PROTEINS - COMFORMATION,CLASSIFICATION,STRUCTURE,ROLES,DENATURATION OF PROTEIN
PROTEINS
PROTEINS:
Proteins are the
most abundant biological macromolecules, occurring in all cells. It is also the
most versatile organic molecule of the living systems and occurs in great
variety; thousands of different kinds, ranging in size from relatively small
peptides to large polymers. Proteins are the polymers of amino acids covalently
linked by the peptide bonds. The building blocks of proteins are the twenty
naturally occurring amino acids. Thus, proteins are the polymers of amino
acids.
CONFORMATION OF
PROTEIN:
1. Biuret test
When 2 ml of test
solution is added to an equal volume of 10% NaOH and one drop of 10% CuSO4
solution, a violet colour formation indicates the presence of peptide linkage.
2. Ninhydrin test
When 1 ml of
Ninhydrin solution is added to 1 ml protein solution and heated, the formation
of a violet color indicates the presence of α-amino acids.
CLASSIFICATION OF
PROTEIN :
Based on the
chemical nature, structure, shape, and solubility, proteins are classified as:
Simple proteins:
They are composed of only amino acid residue. On hydrolysis, these proteins
yield only constituent amino acids. It is further divided into:
Fibrous protein:
Keratin, Elastin, Collagen
Globular protein:
Albumin, Globulin, Glutelin, Histones
Conjugated
proteins: They are combined with non-protein moiety. Eg. Nucleoprotein,
Phosphoprotein, Lipoprotein, Metalloprotein, etc.
Derived proteins:
They are derivatives or degraded products of simple and conjugated proteins.
They may be :
Primary derived
protein: Proteans, Metaproteins, Coagulated proteins
Secondary derived proteins: Proteosesn or albunoses, peptones, peptides.
STRUCTURE OF
PROTEIN :
The linear
sequence of amino acid residues in a polypeptide chain determines the
three-dimensional configuration of a protein, and the structure of a protein
determines its function.
All proteins
contain the elements carbon, hydrogen, oxygen, nitrogen, and sulfur some of
these may also contain phosphorus, iodine, and traces of metals like ions,
copper, zinc, and manganese.
A protein may
contain 20 different kinds of amino acids. Each amino acid has an amine group
at one end and an acid group at the other and a distinctive side chain.
The backbone is
the same for all amino acids while the side chain differs from one amino acid
to the next.
The structure of
proteins can be divided into four levels of organization:
1. Primary
Structure
The primary
structure of a protein consists of the amino acid sequence along the
polypeptide chain.
Amino acids are
joined by peptide bonds.
Because there are
no dissociable protons in peptide bonds, the charges on a polypeptide chain are
due only to the N-terminal amino group, the C-terminal carboxyl group, and the
side chains on amino acid residues.
The primary
structure determines the further levels of organization of protein molecules.
2. Secondary
Structure
The secondary
structure includes various types of local conformations in which the atoms of
the side chains are not involved.
Secondary
structures are formed by a regularly repeating pattern of hydrogen bond
formation between backbone atoms.
The secondary structure
involves α-helices, β-sheets, and other types of folding patterns that occur
due to a regularly repeating pattern of hydrogen bond formation.
The secondary
structure of protein could be :
Alpha-helix
Beta-helix
The α-helix is a
right-handed coiled strand.
The side-chain
substituents of the amino acid groups in an α-helix extend to the outside.
Hydrogen bonds
form between the oxygen of the C=O of each peptide bond in the strand and the
hydrogen of the N-H group of the peptide bond four amino acids below it in the
helix.
The side-chain
substituents of the amino acids fit in beside the N-H groups.
The hydrogen
bonding in a ß-sheet is between strands (inter-strand) rather than within
strands (intra-strand).
The sheet
conformation consists of pairs of strands lying side-by-side.
The carbonyl
oxygens in one strand hydrogen bond with the amino hydrogens of the adjacent
strand.
