2. Proteins
•Proteins are organic molecules of large mol.wt ranging from a
few thousand to a million or more.
•The term protein is derived from Greek word ‘Proteios’ which
means primary or holding first place.
•As the name indicates, this group of compounds is the most
important of cell constituents and constitute 50% or more of
their dry weight.
•The fundamental structural units of proteins is the amino acid.
All proteins are constructed from the same basic set of 20
aminoacids, covalently linked in characteristic sequence
3. Structure
• As each of these aminoacids has a distinctive side chain
which lends its chemical individuality, this group of 20
building- block molecules may be regarded as the
alphabet of protein structure.
• Combination of 20 aminoacids---Different sequence----
Different numbers----Infinte no. of proteins.
ARG LYS VAL ILE PRO ARG GLU LYS
R K V I P R E K
3-letter code
1-letter code
4. Compostition of Proteins
The fundamental structural units of proteins is the amino acid
Proteins are predominantly constituted by five major elements
in the following proportion.
Carbon 50 - 55%, Hydrogen 6 - 7.3% Oxygen 19 - 24%
Nitrogen 13 - 19% Sulfur 0 – 4 %
Besides the above, proteins may also contain
othere lementss uch as P, Fe,C u, l, Mg, Mn, Zn etc
5. Occurence
Proteins are the most abundant biological macromolecules,
occurring in all cells and all parts of cells.
proteins exhibit enormous diversity of biological function and
are the most important final products of metabolic pathways.
Proteins are the molecular instruments through which genetic
information is expressed.
6. Function of Proteins
1. Enzymatic function:
Nearly all chemical rxns in biological systems are catalyzed by
specific enzymes and all known enzymes are proteins
2. Storage and nutrition:
The seeds of many plants store nutrient proteins and similarly
ferritin of animal tissues stores iron in the liver
3. Transport:
Transport protein hemoglobin transport oxygen and
carbondioxide in erythrocytes. Similarly myoglobin transport
oxygen in muscle
4. Defensive function:
Antibodies are highly speicific protiens. Fibrinogen and
thrombin are blood clotting protiens that prevents excess
loss of blood.
7. Function of Proteins
5. Structural:
Around half of the total body is made by proteins only. Eg.
Collagen in skin and bone, elastin in ligaments, keratin in
hair and fingernails.
6. Contraction and motile function
Muscle contraction is accomplished by the sliding motion of
proteins actin and myosin
7. Regulatory functions
Hormones such as insulin, growth hormones have regulatory
functions
8. Generation and transmission of nerve impulse
The response of nerve cells to specific stimuli is mediated by
receptor proteins. Eg rhodopsin
8. Classification of Protein
On the basis of solubility, composition and physical properties
most accepted system of classification
based on the proposals made by the committees of British Physiological
Society (1907) and the American Physiological Society (1908)
9. Classification of Protein
A. Simple Proteins or Holoproteins.
This group includes proteins containing only amino acids, as structural components.
On decomposition with acids, or on complete hydrolysis these liberate the
constituent amino acids.
(a) Globular proteins:
These are spherical or oval in shape, soluble in water or other
Solvents and digestible. Eg. Albumins, Globulins, Prolamines
(b) Fibrous proteins :
These are fiber like in shape, insoluble in water and resistant to digestion,
Albuminoids or scleroproteins constitute the most predominant group of
fibrous proteins. Collagens, elastins
10. Classification of Protein
B. Conjugated or Complex Proteins or Heteroproteins.
These are the proteins linked with a separable nonprotein portion called prosthetic
group. The prosthetic group may be either a metal or a compound.
On decomposition with acids, these liberate the constituent amino acids as well as
the prosthetic group.
Their further classification is based on the nature of the prosthetic group present.
E.g.: Nucleoproteins, Glycoproteins, and lipoproteins
11. Classification of Protein
C. Derived Proteins
action of heat, enzymes or chemical reagents derivatives of proteins
artificially-produced polypeptides.
I. Primary derived proteins
denatured or coagulated or first hydrolysed products of protein No alteration of
size
II. Secondary derived proteins. These are derivatives of proteins Alteration of size
smaller than the original proteins.
12. Structures of Protein
Primary Structure
The linear sequence of aminoacid in polypeptide chain.
The aminoacids are linked to eachother by peptide
bonds formed by combination of amino group of one
amino acid with carboxyl group of another aminoacid.
Secondary Structure
The conformation of polypeptide chain by twisting or folding is referred to as
secondary structure.
Basically the bonds involved are hydrogen & disulfide bonds
Few types of secondary structure that are stable and occur widely in proteins.
– α helix
– β Sheets
14. Structures of Protein
Tertiary Structure
The polypeptide chain with secondary structure is further folded or superfolded
about itself so that the majority of its hydrophobic side-chains are buried in the
interior to form tertiary structure.
15. Structures of Protein
Quaternary Structure
Each polypeptide chain in the molecule has its own characteristic tertiary
structure and is called a subunit or monomer or protomer.
Quaternary structure(4th
level) refers to the spatial arrangement of the
polypeptidesubunits.
These interactions involved in formation of
oligomer (dimer, trimer or tetramer)
16. Physical and Chemical Properties of Protiens
1. Solubility: Proteins form colloidal solutions instead of true
solutions in water. This is due to huge size of protein
molecules. Proteins can be precipitated by dehydration or
neutralization of polar groups.
2. Molecular weight : The proteins vary in their molecular
weight.
3. Shape: There is a wide variation in the
Protein shape. lt may be globular( insulin), oval (albumin)
fibrous or elongated (fibrinogen)
4. zwitterions: At isoelectric pH, the proteins exist as zwitterions
or dipolar ions. They are electrically neutral.
17. Physical and Chemical Properties of Protiens
5. Color reactions of Proteins
a. Biruret test: All proteins and peptides
possessing at least two peptide linkages i.e., tripeptides (with 3
amino acids) give positive biuret test.
All proteins and peptides
possessing at least two peptide linkages i.e.,
tripeptides (with 3 amino acids) give positive.
in The Cu ion present in biuret reagent (Alk. CuSO4) forms the copper co-
cordinated complex with CO-NH (i.e. peptide bond)
b. Ninhydrin Reaction:
Compounds possessing α-aminoacids when heated with ninhydrin in
solution produce purple, blue or pink colour complex
(Ruhemann's purple)
18. Physical and Chemical Properties of Protiens
6. Denaturation:
The phenomenon of disorganization of native protein structure
is known as denaturation. This involves a change structure and
properties of protein molecules.
