl-CoA & Acetoacetyl-CoA are converted into 3-
hydroxy-3-methylglutaryl-CoA(HMG-CoA), an intermediate compound in
cholestrol synthesis, by the help of HMG-CoA synthase enzyme.

23.  Synthesis of Mevalonate:
• In the second step, 3-hydroxy-3-methylglutaryl-CoA(HMG-CoA) is
converted into Mevalonate (by help of an HMG-CoA reductase,
liberating HS-CoA & NADP+ molecules.
• Production of mevalonate is the rate-limiting and irreversible step in
cholesterol synthesis and is the site of action for statins (a class of
cholesterol-lowering drugs).

24.  Production of isoperinoid units:
• In the third step, Mevalonate is finally converted to isopentenyl
pyrophosphate (IPP) through two phosphorylation steps and one
decarboxylation step that requires ATP.

25.  Synthesis of farnesyl pyrophosphate:
• Three molecules of isopentenyl pyrophosphate condense to form
farnesyl pyrophosphate through the action of geranyl transferase.

26.  Synthesis of Squalene:
• In the fourth step, Two molecules of farnesyl pyrophosphate then
condense to form squalene by the action of squalene synthase in
the endoplasmic reticulum.

27.  Conversion of Squalene to Cholesterol:
• Oxidosqualene cyclase then cyclizes squalene to form lanosterol.
Finally, lanosterol is converted to cholesterol through a 19-step
process.
• The final 19 steps to cholesterol contain NADPH and oxygen to help
oxidize methyl groups for removal of carbons, mutases to move
alkene groups, and NADH to help reduce ketones.

28. • Now, once the cholesterol is synthesized, this cholesterol is responsible to
synthesize the bile salt in the liver. It also synthesizes the steroidal
hormones in the Endocrine gland.

29. • Normall plasma level range is 150-220 mg/dl but a level of 200mg/dl is
present as considered to be maximum desirable. Some 140gm of
cholesterol may be present in the ideal body.
• All tissues in the human body can synthesize cholesterol but the most
active one is the Liver, Small intestine, Skin, Aorta, Adrenal cortex and
Gonads etc.
• Liver is quantitatively the most important part in respect. It forms around
to 1-1.5gm of Cholesterol per day.
• All 27 carbons of Cholesterol are originated are Acetyl-CoA. Therefore,
Cholestrol not an essential ingredient of human diet although it is
essential for life.
• The average normal level of Cholesterol is about 20mg/dl. About 70% of
the cholesterol occur in the esterified form. The plasma cholesterol level
in a young man average is 170mg/dl. A progressive rise will occur with
the age.
Q)Why Cholestrol is essential for life?
Ans) Because it can synthesize in body. It is not
necessary that you have to take excessive amount of
cholestrol in the diet.

30. METABOLISMOF CHOLESTEROL:
• Bile salt & Bile acids are the products of Cholesterol.
• The Bile salt or Bile acid are basically derived from Cholesterol.
• Quantitatively, the most important primary Bile acids that are secreted in
bile are Chenodeoxycholic acid & Cholic acid.
• The most abundant bile acids in human bile are Chenodeoxycholic
acid(45%) & Cholic acid acid(31%).
 Cholic acid;
• It is a primary bile acid that is insoluble in water.
Formula; C24H40O5

31.  Chenodeoxycholic acid;
It is a bile acid naturally found in the body. It works
by dissolving cholesterol that makes gallstone &
inhibiting production of cholesterol in liver &
absorption in the intestines which helps to increase
the formation of gallstone.
Formula; C24H40O4

32. • Cholic acid & Chenodeoxycholic acid are conjugated in the Liver
with Glycine and Taurine. Glycine and Taurine are the amino acid,
after conjugation, they become Glycocholic acid and Taurocholic
acid and Glycochenodeoxycholic acid & taurochenodeoxycholic
acid respectively.

33. FormationofGlycocholicAcidfromCholicAcid:
• Cholic Acid reacts with Co-A by the utilization of ATP(Adenosine Tri
Phosphate) to synthesize Cholyl-CoA & one molecule of Pyrophosphate
(PPi), with liberating AMP(Adenosine Mono Phosphate).
• Cholyl-CoA then reacts with Glycine to synthesize Glycocholic Acid & Co-A
is liberated.

34. FormationofTaurocholicAcidfromCholicAcid:
• Cholic Acid reacts with Co-A & ATP(Adenosine Tri Phosphate) to
synthesize Cholyl-CoA & one molecule of Pyrophosphate (PPi),with
liberating AMP(Adenosine Mono Phosphate)
Cholyl-CoA then reacts with Taurine to synthesize Taurocholic Acid &
Co-A is liberated.
• Taurocholic acid also known as cholyltaurine, is a deliquescent
yellowish crystalline bile acid involved in emulsification of fats. It
occurs as a sodium salt in bile.

35. • During conjugation, union between carboxylic acid(COOH) of the
bile acid & NH2 of the Glycine & Taurine will take place.
• Chenodeoxycholic acid is also conjugated in same way as Cholic
Acid is conjugated.
• Taurine is derived from Sulphur (S)
containing amino acid.
At physiological pH, the conjugated Bile Acids
ionize much more easily to react with Na+ than
the primary Bile Acid and occurs as sodium salt
and are termed as therefore Bile salt.

36. ENTEROHEPATIC CIRCULATION
OF BILE SALT:
• The physiological role of Bile salts are that they are mostly reabsorbed
from the intestinal lumen & are resecreted by the Liver into Bile.
• The reabsorption of Bile salt will take place in the upper part of small
intestine. However, less than 10% of Bile salt escape from absorption
and more than 90% are reabsorbed in distal ileum by an active process.
• The Enterohepatic circulation (refers to circulation of biliary acids,
bilirubin, drugs or other substances from Liver to the Bile) of Bile salt is
so marked that Bile salt pass through 5 to 10 such cycles before being
eliminated in the faces.
• After performing a emulsification the primary Bile salts reach the distal
ileum part where they undergo bacterial action & are partly converted
into secondary Bile salts of Deoxycholic Acid & Lithocholic Acids.
These two acids are 3,4-dihydroxy & 3-hydroxy derivatives of
Cholanic Acid.

37. • These changes in the steroidal ring appears to render the products less
absorbable. Thus, the action of intestinal bacteria helps in elimination
of Bile salt from the intestine.
• Normally, about 0.8gm of Bile salt is excreted in the faeces per day.
• The Liver has to synthesize this amount i.e. 0.8 gm/dl of the Bile salt
from Cholesterol.
• The synthetic activity of the liver is limited & in the absence of iliac
absorption; the amount of bile salts reaching the small intestine may
fall considerably & results in an impaired lipid absorption.
• So, here we need to understand the pathological phenomena.
Sometimes, certain iliac problems or certain carcinoma in small
intestine occurs which is an area of absorption of not only Bile salts but
also other components if that is dissected or it is eliminated from the
body.
• If Ileostomy is done i.e. the portion of ileum is removed due to any
surgical intervention or due to any disease that is life threatening. So,
in that case, the reabsorption of Bile salt will not take place because
the
amount whatever is eliminating the liver i.e;0.8gm per day that is
absolutely eliminated out due to unavailability of area of the Ileum
which has been removed out of body.

38. • So, definitely if this pathological condition occurs that will impaired
lipid absorption because lipid absorption basically depends upon the
availability of Bile salts. They are emulsifying whatever the lipid we
are taking in terms of diet.
• So, along with the lipids, bile salts are also reabsorbed & they are
recycled & if this recycling process is not present, ultimately the
deficiency or the reduction of the Bile salts will take place in the body.

39. THEEND