Classification and properties of protein

PROTEIN CAN BE CLASSIFIED BY:
Structure

Biological

function
Shape and solubility
Composition
Nutritional basis

PRIMARY STRUCTURE


The primary structure of proteins is defined as a
linear sequence of amino acids joined together by
peptide bonds.



Peptide bonds and disulfide bonds are responsible
for maintaining the primary structure.

SECONDARY STRUCTURE
The secondary structure of a protein is defined
as a local spatial structure of a certain peptide
segment, that is, the relative positions of
backbone atoms of this peptide segment.
 H-bonds are responsible for stabilizing the
secondary structure.
 Repeating
units
of
Ca-C(=O)-N(-H)-Ca
constitute the backbone of peptide chain.
 Six atoms, Ca-C(=O)-N(-H)-Ca, constitute a
planer peptide unit.


TERTIARY STRUCTURE


The tertiary structure is defined as the threedimensional arrangement of all atoms of a
protein.

QUATERNARY STRUCTURE
The quaternary structure is defined as the
spatial arrangement of multiple subunits of a
protein.
 These subunits are associated through H-bonds,
ionic interactions, and hydrophobic interactions


CLASSIFICATION
BY
BIOLOGICAL FUNCTION

ENZYMES
 Those

proteins which are highly
specialized in their function with
catalytic activity.
 These
proteins regulate almost all
biological reactions going on inside all
living cells.
 There are about 2000 different enzymes
has been recognized; each capable of
catalyzing a different kind of biochemical
reaction.

TRANSPORT PROTEINS
are those proteins which help in transportation of
life sustaining chemicals vital gases and nutrients.
 Carry essential substances throughout the body.
 Example:
- Haemoglobin is a globular protein present in RBC of
blood can binds with oxygen when blood passes
though longs and distributes oxygen through out
the body cells to affect cellular respiration.
- Blood plasma contains lipoprotein which carries
lipids from the liver to other organs.


STORAGE PROTEINS
are those stored inside the cells or tissue as
reserved food and can be mobilized at the time of
nutrient requirement to provide energy.
 Store nutrients.
 Example:
- Casein stores protein in milk.
- Ferritin stores iron in the spleen and liver.


CONTRACTILE/MOTILE PROTEINS
Move muscles.
 the ability to contract to change the shape or to
move about.
 These proteins includes. Actin and myosin; which
are present in form of filamentous protein in muscle
cells for functioning in the contractile systems.


STRUCTURAL PROTEINS
This type of protein form major component of
tendons, cartilages and bones.
 These are fibrous proteins named collagen.
Ligaments are contains special structural protein
capable of stretching in two dimensions called as
elastin.
 Hairs finger nails, feathers of birds consists of tough
insoluble protein named keratin.
 Major component of silk fibers, threads of spider
web contain structural protein named fibroin.


DEFENSE PROTEINS
Many proteins in body of organisms posses
defending action against the invasion and attack of
foreign entities or protect the body from injury.
 Among these proteins special globular protein
named immunoglobulin's or
antibodies in
vertebrate’s body is the most indispensible protein.
 It synthesized by lymphocytes and they can
neutralize the foreign protein produced by bacteria,
virus and other harmful microbes called antigens
through precipitation or glutination.


REGULATORY PROTEIN
Some proteins help to regulate cellular or
physiological activity. Among them are many
hormones, such as insulin; which is a regulatory
protein formed in pancreatic tissue help to regulate
the blood sugar level.
 Growth hormones of pituitary and parathyroid
hormones regulate Ca++ and phosphate transport
in body. Other proteins called repressors regulate
biosynthesis of enzymes.


OTHER FUNCTIONAL PROTEINS
There are number of proteins whose functions are
not yet specified and are rather exotic. These
includes –
 Monelin: - A protein of an African plant has an
intensely sweet taste and used as non toxic food
sweetener for human use.
 Antifreeeze: A protein present in blood plasma of
Antarctic fisher which protect their blood freezing in
ice cold water.
 Resillin: A type of protein present in wing hinges of
some insects with elastic properties.


CLASSIFICATION
BY

SHAPE & SOLUBILITY

FIBROUS PROTEINS


these proteins have a rod like structure. They are
not soluble in water.

