Proteins  Definition, classification of proteins based on composition and solubility with examples  Definition, classification of amino acids based on chemical nature and nutritional requirements with examples  Structure of proteins (four levels of organization of protein structure)  Qualitative tests and biological role of proteins and amino acids  Diseases related to malnutrition of proteins.

1. PROTEINS

2. Proteins are naturally occurring polymers made up of amino acids linked together
by peptide bonds having high molecular weight biomolecules, found in all living
systems.
Functions –
1. Proteins which catalyze biochemical reactions are called enzymes.
2. Proteins responsible for transportation of metabolites or gases (like oxygen,
carbon-di-oxide etc.) are called transport proteins.
3. Proteins, which control metabolic pathways are called regulatory proteins. For
example insulin, hormone receptors etc.
4. Proteins which are responsible to protect from infection and other toxic
substances are called antibodies or defence proteins.
5. Proteins which are required to give strength to cells or tissues are called
structural proteins.
6. Proteins which are required to carry out mechanical work are called muscle
proteins.
PROTEINS

3. AMINO ACID-
Twenty amino acids are found in proteins and they are called standard amino acids. These
amino acids contain the carboxyl group and the amino group attached to α-carbon. Hence,
all these are called as α-amino acids. This is the common structural feature of all α-amino
acids. The general structure of α-amino acids is represented as follows-
PROTEINS

4. AMINO ACID-
Classification-
PROTEINS

5. AMINO ACID-
Classification-
PROTEINS

6. AMINO ACID-
Classification-
PROTEINS

7. AMINO ACID-
Classification-
PROTEINS

8. AMINO ACID-
However some of the amino acids of physiological importance not found in
proteins are as under-
(i) B-Alanine
(ii) g-Aminobutyric acid (GABA)
(iii) d-Amino laevolinic acid (d ALA).
(iv) Creatin
(v) Ornithine
Amino acids are obtained from proteins by hydrolysis, catalysed by acid, base or
enzymes such as pepsin, trypsin and chymotrypsin.
Amino acids are required for the body for various reasons i.e.
(1) For synthesis of various enzymes, hormones, plasma-proteins and
immunoglobulins.
(2) For the growth and repairs of body tissues.
(3) Source of energy when body is having inadequate supply of carbohydrates or
fats.
PROTEINS

9. AMINO ACID-
Classsification of amino acid on the basis of their structure-
1. Aliphatic amino acids :
These amino acids show presence of simple or branched aliphatic
side chains in R group.
e.g. Glycine, alanine, valine, leucine and isoleucine.
2. Aromatic amino acids :
This class of amino acids contain aromatic ring structure in
R group.
e.g. phenyl alanine, tyrosine, and tryptophan.
3. Acidic amino acids :
The amino acids in this class contain carboxylic acid group
(–COOH) in the substituent R group.
e.g. Glutamic acid aspargine. Glutamine and aspargine are their
respective amide forms.
4. Basic amino acids :
These amino acids are basic in nature.
e.g. Histidine, lysine and arginine.
PROTEINS

10. AMINO ACID-
Classsification of amino acid on the basis of their structure-
5. Sulphur containing amino acids :
This class of amino acids contain sulphur atom in the structure.
e.g. Methionine, cysteine.
6. Hydroxyl group containing amino acids :
This type of amino acids show presence of hydroxyl group in the
structure.
e.g. Serine and threonine.
7. Imino acids :
These amino acids contain imino group (=NH) in place of amino
group (– NH2), i.e. a-amino nitrogen is part of a ring structure.
e.g. Proline.
PROTEINS

11. AMINO ACID-
Classsification of amino acid on the basis of nutritional requirements-
Essential and Non-essential amino acids
Essential amino acids :
The amino acids which cannot be synthesized in the body but are required for
normal functioning of body are called as essential amino acids. These amino acids
should be supplied through diet.
Non-essential amino acids :
The amino acids which are synthesized in the body are called as non-essential
amino acids.
PROTEINS