The two strands
can be either parallel or anti-parallel depending on whether the strand
directions (N-terminus to C-terminus) are the same or opposite.
The anti-parallel
ß-sheet is more stable due to the more well-aligned hydrogen bonds.
3. Tertiary
Structure
The tertiary
structure of a protein refers to its overall three-dimensional conformation.
The types of
interactions between amino acid residues that produce the three-dimensional
shape of a protein include hydrophobic interactions, electrostatic
interactions, and hydrogen bonds, all of which are non-covalent.
Covalent disulfide
bonds also occur.
It is produced by
interactions between amino acid residues that may be located at a considerable
distance from each other in the primary sequence of the polypeptide chain.
Hydrophobic amino
acid residues tend to collect in the interior of globular proteins, where they
exclude water, whereas hydrophilic residues are usually found on the surface,
where they interact with water.
4. Quaternary
Structure
Quaternary
structure refers to the interaction of one or more subunits to form a
functional protein, using the same forces that stabilize the tertiary
structure.
It is the spatial
arrangement of subunits in a protein that consists of more than one polypeptide
chain.
ROLE OF PROTEINS:
Proteins are vital
for growth and repair, and their functions are endless. They also have an
enormous diversity of biological functions and are the most important final
products of the information pathways.
Proteins, which
are composed of amino acids, serve in many roles in the body (e.g., as enzymes,
structural components, hormones, and antibodies).
They act as
structural components such as keratin of hair and nail, collagen of bone, etc.
Proteins are the
molecular instruments through which genetic information is expressed.
They execute their
activities in the transport of oxygen and carbon dioxide by hemoglobin and
special enzymes in the red cells.
They function in
the homeostatic control of the volume of the circulating blood and that of the
interstitial fluids through the plasma proteins.
They are involved
in blood clotting through thrombin, fibrinogen, and other protein factors.
They act as the
defense against infections by means of protein antibodies.
They perform
hereditary transmission by nucleoproteins of the cell nucleus.
Ovalbumin,
glutelin, etc. are storage proteins.
Actin, myosin act
as a contractile protein important for muscle contraction.
DENATURATION OF
PROTEINS:
Proteins can be
denatured by agents such as heat and urea that cause the unfolding of
polypeptide chains without causing hydrolysis of peptide bonds.
The denaturing
agents destroy secondary and tertiary structures, without affecting the primary
structure.
If a denatured
protein returns to its native state after the denaturing agent is removed, the
process is called renaturation.
Some of the
denaturing agents include
Physical agents:
Heat, radiation, pH
Chemical agents:
Urea solution which forms new hydrogen bonds in the protein, organic solvents,
detergents.
Coagulation
When proteins are
denatured by heat, they form insoluble aggregates known as coagulum. All the
proteins are not heat coagulable, only a few like the albumins, globulins are
heat coagulable.
Isoelectric point
The isoelectric
point (pI) is the pH at which the number of positive charges equals the number
of negative charges, and the overall charge on the amino acid is zero.
At this point,
when subjected to an electric field the proteins do not move either towards
anode or cathode, hence this property is used to isolate proteins.
REFERENCE:
Smith, C. M.,
Marks, A. D., Lieberman, M. A., Marks, D. B., & Marks, D. B. (2005). Marks’
basic medical biochemistry: A clinical approach. Philadelphia: Lippincott
Williams & Wilkins.
Rodwell, V. W.,
Botham, K. M., Kennelly, P. J., Weil, P. A., & Bender, D. A. (2015).
Harper’s illustrated biochemistry (30th ed.). New York, N.Y.: McGraw-Hill
Education LLC.
John W. Pelley,
Edward F. Goljan (2011). Biochemistry. Third edition. Philadelphia: USA.
https://chemistry.tutorvista.com/biochemistry/proteins.html
http://www.biologydiscussion.com/proteins/proteins-definition-importance-and-classification-biochemistry/41903
https://www.particlesciences.com/news/technical-briefs/2009/protein-structure.html
https://microbenotes.com/proteins-properties-structure-classification-and-functions/
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