2. Tertiary Structure of Protein
o The three-dimensional arrangement of protein structure is
referred to as tertiary structure. It is a compact structure with
hydrophobic side chains held interior while the hydrophilic groups
are on the surface of the protein molecule. This type of arrangement
ensures stability of the molecule.
o Domains : The term domain is used to represent the basic units of
protein structure (tertiary) and function. A polypeptide with 200
amino acids normally consists of two or more domains.
3.
4. Quaternary Structure of Protein
o A great majority of the proteins are composed of single polypeptide chains.
Some of the proteins, however, consist of two or more polypeptides which may
be identical or unrelated. Such proteins are termed as oligomers and possess
quaternary structure. The individual polypeptide chains are known as
monomers, protomers or subunits. A dimer consits of two polypeptides while a
tetramer has four.
oImportance of oligomeric proteins : These proteins play a significant role in the
regulation of metabolism and cellular function.
oExamples of oligomeric proteins : Hemoglobin, aspartate transcarbomylase,
lactate dehydrogenase.
5.
6. Denaturation
The phenomenon of disorganization of native protein structure is known as
denaturation. Denaturation results in the loss of secondary, tertiary and
quaternary structure of proteins. This involves a change in physical, chemical
and biological properties of protein molecules.
Agents of denaturation
Physical agents : Heat, violent shaking, X-rays, UV radiation.
Chemical agents : Acids, alkalies, organic solvents (ether, alcohol), salts of heavy
metals (Pb, Hg), urea, salicylate, detergents (e.g. sodium dodecyl sulfate).
7. Characteristics of Denaturation
1. The native helical structure of protein is lost.
2. The primary structure of a protein with peptide linkages remains intact i.e.,
peptide bonds are not hydrolysed.
3. The protein loses its biological activity.
4. Denatured protein becomes insoluble in the solvent in which it was originally
soluble.
5. The viscosity of denatured protein (solution) increases while its surface
tension decreases.
8. Cont…
6. Denaturation is usually irreversible. For instance, omelet can be prepared
from an egg (protein-albumin) but the reversal is not possible.
7. Careful denaturation is sometimes reversible (known as renaturation).
Hemoglobin undergoes denaturation in the presence of salicylate. By removal
of salicylate, hemoglobin is renatured.
8. Denatured protein cannot be crystallized.
9. Denatured protein is more easily digested. This is due to increased exposure
of peptide bonds to enzymes. Cooking causes protein denaturation and,
therefore, cooked food (protein) is more easily digested. Further,
denaturation of dietary protein by gastric HCl enchances protein digestion by
pepsin.
9. Classification of Proteins
oFunctional classification of proteins
1. Structural proteins : Keratin of hair and nails, collagen of bone.
2. Enzymes or catalytic proteins : Hexokinase, pepsin.
3. Transport proteins : Hemoglobin, serum albumin.
4. Hormonal proteins : Insulin, growth hormone.
5. Contractile proteins : Actin, myosin.
6. Storage proteins : Ovalbumin, glutelin.
7. Genetic proteins : Nucleoproteins.
8. Defense proteins : Snake venoms, Immunoglobulins.
9. Receptor proteins for hormones, viruses.
10. Protein classification based on chemical nature and solubility
This is a more comprehensive and popular classification of proteins. It is based on the
amino acid composition, structure, shape and solubility properties. Proteins are broadly
classified into 3 major groups
1. Simple proteins : They are composed of only amino acid residues.
2. Conjugated proteins : Besides the amino acids, these proteins contain a non-protein moiety
known as prosthetic group or conjugating group.
3. Derived proteins : These are the denatured or degraded products of simple and conjugated
proteins.
11.
12. Plasma Proteins
A) Albumin
o Albumin is the major constituent (60%) of plasma proteins with a
concentration of 3.5–5.0g/dl. Human albumin has a molecular weight of 69,000,
and consists of a single polypeptide chain of 585 amino acids with 17 disulfide
bonds.
oSynthesis of albumin: Albumin is exclusively synthesized by the liver. For this
reason, measurement of serum albumin concentration is conveniently used to
assess liver function (synthesis decreased in liver diseases). Liver produces about
12 g albumin per day which represents 25% of the total hepatic protein
synthesis. Albumin has an half-life of 20 days.
13. Functions of albumin
Plasma albumin performs osmotic, transport and nutritive functions.
