24. Explain
the meaning of the terms primary structure, secondary structure, tertiary
structure and quaternary structure of protein.
Answer:
Primary structure
-describes the
type of amino acids and the specific linear sequence of amino acids in a
polypeptide chains
-the sequence
of amino acids of a protein dictates its biological function
-A
substitution or deletion of even one amino acid can affect the structure and
function of the protein.
e.g. lysozyme
which consists of a single polypeptide chain of 129 amino acid residues. The
enzyme causes lysis of the bacterial cell wall. It is found in many tissues and
secretions of the human body, in plants and in egg whites.
Secondary structure
-the regular
arrangement of the polypeptide chain
-coiled to form a right-handed α-helix
α-helix
*held together by hydrogen bonds between every
fourth
*e.g. the
fibrous protein keratin found in hair, nails, horn and feathers
-folded to form beta-pleated sheets
beta- pleated sheets
*the polypeptide chains lie parallel
to one another and are held together by hydrogen bonds
*e.g. silk
protein fibroin
>Many
hydrogen bonds are formed between C=O and NH-groups from the peptide bond
regions. These maintain the stable structure of α-helix and beta-pleated sheets.
>Sometimes
certain regions of the polypeptide chain have no regular secondary structure
and form a ‘random coil’.
Tertiary structure
-formed by the
folding and coiling of the secondary structure of polypeptide chains to form a
precise, compact globular protein which determines its function
-the compact
three dimensional shape is maintained by hydrogen bonds, ionic bonds,
hydrophobic interactions and Van der Waals interactions,
-e.g. enzymes,
antibodies, protein hormones, and myoglobin, the oxygen-storage red pigments in
muscles
Quaternary structures
-consist of
two or more polypeptide chains joined to form a single functional molecule
e.g. the protein haemoglobin
*red pigment found in red blood cells
*made up of two α-chains and beta-chains
*each polypeptide is coiled to form a globular
tertiary structure with a haem group containing an iron ion
*the four
chains are closely associated to produce the quaternary structure of protein
*the structure
is maintained by weak hydrophobic interactions and Van der Waal interactions.
*a haemoglobin
molecule
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