Classification of protein, Sources of protein, Properties of protein, Structures of protein, Denaturation of protein, Functions of protein

1. Amino Acids and Proteins
Part: 2
Dr. Ifat Ara Begum
Associate Professor
Department of Biochemistry
Dhaka Medical College, Dhaka
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Dr Ifat, dept of Biochemistry, DMC

2. Learning Objectives
 Classification of proteins
 Sources of proteins
 Properties of proteins: Isoelectric pH
 Different structures of proteins
 Denaturation of proteins
 Functions of proteins
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4. Types Function Example Types Function Example
i)
Structural
Protein
Support
framework
of cell
Collagen, Elastin,
etc
v) Gene
regulatory
Protein
Regulate
genetic
functions
Histone,
Protamine
ii)
Catalytic
Protein
Catalysis Enzymes vi) Protective
Protein
Prevent
infection
Immunoglobulin
iii)
Transport
Protein
Transport of
substances
Albumin,
Transferrin
vii) Receptor
Protein
Receptor
function
LDL receptor
iv)
Hormonal
Protein
Regulation of
functions
Insulin, Glucagon viii)
Contractile
Protein
Muscle
contraction
Actin, Myosin
A) Functional Classification of Proteins
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5. Points i) Simple protein ii) Conjugated protein iii) Derived protein
Composition Composed of amino acid
residues without any non
protein substances
Composed of protein along
with non protein prosthetic
substances
Denatured / degraded
products of simple
protein & conjugated
protein
On
hydrolysis
They produce only amino acids They produce amino acid
plus other non amino acid
substances
-
Types 2 types: Fibrous protein &
Globular protein
- -
Example Fibrous: Collagen, elastin,
keratin etc
Globular: Albumin, globulin etc
Nucleoprotein, lipoprotein
etc
Proteose, peptone, PP,
peptides
B) Classification of proteins based on their size, shape & solubility
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Dr Ifat, dept of Biochemistry, DMC

6. Points Fibrous protein Globular protein
Synonym Scleroprotein Spheroprotein
Shape Fibre like with high tensile strength Oval / spherical
MW High Relatively low
Origin Entirely animal origin -
Water solubility Insoluble Soluble
Digestibility Highly resistant to proteolytic enzymes Digestible by proteolytic enzymes
Site Tendons, ligaments, cartilage, hair, nail
etc
-
Example Collagen, elastin, keratin etc Albumin, globulin, histone,
protamine etc
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7. Conjugated protein Prosthetic / Non protein groups
Nucleoprotein Nucleic acid (DNA , RNA)
Lipoprotein Lipid
Glycoprotein Carbohydrate (oligosaccharide)
Hemoglobin, myoglobin,
peroxidase, catalase, cytochrome
Heme
Ferritin Iron
Ceruloplasmin Copper
Carbonic anhydrase Zinc
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Dr Ifat, dept of Biochemistry, DMC

8. Points i) Complete protein ii) Incomplete protein iii) Partially
incomplete protein
Composition Contains all essential
amino acids
Completely lack one or
more essential amino
acids
Partially lack one or
more essential amino
acids
Role on
growth
Promotes growth Can not promote growth -
Example Egg albumin, meat etc Gelatin (lacks Trp) Wheat protein
C) Nutritional classification of proteins
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12. Physical Properties
 Macromolecule
 Shape: variable
 MW: Variable
 Shows differential solubility but
mostly water insoluble
 Forms colloidal solution with water
which is optically active
Chemical Properties
 Amphoteric / Ampholyte in nature
 Acts as buffer
 Heat labile (Denatured / coagulated by
heat)
 Expresses charge on the surface
 Have a definite iso-electric pH
 Gives colour reaction with
 ninhydrin reagent
 biuret reagent
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Proteins exist as Zwitter ion / Dipolar form at isoelectric pH
Isoelectric point (pI)/ Isoelectric pH:
It is the pH of an aqueous solution of an amino acid (or protein) at which the
molecules on average have no net charge.

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15.  Usually described at 4 levels
of organization
 Any alteration in the
structure or sequencing, will
change the properties and
functions of the protein
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17.  Exact linear sequence of amino acids in a protein held together by peptide bonds
 Each Amino acid in a polypeptide chain is termed as amino acid residue
 N-terminal end / Amino end: The free NH2 group of one terminal amino acid,
traditionally assumed to be on left hand side of a PP chain
 C-terminal end / Carboxyl end: The free COOH group of another terminal amino acid,
traditionally assumed to be on right hand side of a PP chain
 Sequence of amino acids is written from N-terminus to the C-terminus
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Dr Ifat, dept of Biochemistry, DMC
A) Primary Structure