12. AMINO ACID-
Classsification of amino acid on the basis of nutritional requirements-
PROTEINS

13. Classification of proteins
I- Simple proteins:
i.e. on hydrolysis gives only amino acids
Examples:
1- Albumin and globulins: present in egg, milk and blood
They are proteins of high biological value i.e. contain all essential amino
acids and easily digested.
Types of globulins:
α1 globulin: e.g. antitrypsin: see later
α2 globulin: e.g. hepatoglobin: protein that binds hemoglobin to
prevent its excretion by the kidney
β-globulin: e.g. transferrin: protein that transport iron
γ-globulins = Immunoglobulins (antibodies) : responsible for
immunity.
PROTEINS

14. 2- Globins (Histones): They are basic proteins rich in histidine amino acid.
They are present in : a - combined with DNA
b - combined with heme to form hemoglobin of RBCs.
3- Gliadines are the proteins present in cereals.
4- Scleroproteins: They are structural proteins, not digested.
include: keratin, collagen and elastin.
a- α-keratin: protein found in hair, nails, enamel of teeth and outer layer of skin.
• It is α-helical polypeptide chain, rich in cysteine and hydrophobic (non polar) amino
acids so it is water insoluble.
b- collagens: protein of connective tissues found in bone, teeth, cartilage, tendons, skin
and blood vessels.
PROTEINS

15. PROTEINS
• Collagen may be present as gel e.g. in extracellular matrix or in
vitreous humor of the eye.
• Collagens are the most important protein in mammals. They form
about 30% of total body proteins.
• There are more than 20 types of collagens, the most common type
is collagen I which constitutes about 90% of cell collagens.
• Structure of collagen: three helical polypeptide chains (trimeric)
twisted around each other forming triplet-helix molecule.
• ⅓ of structure is glycine, 10% proline, 10% hydroxyproline and
1% hydroxylysine. Glycine is found in every third position of the
chain. The repeating sequence –Gly-X-Y-, where X is frequently
proline and Y is often hydroxyproline and can be hydroxylysine.

16. PROTEINS
Solubility: collagen is insoluble in all solvents and not digested.
• When collagen is heated with water or dil. HCl it will be converted into gelatin
which is soluble , digestible and used as diet ( as jelly). Gelatin is classified as
derived protein.
Some collagen diseases:
1- Scurvy: disease due to deficiency of vitamin C which is important coenzyme for
conversion of proline into hydroxyproline and lysine into hydroxylysine. Thus,
synthesis of collagen is decreased leading to abnormal bone development, bleeding,
loosing of teeth and swollen gum.
2- Osteogenesis Imperfecta (OI): Inherited disease resulting from genetic
deficiency or mutation in gene that synthesizes collagen type I leading to abnormal
bone formation in babies and frequent bone fracture in children. It may be lethal.

17. PROTEINS
C- Elastin: present in walls of large blood vessels (such as aorta). It is very important
in lungs, elastic ligaments, skin, cartilage, ..
It is elastic fiber that can be stretched to several times as its normal length.
Structure: composed of 4 polypeptide chains (tetramer), similar to collagen being
having 33% glycine and rich in proline but in that it has low
hydroxyproline and absence of hydroxy lysine.
Emphysema: is a chronic obstructive lung disease (obstruction of air ways) resulting
from deficiency of α1-antitrypsin particularly in cigarette smokers.
Role of α1-antitrypsin: Elastin is a lung protein. Smoke stimulate enzyme called
elastase to be secreted form neutrophils (in lung).
Elastase cause destruction of elastin of lung.

