SIF,CUSAT

1. Denaturation of Proteins
Koushik Das
M.Sc. 1st
sem
School of Industrial Fisheries
Cochin University of Science&
Technology

2. Denaturation of Proteins
Can be defined as any change that alters
the unique 3D conformation of a protein
molecule without causing concomitant
cleavage of the peptide bonds
It leads to temporary or permanent loss of
activity.

3. Denaturation is a process that disrupts
secondary, tertiary, and quaternary
structures.
The primary structure is not destroyed
during denaturation.

4. ∗ Denaturation
∗ Any physical or chemical agent that destroys the
conformation of a protein is said to “denature” it.
∗ Examples:
∗ Heat (boil an egg) Frying an egg is an example of
denaturation by heat. Heat disrupts intermolecular
forces such as hydrogen bonding, and other polar
interactions.
∗ Addition of 6M Urea (breaks H bonds)
∗ Detergents (surface-active agents)
∗ Reducing agents (break -S-S- bonds)
∗ Acids/Bases/Salts (affect salt bridges)
∗ Heavy metal ions (Hg2+
, Pb2+
)

5. Other denaturing agents:
• Changes in pH, which alters the ability of the acidic and
basic side chains to form salt bridges
• Organic compounds, which will disrupt the disulfide
bonds
• Heavy metals that disrupt salt bridges and disulfide bonds
• Mechanical agitation, which disrupts hydrogen bonds
Denaturation of Proteins

6. A protein will lose its biological activity if it loses its
three-dimensional shape.
Denaturation of Proteins

7. © 2011 Pearson Education, Inc.Chapter 9 7
Denaturation of Proteins

8. ∗ At Very High or Low pH.
∗ At Very High Temperatures.
∗ By Heavy Metal Ions.
∗ By Small Polar Molecules.
When and How are Proteins Denatured?

9. ∗ Affects Ionic Bonds.
∗ A High (>9) or Low pH (<3) will neutralise the charge on one
of the ionically bonded ions.
∗ Hence the ionic bond is broken.
Denaturation at HIGH or LOW pH

10. ∗ As the temperature increases the energy of the
protein increases.
∗ As the energy increases bonds are broken in the
following order:
Denaturation at High Temperatures
• Van der Waal’sVan der Waal’s
• Hydrogen BondsHydrogen Bonds
• Ionic BondsIonic Bonds

11. ∗ Certain metal ions will disrupt the Van der Waals’
forces in proteins.
∗ Heavy Metal Ions, such as Ag+
and Hg2+
, also inhibit
enzyme activity.
∗ They do so by reducing the SH groups in cysteine.
Denaturation by Metal Ions
PLEASE NOTE: Spelling mistake on
pictures: --- The S-S bond is a
DISULPHIDE bond.

12. ∗ Urea ( CO(NH2)2 ) in concentrated solution will denature
proteins.
∗ It disrupts the Hydrogen Bonds.
∗ This causes complete denaturation.
Denaturation by Small Polar Molecules

13. Denaturation at the Quaternary Level
- Dissociation of the subunits
- Dissociation is usually accomplish by disrupting ionic
interaction between certain groups
- Urea, guanidine hydrochloride or dodecyl sulfate
(detergent)

14. Denaturation at the Tertiary Level
- Heat and ultraviolet radiation
- Extremes of pH (acidic and basic reagents)
- Ions of heavy metals (e.g. Hg2+
, Ag+
, Pb2+
) forms
mercaptides with thiol groups
- Organic solvents or non-polar additives (e.g. ethyl
alcohol, formaldehyde and rubbing alcohol)

15. Denaturation at the Tertiary Level
- Urea and guanidinium chloride
- Amphipaths can destabilize polar and non-
polar interactions
- Polyethylene glycol – can destabilize
hydrogen bonds
- Performic acid – can cause oxidation of
disulfide bonds
- Mercaptoethanol –can cause disruption of
disulfide bonds

16. Denaturation of the Secondary Level
High temperature - will increase the vibration
of atoms along hydrogen bonds

17. Denaturation at the Primary Level
- Hydrolysis
- Enzymes (peptidases)
- Cyanogen bromide, N-
bromosuccinimide

18. Why is Denaturation Sudden?
Concentration of denaturant or temperature
100%
0%
NativeStructure
Critical value
COOPERATIVE PROCESS
Partly denatured structure is weaker so
begins to change faster

19. ∗ Some denaturation is reversible
∗ Urea (6M) then add to H2O
∗ Some is irreversible
∗ Hard boiling an egg
Denaturation

20. ∗ Denaturation
Denaturation

21. Unfolding and
Renaturation (refolding)
Anfinsen’s Experiment:
Spontaneous folding of
denatured ribonuclease
The first proof for
“sequence determines structure”

22. ∗ Curling straight hair or straightening curly hair
requires protein denaturation.
∗ Both processes require disruption of disulfide
bonds found in the hair protein keratin.
∗ The disruption of disulfide bonds reshapes the hair
and forces the reformation of disulfide bonds in
new places.
.
Cont..

23. Ammonium
thioglycolate, which
reduces (breaks)
disulfide bonds, and
hydrogen peroxide
(reforms disulfide
bonds) are two
chemical agents used
in hairstyling.
23
Denaturation of Proteins

24. ∗ The process of denaturation is used as an antidote
for lead or mercury poisoning.
∗ Egg whites can be given to an individual who has
ingested a heavy metal. Egg whites are denaturated
by the heavy metals and a precipitate is formed.
∗ Vomiting is induced to eliminate the metal-protein
precipitate.
Denaturation of Proteins

25. Behavior of Denatured Protein
Hydrophobic core
Hydrophilic surface
NATIVE
AGGREGATED
or other ingredient interactions
DENATURED
Unfolding forces som
hydrophobic AA to
surface
Fast under non-physiological conditions
Slow under physiological conditions

26. ∗ Loss of enzymatic activity (death)/ Loss of biological
activity
∗ Destruction of toxins
∗ Improved digestibility
∗ Loss of solubility
∗ Changes in texture
∗ Altered water binding capacity
∗ Increased intrinsic viscosity
∗ Inability to crystallize
Consequences of Denaturation

27. ∗ reduce the protein to a simple polypeptide chain
∗ slow freezing and variability of storage conditions, cause this
denaturation.
∗ A denatured protein has not only lost its ability to function as an
enzyme, but also its "water-holding“ ability.
∗ This results in denatured fish flesh dripping excessively when thawed (a
situation known as "drip-thaw"), and appearing white, dull and spongy,
and upon chewing becoming
fibrous and tasteless.
Protein Denaturation
- chemical change during fish autolysis

28. ∗ The conversion of a biologically functional molecule
into a non-functional form
∗ There are many denatured states but one native state
∗ Proteins can regenerate to their native state but
slowly
∗ Denatured proteins have a greater tendency to
aggregate.
Denaturation