18.  Primary structure of a protein is
determined genetically and this
in turn determines the secondary,
tertiary & quaternary structure
of that protein
 Any change in the sequence of
amino acids in a peptide / PP
chain is abnormal & may affect
the properties and functions of
the protein
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Dr Ifat, dept of Biochemistry, DMC

19. Why called Amino acid residue ?
Because, amino acids in the peptide / polypeptide chains have already lost
the atoms of water during formation of peptide bonds among them
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Helical Structure:
It is produced like a
spiral staircase
around the central
axis.
Pleated sheet :
It is formed by
hydrogen bond
between 2 extended
polypeptide chains
B) Secondary Structure:Formed by periodic folding, twisting or coiling of its
primary structure

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Dr Ifat, dept of Biochemistry, DMC
D) Quaternary Structure
C) Tertiary Structure

23. Points A) Primary
structure
B) Secondary
structure
C) Tertiary
structure
D) Quaternary
structure
What is it? Exact linear
sequence of amino
acids in a protein
held together by
peptide bonds
The Helical or
pleated sheet
like protein
The three dimensional
globular form of protein
A multi chain oligomeric
protein
How
formed?
By joining a
number of amino
acids whose
sequence is
determined
genetically
By periodic
folding, twisting
or coiling of its
primary
structure
By further folding &
twisting of a secondary
structure of protein
about itself keeping the
hydrophobic side chains
buried interiorly &
hydrophilic groups
exposed outside
By joining of 2 or more
polypeptide chains (same /
different type) at a definite
spatial relationship.
(Individual PP chain is
called monomer/ subunit)
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Dr Ifat, dept of Biochemistry, DMC

24. Points A) Primary
structure
B) Secondary
structure
C) Tertiary
structure
D) Quaternary
structure
Any
specialty?
It determines the
secondary,
tertiary &
quaternary
structure of a
specific protein
2 types of conformation:
i) Helical form
ii) Pleated sheet form
It is the overall shape
of a single protein
unit.
The protein is
functionally active at
this level
Collection of single
protein units at tertiary
level.
Specific
Bond (S)
- Peptide bond
(Mainly)
- Di sulfide bond
- Peptide bond
- Di sulfide bond
- Hydrogen bond
- Peptide bond
- Di sulfide bond
- Hydrogen bond
- Ionic bond
- Hydrophobic bonds
- Van der walls force
Same as tertiary
structure :
- Peptide bond
- Di sulfide bond
- Hydrogen bond
- Ionic bond
- Hydrophobic bonds
- Van der walls force
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Dr Ifat, dept of Biochemistry, DMC

25. Points Primary
structure
Secondary
structure
Tertiary
structure
Quaternary
structure
Example Insulin Fibrous protein
(collagen, elastin,
keratin)
Albumin, globulin,
antibody etc.
Hb, Insulin, CPK,
LDH etc
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Disulfide bond:
(–S–S–) formed by the coupling of two thiol (–SH) groups.
Cysteine has thiol group in its side chain.

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29.  It is the process of disruption &
possible destruction of of protein’s
secondary, tertiary & quaternary
structures while preserving the
primary structure.
 It is not accompanied by hydrolysis
of peptide bond.
 No intra molecular change (no
change in MW)
 Change occurs in properties of
proteins
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Dr Ifat, dept of Biochemistry, DMC

30. Denaturation
Usually Irreversible
(e.g. Boiled egg, cooked
meat / fish etc )
Reversible
(Under ideal condition.
e.g. Ribonuclease at high urea
conc. )
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Dr Ifat, dept of Biochemistry, DMC

31. Denaturing agents:
 Physical agent: Extremes of temperature,
ultra sound, UV radiation etc
 Chemical agents: Extreme pH, strong acid /
alkali, detergent, heavy metals etc
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Dr Ifat, dept of Biochemistry, DMC

32. Changes in protein following denaturation
 Physical change:
 Increased viscosity
Decreased diffusibility
 Chemical change:
 Increased chance of precipitation
 Decreased solubility
 Biological change :
 Loss of function
Increased digestibility by proteolytic enzymes
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34.  Source of energy
 Formation of cytoskeleton
(Flexible structural
framework for cells / tissues)
 Provides :
 Mechanical support: By structural protein
like collagen, elastin
 Defense against infection: By antibody like
immunoglobulin
 Helps in:
 Muscle contraction : By
contractile protein like actin,
myosin
 Coagulation : By clotting factors
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 Acts as:
 Vehicle for transport of diff molecules
like hormones, drugs, vitamins etc
 Buffer

35.  Promotes:
 Catalytic function by enzymes
 Hormonal functions by hormones
 Plasma proteins maintain COP of
plasma to maintain blood volume
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Dr Ifat, Dept of Biochemistry, DMC
 Storage, expression & transmission of genetic information by
nucleoprotein

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Dr Ifat, Dept of Biochemistry, DMC