18. PROTEINS
α1-antitrypsin is an enzyme (secreted from liver) and inhibit elastase
and prevent destruction of elastin. So deficiency of α1-antitrypsin
especially in smokers leads to degradation of lung and destruction of
lung ( loss of elasticity of lung, a disease called emphysema.
Conjugated proteins
i.e. On hydrolysis, give protein part and non protein part and
subclassified into:
1- Phosphoproteins: These are proteins conjugated with phosphate
group. Phosphorus is attached to oh group of serine or threonine.
e.g. Casein of milk and vitellin of yolk.

19. PROTEINS
2- Lipoproteins:
These are proteins conjugated with lipids.
Functions: a- help lipids to transport in blood
b- Enter in cell membrane structure helping lipid soluble substances
to pass through cell membranes.
3- Glycoproteins:
proteins conjugated with sugar (carbohydrate)
e.g. – Mucin
- Some hormones such as erythropoeitin
- present in cell membrane structure
- blood groups.
4- Nucleoproteins: These are basic proteins ( e.g. histones) conjugated with
nucleic acid (DNA or RNA).
e.g. a- chromosomes: are proteins conjugated with DNA
b- Ribosomes: are proteins conjugated with RNA

20. PROTEINS
5- Metalloproteins: These are proteins conjugated with metal like iron, copper, zinc,
……
a- Iron-containing proteins: Iron may present in heme such as in
- hemoglobin (Hb)
- myoglobin ( protein of skeletal muscles and cardiacmuscle),
- cytochromes,
- catalase, peroxidases (destroy H2O2)
- tryptophan pyrrolase (desrtroy indole ring of tryptophan).
Iron may be present in free state ( not in heme) as in:
- Ferritin: Main store of iron in the body. ferritin is present in liver, spleen and bone
marrow.
- Hemosidrin: another iron store.
- Transferrin: is the iron carrier protein in plasma.

21. PROTEINS
b- Copper containing proteins:
e.g. - Ceruloplasmin which oxidizes ferrous ions into ferric ions.
- Oxidase enzymes such as cytochrome oxidase.
c- Zn containing proteins: e.g. Insulin and carbonic anhydrase
d- Mg containing proteins:e.g. Kinases and phosphatases.
6-Chromoproteins: These are proteins conjugated with pigment. e.g.
- All proteins containing heme (Hb, myoglobin, ………..)
- Melanoprotein:e.g proteins of hair or iris which contain melanin.
Derived proteins
Produced from hydrolysis of simple proteins.
e.g. - Gelatin: from hydrolysis of collagen
- Peptone: from hydrolysis of albumin

22. PROTEINS
Protein structure:
There are four levels of protein structure (primary,
secondary, tertiary and quaternary)
Primary structure:
• The primary structure of a protein is its unique
sequence of amino acids.
– Lysozyme, an enzyme that attacks bacteria,
consists of a polypeptide chain of 129 amino acids.
– The precise primary structure of a protein is
determined by inherited genetic information.
– At one end is an amino acid with a free amino
group the (the N-terminus) and at the other is an
amino acid with a free carboxyl group the (the C-
terminus).

23. PROTEINS
High orders of Protein structure
• A functional protein is not just a polypeptide chain, but one or more polypeptides
precisely twisted, folded and coiled into a molecule of unique shape (conformation).
This conformation is essential for some protein function e.g. Enables a protein to
recognize and bind specifically to another molecule e.g. hormone/receptor;
enzyme/substrate and antibody/antigen.

24. PROTEINS
2- Secondary structure:
Results from hydrogen bond
formation between hydrogen of –NH
group of peptide bond and the carbonyl
oxygen of another peptide bond.
According to H-bonding there are two
main forms of secondary structure:
α-helix: It is a spiral structure resulting
from hydrogen bonding between one
peptide bond and the fourth one
β-sheets: is another form of secondary
structure in which two or more
polypeptides (or segments of the same
peptide chain) are linked together by
hydrogen bond between H- of NH- of one
chain and carbonyl oxygen of adjacent
chain (or segment).

