Proteins lecture 12

1. Lecture 12
Proteins

2. Bonds responsible for protein
structure
Protein structure is stabilize by two types of bonds
Covalent and non-covalent bond
1. Covalent bonds
 Disulphide bonds
2. Non-covalent bonds
 Hydrogen bonds
 Hydrophobic bonds
 Electrostatic bonds
 Van der Waals forces

4. Denaturation
 The phenomenon of disorganization of native
protein structure is known as denaturation.
 Denaturation results in the loss of secondary and
quaternary structures of proteins
 This involves a change in physical, chemical and
biological properties of protein molecules

5. Agents of denaturation
 Physical agents:
Heat, violent shaking, X-rays, UV radiation
 Chemical agents:
Acids, alkalies, organic solvents, salts of heavy
metals (Pb, Hg), urea, salicylate, detergents
(SDS)

6. Characteristics of denaturation
1. The native helical structure of protein is lost
2. The primary structure of protein with peptide
linkages remains intact
3. The proteins loses its biological activity
4. Denatured proteins becomes insoluble in the
solvent in which it was originally soluble
5. The viscosity of the denatured protein increases
while its surface tension decreases
6. Denatured protein is more easily digested
7. Denaturation is usually irreversible

7. Cont…
8. Careful denaturation is some times reversible
(known as renaturation). Hemoglobin undergoes
denaturation in the presence of salicylate. By
removal of the salicylate hemoglobin is renatured
9. Denatured protein cannot be crystallized

8. Coagulation
 The term “coagulum” refers to as semi solid
viscous precipitate of proteins
 Irreversible denaturation results in coagulation
 e.g. albumin and globulins
 Heat coagulation test is commonly used to detect
the presence of albumin in urine

9. Flocculation
 It is the process of precipitation of proteins at
isoelectric pH.
 The precipitate is referred to as flocculum
 Casein can be easily precipitated when adjusted
to isoelectric pH (4.6) by dilute acetic acid
 Flocculation is reversible
 On application of heat, flocculum can be
converted into an irreversible mass, coagulum