(a) These are made up of polypeptide chain that
are parallel to the axis & are held together by strong
hydrogen
and
disulphide
bonds.
(b)
They can be stretched & contracted like
thread.
 Examples:
-Collagen
-Keratin
-Fibrinogen
-Muscle protein

GLOBULAR PROTEINS




these proteins more or
less spherical in nature.
Due to their distribution of
amino acids (hydrophobic
inside,
hydrophillic
outside) they are very
soluble
in
aqueous
solution.
Examples
Myoglobin, albumin, globu
lin, casein, haemoglobin,
all of the enzymes, and
protein hormones.

MEMBRANE PROTEINS
These are protein which are in association with lipid
membranes.
 Those membrane proteins that are embedded in
the lipid bilayer have extensive hydrophobic amino
acids that interact with the non-polar environment of
the bilayer interior.
 Membrane proteins are not soluble in aqueous
solution.


SIMPLE PROTEINS
are those which on hydrolysis yield only amino
acids and no other major organic or inorganic
hydrolysis products. They usually contain about
50% carbon,7% hydrogen, 23% oxygen, 16%
nitrogen and 0–3% sulphur.
 Example:
-Egg (albumin)
-Serum (globulins)
-Wheat (Glutelin)
-Rice (Coryzenin)


CONJUGATED PROTEINS
are those which on hydrolysis yield not only amino
acids but also organic or inorganic components.
The non-amino acid part of a conjugated protein is
called prosthetic group.
 Conjugated proteins are classified on the basis of
the chemical nature of their prosthetic groups.


COMPLETE PROTEINS
A complete protein contains an adequate amount of
all of the essential amino acids that should be
incorporated into a diet.
 Some protein contains all the amino acids needed
to build new proteins, which generally come from
animal and fish products. A complete protein must
not lack even one essential amino acid in order to
be considered complete.
 Sources: The following foods are examples of
complete proteins, which need not be combined
with any other food to provide adequate protein:
Meat, Fish, Poultry, Cheese, Eggs, Yogurt, Milk


INCOMPLETE PROTEINS
An incomplete protein is any protein that lacks one
or more essential amino acids in correct
proportions. These can also be referred to as partial
proteins.
 Even if the protein contains all the essential amino
acids, they must be in equal proportions in order to
be considered complete. If not, the protein is
considered incomplete.
 Sources of Incomplete Proteins: Grains, Nuts,
Beans, Seeds, Peas, Corn


COMBINING INCOMPLETE PROTEINS TO
CREATE COMPLETE PROTEINS
By combining foods from two or more incomplete
proteins, a complete protein can be created.
The amino acids that may be missing from one type
of food can be compensated by adding a protein
that contains that missing amino acid.
 When eaten in combination at the same meal, you
are providing your body with all the essential amino
acids it requires. These are considered
complementary proteins when they are combined to
compensate for each other's lack of amino acids.


SAMPLES OF COMPLEMENTARY
PROTEINS
create a complete protein in one meal include:
 Grains with Legumes - sample meal: lentils and rice
with yellow peppers.
 Nuts with Legumes - sample meal: black bean and
peanut salad.
 Grains with Dairy - sample meal: white cheddar and
whole wheat pasta.
 Dairy with Seeds - sample meal: yogurt mixed with
sesame and flax seeds.
 Legumes with Seeds - sample meal: spinach salad
with sesame seed and almond salad dressing.


PROPERTIES
OF
PROTEINS

•Physical

Properties
•Chemical Properties

PHYSICAL PROPERTIES
contains carbon, hydrogen, oxygen, nitrogen and
small amount of sulphur.
 composed of amino acids that are linked together
by peptide bonds
 act as catalysts, enzymes that speed up the rate of
chemical reactions
 provides structural support for cells
 transports substances across cell membrane
 provides a defense mechanism against pathogens
(antibodies)
 responds to chemical stimuli
 secretes hormones.


TO DETERMINE MOLECULAR NATURE
•In order to determine the nature of the molecular and
ionic species that are present in aqueous solutions at
different pH's, we make use of the Henderson Hasselbalch Equation.

ISOELECTRIC POINT


the negatively and positively charged molecular
species are present in equal concentration. This
behavior is general for simple (difunctional) amino
acids.

ELECTROPHORESIS


The distribution of charged species in a sample can
be shown experimentally by observing the
movement of solute molecules in an electric field,
using the technique of electrophoresis.