Osmotic function : Due to its high concentration and low molecular weight, albumin contributes
to 75–80% of the total plasma osmotic pressure (25 mm Hg). Thus, albumin plays a predominant
role in maintaining blood volume and body fluid distribution. Decrease in plasma albumin level
results in a fall in osmotic pressure, leading to enhanced fluid retention in tissue spaces, causing
edema.
Transport functions : Plasma albumin binds to several biochemically important compounds and
transports them in the circulation. These include free fatty acids, bilirubin, steroid hormones,
calcium and copper.
Nutritive functions : Albumin serves as a source of amino acids for tissue protein synthesis to a
limited extent, particularly in nutritional deprivation of amino acids.
Buffering function : Among the plasma proteins, albumin has the maximum buffering capacity.
However, the buffering action of albumin in plasma is not significant compared to bicarbonate
buffer system.
14. Cont…
B) Globulins
The globulins are a family of globular proteins that have higher molecular weights than albumins
and are insoluble in pure water but dissolve in dilute salt solutions. Some globulins are produced
in the liver, while others are made by the immune system.
Alpha Globulins:
◦ Examples include alpha-1 antitrypsin and alpha-2 macroglobulin.
◦ Function as enzyme inhibitors, protecting tissues from damage.
◦ Act as acute phase proteins, responding to inflammation.
Beta Globulins:
◦ Examples include beta-2 microglobulin and some clotting factors like beta fibrinogen.
◦ Beta globulins are involved in the transport of substances in the blood.
◦ Some play a role in the blood clotting process.
15. Cont…
Gamma Globulins:
oMainly composed of immunoglobulins (antibodies).
oEssential components of the immune system.
oRecognize and neutralize pathogens, contributing to the body's defense against infections
Immunoglobulins
The higher vertebrates, including man, have evolved a defense system to protect themselves
against the invasion of foreign substances e.g, a virus, a bacterium or a protein. The defense
strategies of the body are collectively referred to as immunity, and are briefly described under
immunology.
16. Classes of Immunoglobulins
Humans have five classes of immunoglobulins namely IgG, IgA, IgM, IgD and IgE.
1- Immunoglobulin G (IgG)
IgG is the most abundant (75–80%) class of immunoglobulins. IgG is composed of a single Y-
shaped unit (monomer). It can traverse blood vessels readily. IgG is the only immunoglobulin
that can cross the placenta and transfer the mother’s immunity to the developing fetus. IgG
triggers foreign cell destruction.
2- Immunoglobulin A (IgA)
IgA occurs as a single (monomer) or double unit (dimer) held together by J chain. It is mostly
found in the body secretions such as saliva, tears, sweat, milk and the walls of intestine. IgA is
the most predominant antibody in the colostrum, the initial secretion from the mother’s breast
after a baby is born. The IgA molecules bind with bacterial antigens present on the body (outer
epithelial) surfaces and remove them. In this way, IgA prevents the foreign substances from
entering the body cells.
17. Cont…
3-Immunoglobulin M (IgM)
IgM is the largest immunoglobulin composed of 5 Y-shaped units (IgG type) held together by a J
polypeptide chain. Thus IgM is a pentamer. Due to its large size, IgM cannot traverse blood
vessels, hence it is restricted to the blood stream. IgM is the first antibody to be produced in
response to an antigen and is the most effective against invading microorganisms. It may be
noted that IgM can simultaneously combine with 5 antigenic sites due to its pentameric
structure.
4-Immunoglobulin D (IgD)
IgD is composed of a single Y-shaped unit and is present in a low concentration in the
circulation. IgD molecules are present on the surface of B cells. Their function, however, is not
known for certain. Some workers believe that IgD may function as B-cell receptor.
5-Immunoglobulin E (IgE)
IgE is a single Y-shaped monomer. It is normally present in minute concentration in blood. IgE
levels are elevated in individuals with allergies as it is associated with the body’s allergic
responses. The IgE molecules tightly bind with mast cells which release histamine and cause
allergy.
18. C- Fibrinogen
Fibrinogen is a soluble plasma glycoprotein, one of the key components of the
blood clotting system. It plays a crucial role in the blood coagulation process,
which is essential for wound healing and preventing excessive bleeding.
Fibrinogen is produced by the liver and circulates in the blood in an inactive
form.
When there is tissue injury or damage to blood vessels, a series of events
known as the coagulation cascade is triggered. Fibrinogen is converted into
fibrin through enzymatic reactions involving thrombin. Fibrin molecules then
aggregate and form a mesh-like structure, which stabilizes the blood clot and
helps to stop bleeding.