25. PROTEINS
Hydrogen bonding in α-helix: In the α-helix CO of the one amino acid residue forms H-bond
with NH of the forth one.
Supersecondary structure or Motifs :
occurs by combining secondary structure.
The combination may be: α-helix- turn- α-helix- turn…..etc
Or: β-sheet -turn- β-sheet-turn………etc
Or: α-helix- turn- β-sheet-turn- α-helix
Turn (or bend): is short segment of polypeptides (3-4 amino acids) that connects successive
secondary structures.
e.g. β-turn: is small polypeptide that connects successive strands of β-sheets.

26. PROTEINS
• Tertiary structure is
determined by a variety of interactions (bond
formation) among R groups and between R
groups and the polypeptide backbone.
a. The weak interactions include:
 Hydrogen bonds among polar side chains
 Ionic bonds between
charged R groups ( basic and acidic amino
acids)
 Hydrophobic
interactions among
hydrophobic ( non polar) R
groups.

27. PROTEINS
b. Strong covalent bonds include disulfide bridges, that form
between the sulfhydryl groups (SH) of cysteine monomers,
stabilize the structure.

28. PROTEINS
• Quaternary structure: results from the aggregation (combination) of two or more
polypeptide subunits held together by non-covalent interaction like H-bonds,
ionic or hydrophobic interactions.
• Examples on protein having quaternary structure:
– Collagen is a fibrous protein of three polypeptides (trimeric) that are
supercoiled like a rope.
• This provides the structural strength for their role in connective tissue.
– Hemoglobin is a globular protein with four polypeptide chains (tetrameric)
– Insulin : two polypeptide chains (dimeric)

29. PROTEINS

30. PROTEINS
Biochemical Importance of proteins :
Proteins are the structural component of protoplasm, cells and tissues. Enzymes and few
hormones are proteinous in nature; antibodies, haemoglobin are also proteins. Proteins play
important role in cell-mediated immunity mechanism.
Protein is one of the important components of diet. It is required to maintain growth and
healthy functioning of the body. Proteins are also classified on a nutritional basis.
Complete proteins : Proteins which contain all the essential amino acids in required
quantities are called complete proteins.
For example, milk proteins and egg proteins are complete proteins.
Incomplete proteins : Proteins not containing all essential amino acids are called
incomplete proteins.
For example : gelatin, zein of maize etc. Thus complete proteins have higher biological
value than incomplete proteins. Basically, dietary proteins are utilised by the body to
synthesize new proteins such as enzymes, transport proteins, structural proteins, defence
proteins etc. Under certain conditions, dietary proteins or body proteins are diverted for the
production of energy. This may be due to an inadequate supply of carbohydrate and fat or
because protein intake itself is very high.

31. PROTEINS
QUALITATIVE TESTS FOR PROTEINS :
(1) Heat test : When a protein solution is heated in a boiling water bath, the proteins
get coagulated and loose their biological activity. This is called thermal denaturation
of proteins e.g. boiling of eggs. Some seeds and leaves of plants contain toxic
polypeptides.
Their toxicity is reduced by heat treatment in boiling water.
(2) Test with trichloroacetic acid (TCA) : TCA is normally used to precipitate
proteins from their solution. TCA denatures the protein. A precipitate is formed when
TCA is added to a protein solution hence, TCA is used for deproteinization.
(3) Biuret test : Biuret reagent consists of copper sulphate in an alkaline medium.
When proteins are treated with Biuret reagent, it shows a violet colour. This test is
used for the identification of the proteins and also for protein estimation.
(4) Hydrolysis test : Protein, on hydrolysis, gives free amino acids. Hydrolysis can
be carried out by acids like HCl, H2SO4 etc. or alkalis like NaOH, KOH etc. Protein
hydrolysis is a conventional source of amino acids. Extent of hydrolysis is tested by
carrying out ninhydrin and Biuret tests with respect to time for which the hydrolysis
reaction is carried out. Ninhydrin reaction is negative for protein but after hydrolysis,
it is positive as free amino acids are formed.