CHEMICAL PROPERTIES


Denaturation of Proteins
Denaturation
is
a
process
in
which proteins or nucleic acids lose the quaternary
structure, tertiary structure and secondary
structure which is present in their native state, by
application of some external stress or compound
such
as
a
strong
acid
or
base,
a
concentrated inorganic salt, an organic solvent
(e.g., alcohol or chloroform), radiation or heat.

Denaturation occurs because the bonding interactions
responsible for the secondary structure (hydrogen
bonds to amides) and tertiary structure are disrupted.
 In tertiary structure there are four types of bonding
interactions between "side chains" including: hydrogen
bonding, salt bridges, disulfide bonds, and non-polar
hydrophobic interactions. which may be disrupted.
 Therefore, a variety of reagents and conditions can
cause denaturation. The most common observation in
the denaturation process is the precipitation or
coagulation of the protein.


HEAT
Heat can be used to disrupt hydrogen bonds and
non-polar hydrophobic interactions. This occurs
because heat increases the kinetic energy and
causes the molecules to vibrate so rapidly and
violently that the bonds are disrupted. The proteins
in eggs denature and coagulate during cooking.
Other foods are cooked to denature the proteins to
make it easier for enzymes to digest them. Medical
supplies and instruments are sterilized by heating
to denature proteins in bacteria and thus destroy
the bacteria.

ALCOHOL DISRUPTS HYDROGEN BONDING:




Hydrogen bonding occurs between amide groups in
the secondary protein structure. Hydrogen bonding between
"side chains" occurs in tertiary protein structure in a variety of
amino acid combinations. All of these are disrupted by the
addition of another alcohol.
A 70% alcohol solution is used as a disinfectant on the skin.
This concentration of alcohol is able to penetrate the bacterial
cell wall and denature the proteins and enzymes inside of the
cell. A 95% alcohol solution merely coagulates the protein on
the outside of the cell wall and prevents any alcohol from
entering the cell. Alcohol denatures proteins by disrupting the
side chain intramolecular hydrogen bonding. New hydrogen
bonds are formed instead between the new alcohol molecule
and the protein side chains.

ACIDS AND BASES DISRUPT SALT BRIDGES:
Salt bridges result from the neutralization of an
acid and amine on side chains. The final interaction
is ionic between the positive ammonium group and
the negative acid group. Any combination of the
various acidic or amine amino acid side chains will
have this effect.
 The denaturation reaction on the salt bridge by the
addition of an acid results in a further straightening
effect on the protein chain as shown in the graphic
on the left.


HEAVY METAL SALTS






Heavy metal salts act to denature proteins in much the same
manner as acids and bases. Heavy metal salts usually
contain Hg+2, Pb+2, Ag+1 Tl+1, Cd+2 and other metals with high
atomic weights. Since salts are ionic they disrupt salt bridges in
proteins. The reaction of a heavy metal salt with a protein
usually leads to an insoluble metal protein salt.
This reaction is used for its disinfectant properties in external
applications. For example AgNO3 is used to prevent gonorrhea
infections in the eyes of new born infants. Silver nitrate is also
used in the treatment of nose and throat infections, as well as
to cauterize wounds.
Mercury salts administered as Mercurochrome or Merthiolate
have similar properties in preventing infections in wounds.

Acids
 Urea 6 – 8 mol/l
 Acidic protein denaturants
 Guanidinium chloride 6 mol/l
include:
 Lithium perchlorate 4.5 mol/l
 Acetic acid[8]
Disulfide bond reducers[edit]
 Trichloroacetic acid 12% in water  Agents that break disulfide
 Sulfosalicylic acid
bonds by reduction include:[citation
needed]
Solvents
 2-Mercaptoethanol
 Most organic solvents are
denaturing, including:
 Dithiothreitol
 Ethanol
 TCEP (tris(2carboxyethyl)phosphine)
 Methanol
Other
Cross-linking reagents
 Cross-linking agents for proteins  Picric acid
include:[citation needed]
 Radiation
 Formaldehyde
 Temperature
 Glutaraldehyde
 Chaotropic agents
 Chaotropic agents include:

Example of denaturation that occurs in our
living:
1. Denaturation of human hair
 The extent to which fatty acid oxygenases are
activated in the normal epidermis is not known
2. In cooking eggs
 cooking eggs turns them from runny to solid
 cooking food makes it more digestible.
3. Milk forms a solid curd on standing
·
bacteria in milk grows
·
forms lactic acid
·
protonates carboxylate groups
·
becomes isoelectric
·
coagulates into a solid curd

Classification and properties of protein

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