Proteins
•Proteins are synthesized from amino acid through a process called
translation that is carried out by the ribosomes.
•The information for protein synthesis comes from the DNA in the
nucleus.
•The segment of DNA coding for the synthesis of a protein is called a
gene.
•The gene is first used to synthesize an RNA molecule called messenger
RNA (mRNA), which is then transported to the cytoplasm where
protein synthesis occur.
•Protein synthesis involved joining of different amino acids together
in a particular order through formation of bonds called peptide bonds.
•After formation of peptide bonds, functional proteins are formed
through folding, removal of sequences, combination of different
peptides chains and addition of other compounds such as
carbohydrates.
Peptides are short polymers of amino acids.
•Peptide bond is formed when the α- carboxyl group of one
amino acid reacts with the α- amino group of another amino
acid with the release of water molecule.
•Polypeptides (polymer of amino acids joined together by
peptide bonds) vary in size from small to very large ones
comprising of thousands of amino acid residues of the chain.
•A peptide contains a free amino group on one end (the amino
terminal) and a free carboxyl group on the other end (the
carboxy terminal) .
Types of protein and their functions
•Antibodies protein (in animals): are specialized proteins that defend
the body against antigens or foreign invaders. Their ability to travel
through the bloodstream enables them to be utilized by the immune
system to identify and defend against bacteria, viruses, and other
foreign intruders in blood. One way antibodies counteract antigens is
by immobilizing them so that they can be destroyed by white blood
cells.
•Contractile Proteins: Contractile proteins are responsible for muscle
contraction and movement
•Enzymes: all enzymes identified thus far are proteins. Enzymes, which
are the catalysts of all metabolic reactions, enable an organism to build
up the chemical substances necessary for life—proteins, nucleic acids,
carbohydrates, and lipids—to convert them into other substances, and
to degrade them.
•Hormonal Proteins: Hormonal proteins are messenger proteins that
help coordinate certain bodily functions.eg: insulin, cytokines
•Structural Proteins: A large group of structural proteins
maintains and protects the structure of the animal body. The
most common example of a structural protein is collagen
which is found in the bones, cells and skin.
•Storage Proteins: Storage proteins reserve amino acids for the
body until ready for use. Examples of storage proteins include.
Ferritin a storage protein that stores iron.
•Transport Proteins: Transport proteins are carrier proteins
that move molecules from one place to another in the body e.g.
haemoglobin in animals.
Structural organization of proteins
•Proteins are built up with increasingly complex organizational
units.
Primary structure of protein
•This is the linear sequence of amino acids and the location of
di-sulphide bonds. The amino acid sequence of a protein
dictates the three-dimensional structure, which is biologically
active confirmation and more stable than the unfolded state.
The length of the primary protein varies from protein to
protein
Levels of protein structure
•Primary Structure of Protein
•The Primary structure of proteins is the
exact ordering of amino acids forming
their chains.
•The exact sequence of the proteins is very
important as it determines the final fold
and therefore the function of the protein.
•The number of polypeptide chains
together form proteins.
•These chains have amino acids arranged
in a particular sequence which is
characteristic of the specific protein. Any
change in the sequence changes the entire
protein.
Secondary structure of protein
•It refers to local folded structures that form
within a polypeptide due to interactions
between atoms of the backbone.
•The proteins do not exist in just simple
chains of polypeptides.
•These polypeptide chains usually fold due to
the interaction between the amine and
carboxyl group of the peptide link.
•The structure refers to the shape in which a
long polypeptide chain can exist.
•They are found to exist in two different
types of structures α – helix and β – pleated
sheet structures.
•This structure arises due to the regular
folding of the backbone of the polypeptide
chain due to hydrogen bonding between -
CO group and -NH groups of the peptide
bond.
•(a) α – Helix:
•α – Helix is one of the most common ways in which a polypeptide
chain forms all possible hydrogen bonds by twisting into a right-
handed screw with the -NH group of each amino acid residue
hydrogen-bonded to the -CO of the adjacent turn of the helix. The
polypeptide chains twisted into a right-handed screw.
•(b) β – pleated sheet:
•In this arrangement, the polypeptide chains are stretched out beside
one another and then bonded by intermolecular H-bonds.In this
structure, all peptide chains are stretched out to nearly maximum
extension and then laid side by side which is held together by
intermolecular hydrogen bonds. The structure resembles the pleated
folds of drapery and therefore is known as β – pleated sheet.
Tertiary Structure of Protein
•This structure arises from further
folding of the secondary structure of
the protein.
•H-bonds, electrostatic forces,
disulphide linkages, and Vander
Waals forces stabilize this structure.
•The tertiary structure of proteins
represents overall folding of the
polypeptide chains, further folding of
the secondary structure.
•It gives rise to two major molecular
shapes called fibrous and globular.
•The main forces which stabilize the
secondary and tertiary structures of
proteins are hydrogen bonds,
disulphide linkages, van der Waals
and electrostatic forces of attraction.
Quaternary Structure of Protein
•The spatial arrangement of various
tertiary structures gives rise to the
quaternary structure.
•Some of the proteins are composed of
two or more polypeptide chains
referred to as sub-units.
•The spatial arrangement of these
subunits with respect to each other is
known as quaternary structure.