32. PROTEINS
QUALITATIVE TESTS FOR PROTEINS :
(5) Xanthoproteic test : Nitration of aromatic amino acids of proteins gives yellow
colour. Concentrated nitric acid is used for nitration. On the treatment of nitric acid,
proteins give yellow precipitate which turns to orange colour on the treatment with
alkali.
(6) Millon's test : Phenolic group of tyrosine of proteins react with mercuric sulphate
in the presence of sodium nitrite and sulphuric acid to give red colour.
(7) Precipitation test : Proteins are precipitated by using different agents, the
common precipitating agents are salt, organic solvents, heavy metal ions, acids etc.
The following reagents are used for the protein precipitation.
(i) Salts : Ammonium sulphate, Sodium chloride.
(ii) Organic Solvents : Acetone, alcohol.
(iii) Heavy metal ions : Sodium tungstate, ammonium molybdate, copper or mercury
salts.
(iv) Acids : Trichloroacetic acid (TCA), acetic acid, hydrochloric acid.

33. PROTEINS
Protein deficiency :
We know that proteins are required for several vital processes in the body. Naturally, a low
intake of proteins results in deficiency symptoms. Such conditions that are developed may
be due to low dietary intake or malfunctioning of the body such as mal-absorption and non-
conversions.
PROTEIN DEFICIENCY DISEASES :
Protein deficiency particularly in early childhood is regarded as the disease. The qualitative
and quantitative deficiency of proteins in children results in the disease kwashiorkor. It
appears most commonly in children between the ages of 1 to 4 years. The protein and other
nutritional deficiencies especially in infants below one year is called marasmus, to
differentiate it from kwashiorkor. In adults, the protein deficiency disease is very rare as
proteins are no longer required for growth. But long continued deprivation of proteins may
result in Nutritional edema.

34. PROTEINS
(i) Kwashiorkor : The symptoms of the disease are retarded growth, edema and
alterations in skin, hair pigmentation and texture. Frequently there is liver
enlargement. There is vomiting and diarrhoea and stools contain much undigested
food. The contributory factors for the disease are large family size, poor maternal
health, premature termination of breast feeding, poor environmental conditions and
delayed supplementary feeding. The adverse cultural practices such as use of over-
diluted cow's milk contributes for the disease. Due to poor health, the child may
become the victim of secondary infections by parasites or other germs.
Diet rich in proteins such as milk and eggs and in severe cases serum transfusion
will assist in curing the disease. Soya-beans are the best known vegetable source of
complete protein and will also be helpful in the treatment of this disease.
Implementation of preventive measure is the best way of escape from the disease.

35. PROTEINS
(ii) Marasmus : It is a disease of infants below one year of age. It's cause is protein and
carbohydrate or other nutritional factor deficiencies. Protein and energy deficiency disease is
also known as 'marasmic kwashiorkor'. Marasmus is more likely to occur in poor people. The
symptoms are more or less as in the kwashiorkor. The most important cause of marasmus is
stoppage of early breast feeding in contrast to the delayed breast feeding in kwashiorkor.
Providing diet rich in calories, proteins and other nutritional factors is the best course in
prevention and cure of the marasmus.
(iii) Nutritional Oedema : It results from long continued deprivation of proteins and usually
occurs in famine areas. The protein deficiency in adults is very rare. The deficiency
symptoms in adults include loss of weight, reduced subcutaneous fat, anaemia, greater
susceptibility to infection, frequent loose stools, general lethargy, incapacity to sustained hard
work, delay in healing of wounds and oedema. Use of soyabean, milk and eggs and other
measures appropriate to symptoms will prevent or cure the protein deficiency syndrome in
adults.
Deficiency of proteins shows different changes in the body. The liver loses proteins rapidly
and prolonged protein deficiency can cause permanent damage to a large number of liver
cells resulting into necrosis. It is also observed that plasma albumin levels fall.